# Modifying the Xiang Sheng 708B [56K!!!!]



## dcheming (Aug 25, 2017)

Well I figured it was about time to condense all the info on modifying this amp into a dedicated thread. If you're interested to see where the majority of this info came from please see the main Xiang Sheng thread: http://www.head-fi.org/forums/f5/xia...be-amp-189198/

*Intro*

While the overall build quality of this amplifier isn't that bad, the quality of most of the parts is pretty low, but that's exactly why there is so much room for sonic improvement by doing modifications. One of the great things about this amp is that there is quite a bit of room inside, unlike most of the other tube amp offerings in this price range, and this makes it quite easy to add big parts such as film caps, electrolytic caps, and volume pots.

So to start off here's an inside pic of a bone stock 708B courtesy of *Nugget*:









Here's the schematic for the audio section courtesy of *zer061zer0*:







Here's the parts list:

 Capacitors

C1,2 330uF 450V electrolytic - Main filter.
C6,7 150uF 200V electrolytic - Headphone output.
C8,9 33uF 450V electrolytic - Main filter bypass.
C101,201 0.1uF 50V film - Input.
C104,204 0.22uF 400V film - Interstage coupling.
C105,205 0.22uF 400V film - Preamp output.


Resistors

R101,201 1M 1/2W
R102,202 33K 2W - Anode load for 6N3.
R103,203 1K 1W - Cathode bias for 6N3.
R104,204 200R 1/2W
R105,205 220R 1W - Cathode bias for 6922.
R106,206 1M 1/2W
R107,207 1k 1w - Input.
R1 1K 2W
R2 2K 2W
R3,4 100R 1W
R7,8 100R 1W
R9,10 330K 1/4W - The value was hard to read so I'm not 100% positive on this one.
R1,2 10R 1W - Headphone output.


Misc

Vol Pot - 100K stereo audio/log taper
D1,2 1N4007 1A 1000V diodes
Screws - M3 x 0.5mm



*Safety*

Before I get into the main mods, I would like to touch upon a few things concerning safety while working on this amp. First and foremost is the fairly HV (~220VDC) that is present while the amp is powered up. If you are ever taking measurements in the circuit with your DMM while the power is on, it is recommended to keep one of your hands behind your back. Doing this eliminates the possibility of your body forming a path for HV to travel through, which could be fatal. It's a good habit to get into while measuring tube amps or anything electrical for that matter.

The other main thing to keep in mind is that the big 330uF filter capacitors in the power supply will remain charged for quite a while. So even if you unplug the amp there is still a considerable shock hazard with these big caps. Many amps employ bleeder resistors on the filter caps to safely drain them as soon as the power is tuned off. Unfortunately the XS 708B doesn't have these, so I'd highly recommend adding these as your first mod, especially if you plan on taking this amp apart a lot to do extensive mods. Anything from about 50KΩ to 200KΩ will be okay for this purpose. I soldered a 2W resistor to both of the caps to divide the heat to be dissipated into two resistors. If you used a 100KΩ resistor on each cap, then they will dissipate about 1/2 watt each. If you used a 150KΩ resistor on each cap, then they will dissipate about 1/3 watt each. Adding these resistors will drain the filter caps within a few minutes of the power being turned off. In the mean time you could just use something like a 25KΩ resistor with a pair of alligator clip wires to manually drain these caps.

Bleeder resistors added:








 Another simple power supply mod that I recommend is replacing R1 and R2 in the power supply with higher wattage resistors. This isn't really necessary, but in my amp these two resistors were getting pretty hot. For R2 (2KΩ) I used a 3W NTE with plenty of space around it for good air circulation. R1 (1KΩ) is situated in between the two filter caps so I wanted to do something different here since the larger size of a 3W would bring it pretty close to the caps themselves, which could potentially heat them up. So for this I crafted an equivalent 1KΩ 4W resistor using four 1KΩ 1W resistors in series/parallel. This is ideal here since it gives plenty of power handling, good clearance in between the caps, and much better air circulation.








1KΩ 4W combo resistor:








*Crucial Mods*

The mods in this group are the ones that I feel make the biggest improvement in the sound. If you want to know what will give the biggest bang for the buck, this is where to start. This crucial group includes the volume pot, the input resistors, the interstage coupling caps, the output caps, and the output resistors.


Volume Pot

The stock volume pot is a very cheap carbon type and it suffers from pretty severe channel imbalance. This is especially a problem in the 7 to 8 o'clock range, which is right in the range where you'll be the majority of the time due to the very high gain of the 708B. I measured how much difference there was between the two channels for this pot at a few different rotations and it averaged about 2000Ω, which is why it has such horrible channel balance. I upgraded this to a 100KΩ ALPS stereo pot and I was very surprised by how much this helped with the clarity. The size of the soundstage increased a bit as well, which was especially noticeable with instruments that are panned far to one side.

There's a few notes about doing this mod. The threaded barrel of the ALPS is slightly larger than the stock pot so you will have to enlarge the mounting hole a little bit. I did this by hand so I don't know what size drill bit is required. A new hole will also have to be drilled to locate the little tab on the ALPS that keeps it from rotating and I would recommend locating this hole to orient the pot with the pins facing up since this gives good access to them for later when upgrading the signal wire. The knob that I used is from Rat-Shack and since it's made for a 1/4" shaft I had to wrap a single layer of electrical tape around the shaft of the ALPS 6mm shaft for the knob to be centered. This particular knob almost fits into the circular recess milled into the faceplate and with a little bit of sanding to the knob's outer edge it'll fit inside.

Detail of new hole for tab and tape wrap:












Input & Output Resistors

Both pairs of these resistors are directly in the audio path and since low quality film resistors have the potential to be noisy I'd definitely change these four resistors. This is a very simple mod and there is a wide choice of options for these. You could just play it safe and use some nice metal films like PRP or Vishay/Dale, you could try something different like some RIKEN carbon films, or you could get crazy and go with some Audio Note tantalum film resistors here like some of us have. The input resistors are 1KΩ 1/2W (R107,207) and the headphone output resistors are 10Ω 2W (R1,2 *note that these two resistors share the same part number as the two resistors previously mentioned in the power supply so be careful).

RIKEN input resistors:






NTE output resistors on HP jack PCB:








Interstage Coupling Caps

The stock caps here are WIMA metallized mylar so there is definitely room for improvement. Changing these caps (C104,204) has a pretty big affect on the sound, right up there with the changing the volume pot. I went with Auricaps for these, but I would recommend the Mundorf M-Cap ZN as a better choice for around the same price. Like the input/output resistors this is a very easy mod. For those who also plan on using the preamp output of the 708B I would upgrade C105,205 as well.








Headphone Output Caps

The stock ones are just some cheap 150uF electrolytic caps and needless to say these can be significantly improved upon. If you are using lower impedance cans I would increase their uF value, which lowers the frequency at which the bass rolls off. Since I use 300Ω cans I just went with 200uF here in the form of two ELNA 100uf 250V caps paralleled per channel. Whatever you end up using just make sure it's rated for at least 250V to be safe and make sure you observe polarity when installing these.













*Other Signal-Path Mods*

This is all the other stuff in the signal path that could be upgraded after the crucial mods above. This includes the RCA jacks, signal wire, jumper, input caps, and HP jack. Upgrading these still helps, just not nearly as much as the above group.


RCA Jacks

The RCA jacks in my amp were of very low quality and one was undersized enough to let the IC easily fall off. The solder job on these was also horrible. I replaced these with some Teflon Calrad RCA jacks. The mounting holes are 3/8" so any standard RCA jack will work.

Stock jacks:







Calrad jacks:








Signal Wire

The signal wire used in this amp is of fairly low quality as is the solder job on them. I used some Belden 7882A CAT6 Teflon wire to replace this since it's easy to work with and very affordable. Even though the stock wire is shielded I haven't had the slightest problem with hum by using this wire. Of course there are many options for wire here, but I would use something smaller than 20ga to avoid having to enlarge the holes in the PCB with a small drill bit.

Stock vs. Belden CAT6 wire:






Amp with Belden CAT6 wire throughout:








Jumper

There is also a little jumper that is in the direct signal of the right channel. I believe the stock one is made of steel so it should definitely be replaced. I used a small piece of the CAT6, but a piece of lead-wire left over from a cap or resistor would work too.


















Input Caps

The input caps (C101,201) can also be replaced with jumpers if you are sure that your source puts out an acceptable amount of DC. I just used jumpers made out of CAT6 for this.








Headphone Jack

The stock headphone jack is okay, but the way it's mounted kind of sucks. I replaced mine with a SwitchCraft open-frame type designed for thick panels and I soldered the 10Ω NTE output resistors directly to it.


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## cotdt

Also the CAT6 wires should be jacketed to lower the capacitance of the wiring. you get better bass response that way.


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## dcheming (Aug 26, 2017)

*Miscellaneous Mods*

This group includes adding a toggle power switch, replacing the power wire, mounting the transformer on standoffs, rotating the power supply, adding a source selector switch, and reducing the heater voltage. These mods don't really improve the sound they just fix problem areas, add features, or just make things look nicer.


Toggle Power Switch

I'm not really a big fan of push button switches. The main problem with the one in the 708B is that both it and the HP jack are held up to the front panel by a very flimsy aluminum bracket. This bracket flexes back when you insert headphones and eventually the switch can become off center a bit. So I replaced the push button with a 15 amp medium duty toggle from ACE Hardware. I used some plastic washers to protect the finish of the front panel and to electrically isolate the switch from the front panel in case of internal failure. If you are leaving the stock HP jack but putting in a toggle switch, you'll have to cut the circuit board a bit to give some clearance for the switch.













Power Wire

This mod is completely unnecessary since the amp only draws about 20 watts while in use, but I was already adding the toggle switch so I did the power wire too just to make it look better. I used 12ga because that's all ACE had in stock at the time, but I would recommend 14ga as the biggest since I had to slightly modify the solder pins of the IEC jack to be able to fit the massive 12ga through. I also mounted the IEC jack with stainless fasteners that I counter-sunk to be flush on the outside.

Stock vs 12ga power wire:


















Raising the Transformer

This was mostly done for better air circulation under the transformer, but I also think it looks a bit better having the transformer raised up like this. I used stainless steel fasteners and 1/2" aluminum standoffs.








Rotating the Power Supply

I did this for a few reasons, but mainly because the wiring works out better IMO. I also think it looks much better having the rectifier tube more on the left side of the amp since it becomes symmetrical with the audio tubes. The four mounting holes all line up so rotating the PCB 180° is straight forward. The yellow and blue twisted pairs are soldered to the bottom side of the PS PCB stock, so I moved these to the top like one would normally do and also shortened them for a cleaner look. For the DC out twisted pair you'll have to use some new wire since the stock one is too short now. Make sure you observe polarity with this twisted pair since it's DC. I used red and black to make it easier for future reference.








Source Selector Switch

This is by far the hardest mod I did due to all the metal cutting required, but as long as you have proper tools it's not that bad. First you'll need to figure out how you want to do the RCA jacks. You have two options here: if you don't ever plan on using the 708B as a preamp then you can just use the existing pre-out holes for the new input jacks. If you do want to keep the preamp function you can drill an additional pair of 3/8" holes to mount the new input RCA jacks wherever you like. For the switch I used a 3PDT mini toggle made by GC Switches. There are better mini toggles out there with silver contacts like those from TOCOS, but I didn't want to wait for shipping. A 2PDT will also work, but I wanted to switch the grounds as well to avoid any possible crosstalk or ground loops. You have two options for where to mount the switch on this amp. The easy way is to mount it on the back panel by the RCA jacks since there is so much room. The more functional way is to mount it on the front panel, but since the panel is almost 8mm thick this requires a threaded hole for mounting. The thread required is 1/4"-40 which is pretty hard to find a tap for. I got two of these special taps from ICS Cutting Tools for about $6 each.























Here's how I wired the switch:













Reducing the Heater Voltage

Unfortunately since the transformer in this amp is rated for 110V when we use it here in the US with 120V all of the voltages will be too high. With the tubes I use in my amp, the heater voltage measures around 7.42VAC for the audio tubes instead of the ideal 6.3V. The heaters in these three tubes draw right around 1 amp total so to drop about 1 volt requires a resistor of around 1Ω. The only place that sells electronic parts here is Rat-Shack unfortunately and all they had in this range for power resistors was a 1Ω 10W so I couldn't try any other values. Using this resistor in series with the heaters has brought the voltage down to about 6.63VAC which is still a bit high but much better than before. I'll have to go to a _real_ electronics store to get some more power resistors to play with. I'm thinking a 1.2Ω 10W would be pretty close, but most likely I'll have to use two 5W in series to get it just right. I'll update this mod when I do more calculations and get the new resistors.








Here's the current state of my 708B as of 7-30-2008:







Here's the bone stock pic again for comparison:


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## Chops

And now I will post about the tweaks and mods that I have performed to my 708B in the order that I did them in. Of course, I wouldn't have done any of this if it weren't for *dcheming* and his excellent research and testing through all of this. So here it goes.

 First, a couple of tweaks...

*Power Delivery:*
 First off was upgrading the nasty, tiny stock power cord. Luckily, I had a nice 3' long, 3 conductor, 14awg multi-strand power cord laying around unused, so it was a natural choice to go with.







*Tube Rolling:*
 The stock Chinese tubes that come in this amp are not impressive at all. They have a rather thin midrange, are pretty bassless, have a brittle top end, and like to run out of steam early on and distort before the music even begins to get loud. 

 So for the first run of tube rolling, I first went with *drarthurwells'* recommendations of replacing the stock Chinese tubes with a NOS Russian Military 6N3P EB driver tube and a pair of matched NOS RCA 6DJ8 output tubes, but then switchd out the Russian 6N3P with a NOS GE JAN 5670. 

 These tubes really brought the stock 708B up to speed. No more distortion, a bit more bass, a fuller midrange and a tamed top end.












*Upgrading and adding parts:*
 Next in line is my first batch of mods. This includes replacing the volume pot, coupling caps, output caps and adding a bleeder resistor to the PS filter caps. 

 1) I pulled the stock WIMA 0.22uF - 250V coupling caps in favor of a pair of Hovland 0.22uF - 400V Musicaps.

 2) I removed the stock 150uF - 200V electrolytic output caps and upgraded to a pair of huge, monstrous Mundorf 100+100uF - 450V M-Lytic HV electrolytics. These are basically TWO 100uF capacitors in one body with a single (-) terminal and two (+) terminals. With the two (+) terminals wired in parallel, each one of these caps equals 200uF.

 3) I replaced the stock cheap carbon volume pot with a quality ALPS 100k Blue Velvet. I won't go into the installation details and/or the "why" since dcheming has already done so in his first post.

 4) I added a 68k - 2W resistor to just one of the PS 330uF - 450V filter caps. Since these two caps are wired in parallel along with the two 22uF - 400V filter bypass caps on the audio board, this single 68k resistor completely bleeds off the stored power in ALL four of these caps within a couple of minutes after shutting down the amp. This allows for a safe environment when working on the amp.

*NOTE:* THE OUTPUT CAPACITORS WILL NOT BE DISCHARGED FROM THE INSTALLED PS RESISTOR(S), AND THEY WILL PACK QUITE A PUNCH IF YOU SHORT THEM OUT, SO PLEASE BE VERY CAREFUL WHEN WORKING ON THIS AMP.

 Refers to #'s 1 & 2





 Refers to # 3





 Refers to # 4





 And one overall shot of the first batch of mods completed.


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## Chops

This second batch of mods was to clean things up a bit, but also to help in the sound quality as well. There are as follows...

*A little tweaking and tidying up:*
 1) Replaced the 0.1uF - 50V input caps and steel jumper with 20awg OFC wire jumpers. Since my cdp doesn't put out any DC of any kind, I decided instead of upgrading these caps to better ones, to just completely ditch them and use wire jumpers instead, as well as replacing the nasty 24awg steel jumper in the right channel signal chain.

 2) Placed PS transformer on nylon standoffs. The standoffs came from the LED brackets in the window display that I removed.

 3) Omission of pre-out connections to RCA jacks, pre-out caps & garish window tube display. I don't ever plan on using this amp as a pre-amp, I removed these components to clean up the interior a little and free up some space. Plus, that window display just yelled out "CHEAP!" That was just taking up usefull room, probably adding a bit of noise and using up unneccissary power from the PS.

 4) Moved Mundorf caps up front in window. This frees up room in the back of the unit. Plus, these caps are so large that they overlapped the audio board some, not allowing me to easily remove the mounting nut and/or board from the chassis. Now they are out of the way, and much closer to the headphone output jack, which will be perfectly convenient for a future upgrade.

 5) Added a ground wire to the ALPS volume pot. I had to do this as there was a slight 60 cycle hum coming through when the volume was turned up anywhere between the 12 o'clock and 4 o'clock position. Not that I ever go up that high (the most I ever go is about the 11:30 position), but knowing that _that_ noise was there bothered me, hence the ground wire. Now there's absolutely no noise anywhere throught the range.

 Refers to #1: This...





 And this...





 Replaced with this...





 And this.





 Refers to #2 & 3










 Refers to #4





 Refers to #5





 Here's a couple extra shots of the 2nd batch of completed mods and new "window treatments". 
	

	
	
		
		

		
		
	


	














*More Tube Rolling:*
 I decided to go out on my own this time and try some tubes that interested me. One of which others have tried on a "stock" 708B, but never on a modded one. This is the NOS GE JAN 5670. This one in particular is from 1968. The reason no one liked this tube is because it created too much hum or noise into the signal. Again, this was with all stock 708B amps. However, I put this 5670 tube in my amp and it is totally, completely and 100% dead silent, even at max volume and everywhere in between.

 I also want to try some different output tubes as well. I've heard that the Amperex-Japan 6922/E88CC tubes were actually made by Matsu****a, which are very VERY good tubes.

 Here they are...





 As for the sound, I feel that they have a better "grip" on the music than the UK made RCA 6DJ8's and Russian 6N3P EB that drarthurwells recommended. They DO have a wider soundstage, the dynamics are more dynamic, bass seems to be a bit tighter and definately deeper, the midrange is full and natural, and the treble is still smooth, crisp and extended.


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## Chops

Now for the final mods that I made to the 708B. This mod put this amp well over the top, especially after a healthy, constant 92 hour burn-in time. 

*Resistor & Diode Upgrades:*
 I replaced every single resistor on the audio board that has to do with the sound getting from the input jacks to my ears, which means the ONLY stock resistors left on the audio board are the two very small, blue ones all the way in the back of the board going to the pre-amp outputs, one for left and one for right channel, which are disconnected anyway.

 Almost all of the resistors used on the board are PRPs. There are three exceptions. For the anode load resistors, I used a pair of Kiwames. For the input resistors, I used a pair of Audio Note Tantalums. I also used another pair of Audio Note Tantalums for the outputs which is mounted directly on the headphone jack.

 I also upgraded the stock cheap PS rectifier diodes with Vishay-Telefunken ultrafast soft recovery diodes. 

 Here's a few pics of the entire process.


















































 I'm not sure what kind of sound improvement the diodes made, if any. But as far as the resistors are concerned, WOW! This amp has totally transformed into something special, leaving my LDM+ and my brother's LD II++ lightyears back in the dark ages!

 This is what I wrote dcheming in a couple of PM's...

 This one was only a few hours after performing the resistor and diode upgrades.
_"And as for an update on the amp... Man, this thing is blowing me away! Especially when you have a good quality recording spinning! This MH cdp is excellent also, but just to be safe, I reconnected the Sony cdp for a couple of hours tonight after work. These improvements I am hearing are definately coming from the amp itself. All I can say is "WOW"! And then when I put the MH back up to it, it's another "WOW" all over again! I'm telling ya, you need to upgrade all of those resistors when you get a chance. They really make this little amp come to life."_

 And then this morning.
_"BTW, on a side note, I have had this amp and MH cdp playing constantly since Wed afternoon (4pm) when I finished the last batch of mods and installed the opamps. I now have roughly 89 hours on both the amp and cdp. 

 All I can say is WOW! Everything is working flawlessly, everything sounds excellent, and I am just loving this entire setup. For a couple hours last night, I had the Sony player on the amp again and I couldn't get over how excellent that little cd/sacd player sounds now. The amp was definately the weak link, but not anymore! However, once I reconnected the Music Hall cdp, I could certainly hear another huge improvement in sound.

 So I can safely say that the mods to the amp are definately worth it, especially replacing all of those resistors. You'll see what I'm talking about whenever you get your resistors upgraded.

 Imaging, soundstaging, air, space, depth, detail, bass... Now I can finally see (hear) what everyone has been raving about with these K701 cans!!"_


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## zer061zer0

Hi guys,

 I am also thinking of changing all my resistors on my XS. However, I do not have my multimeter with me now. Would it be possible for you to give me the values and voltages for the resistor values of R3 & R4 on the PS board and R102, R105, R202, R205 and R7, R8 on the audio board. thanks.


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## laxx

Wow, what a write up. I don't think I can do something like this, as I'm not very knowledgable in this area, but the documentation in this thread does inspire me since it seems so laid out. =T


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## penguindude

[size=medium]WOW that is one NASTY modded amp. [/size]

 Note: "Nasty" meaning "sick" meaning "AWESOME!"

 Great work there!


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## dcheming

Hey laxx,

 So do you know how to solder and desolder? This amp is easy to mod since it has so much room inside. You could just start with the simple mods like the 4 input/output resistors and the 2 coupling caps. If you plan on working inside this amp at all I'd highly recommend adding the bleeder resistors to the big 330uF filter caps first for safety reasons. I'll help you out as much as I can with your mods if you run into any problems or if you have any questions. 


 zer061zer0,

 The full list of all the resistors and their wattage ratings are near the top of post# 1 right under the schematic. For R102,202 I'd recommend using a 5 watt non-inductive wire-wound (NIWW) type since they have very low noise. Vishay Dale makes a 33K 5W in their CW-5 series, but I haven't found a source for these yet over here. All I've been able to find in 33K are some 12W Mills which are way to big, but you might have better luck finding a 33K 5W NIWW where you live.


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## penguindude

Hey dcheming, how is the ALPS 100kax2 potentiometer treating you? Is it much better than the stock one?

 Nvm, already answered.


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## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Hey laxx,

 So do you know how to solder and desolder? This amp is easy to mod since it has so much room inside. You could just start with the simple mods like the 4 input/output resistors and the 2 coupling caps. If you plan on working inside this amp at all I'd highly recommend adding the bleeder resistors to the big 330uF filter caps first for safety reasons. I'll help you out as much as I can with your mods if you run into any problems or if you have any questions. 


 zer061zer0,

 The full list of all the resistors and their wattage ratings are near the top of post# 1 right under the schematic. For R102,202 I'd recommend using a 5 watt non-inductive wire-wound (NIWW) type since they have very low noise. Vishay Dale makes a 33K 5W in their CW-5 series, but I haven't found a source for these yet over here. All I've been able to find in 33K are some 12W Mills which are way to big, but you might have better luck finding a 33K 5W NIWW where you live._

 


 Oops sorry i miss that..thanks anyway


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## Miller-8

Does removing the the front tube improve sound?


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## dcheming

I didn't notice any improvement in sound when I took mine out. The 6N1 in the front window shares it's heater supply with the 6Z4 rectifier tube only and the three audio tubes have there own dedicated heater winding.


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## dcheming (Aug 25, 2017)

I finally picked up a 6X4 rectifier tube today. I was planning on building an adapter to use this tube in place of the 6Z4, but they didn't have any 7-pin sockets there. Since I really want to try this much better tube today I've decided to just modify the socket pins on the PS board itself. This mod is pretty easy since the two pins that need to be swapped are in the perfect place to do so. The only thing that I'm worried about is that the heater in the 6X4 draws 600mA which is _right_ at the limit of what the heater winding is rated for. Hopefully the transformer can handle it.

 I'll post my listening impressions later on tonight when I get it done.


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## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_I finally picked up a 6X4 rectifier tube today. I was planning on building an adapter to use this tube in place of the 6Z4, but they didn't have any 7-pin sockets there. Since I really want to try this much better tube today I've decided to just modify the socket pins on the PS board itself. This mod is pretty easy since the two pins that need to be swapped are in the perfect place to do so. The only thing that I'm worried about is that the heater in the 6X4 draws 600mA which is right at the limit of what the heater winding is rated for. Hopefully the transformer can handle it.
	

	
	
		
		

		
			





 I'll post my listening impressions later on tonight when I get it done._

 

Pictures pictures pictures


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## Chops

Cool dcheming! I hope that transformer doesn't give you any problems. I'd really hate to hear that it burned up or something.

 BTW, look how serious I'm getting with all of this modding! Donated by my father who has about half a dozen different types of these MMs.


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## Chops

And the various clip-on tips for the probes.


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## jink

What an amazing thread, you are a wonderful tutorial writer! I did not understand about 70% of the posts, but the pictures are nothing short of incredible. What a high level of detail, this amp doesn't have anything left to improve now! Thanks for the good read. =)

 That must sound amazing, the best amp I have heard is a go-vibe. I have yet to spend the money to experience a tube amp or a spendy solid state amp.


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## dcheming

Thanks for the kind words guys. 
	

	
	
		
		

		
		
	


	




 I was worried that I posted too many pics, but this page doesn't take that long at all to load, with a cable connection at least. My goal with this thread is to make it as easy as possible for anyone looking to mod this amp to find the info that they need. I still have a few more mods that I want to do so I'll be adding additional sections as those progress. If anyone needs more detailed info on what I've already done or if you just have a question feel free to ask.


 zer061zer0,

 I didn't get a chance to play around with the new rectifier since I was working on another project all night. I'm definitely doing it tomorrow though and I'll be posting detailed pics of the mod procedure to use the 6X4.


 Chops,

 If the transformer does burn up then I'll _have_ to get one of those Hammonds that we were talking about earlier. 
	

	
	
		
		

		
		
	


	




 That's a pretty sweet DMM you got there; blows away my cheap Velleman for sure. You're lucky your dad has so many of them. Even though it's huge I prefer them that way since you can stack it neatly on your workbench shelf as you get more test equipment later.

 I also picked up a bunch of different valued 5W and 10W resistors from Norvac today. After I get the 6X4 up and running tomorrow I'm going to play around with adjusting the heater voltage for the audio tubes some more. I'm sure the heater voltage feeding the 6X4 will be too high also so I'll adjust it as well.


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## Miller-8

Guys I have a question for some of you. You are spending quite a bit on things like caps and resistors but all of you seem to have fitted an Alps Blue pot. Whilst it's a decent pot it is by no means the best. Why has nobody tried a Noble or a TKD or an Alps Black? The volume pot is one of the major components that affects sound quality of a pre-amp after all.


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## fishcarver

Excellent tutorial! Very well written and with all the photos it is first rate. Thank you for posting this thread.


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## dcheming

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Guys I have a question for some of you. You are spending quite a bit on things like caps and resistors but all of you seem to have fitted an Alps Blue pot. Whilst it's a decent pot it is by no means the best. Why has nobody tried a Noble or a TKD or an Alps Black? The volume pot is one of the major components that affects sound quality of a pre-amp after all._

 

I got the Blue Velvet way back when I first started modding this amp and I wasn't really sure how much money I wanted to put into it at the time. Considering how much the ALPS helped I think it would definitely be worth it to get an even better one and I would love to try a TKD when I have the money.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_I got the Blue Velvet way back when I first started modding this amp and I wasn't really sure how much money I wanted to put into it at the time. Considering how much the ALPS helped I think it would definitely be worth it to get an even better one and I would love to try a TKD when I have the money._

 


 x2

 I too wanted to wait and see how much the amp improved with the mods I did before deciding to get a TKD or Black Beauty, which BTW I was originally planning on getting before deciding on the Blue Velvet.

 However, with the way things have turned out so far with this amp, I'm now very seriously considering the TKD for an upgrade. This amp really IS that good.


----------



## daggerlee

quite and outstanding writeup guys!!


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## dcheming (Aug 25, 2017)

Well apparently I had looked at the pinout diagram of the 6X4 wrong. I thought this mod would only require two wire jumpers, but one of the two pins that needs to be swapped has to actually be moved to a new spot in the socket itself. Thankfully this extra step is very easy due to the design of the socket.


*6X4 Rectifier Modification*

Rectifier Pinouts

The difference between the 6Z4 and the 6X4 is that one of anodes and the cathode are connected to different pins. The 6X4 draws 600mA for it's heater and I assume the 6Z4 draws 300mA, but I have no idea since I can't find a data sheet for it.








Stock Layout

Here is the stock layout using the 6Z4 for reference.








Step 1

The first thing you'll have to do is de-solder pin 5 from the PCB. Try to clean the majority of the solder off it too.








Step 2

Bend the pin straight up and remove if through the other side of the socket.








Step 3

Next de-solder pin 7 from the PCB and bend it a bit to the left like shown in the pic.








 Step 4

This step is just a safety precaution against possible shorts. Scrape away the corner of the trace near pin 7 as shown in the pic using a heavy Xacto knife or a small file.








Step 5

Craft a little wire jumper similar to the one in the pic. This will be used in step 6.








Step 6

The jumper will be connecting pin 7 to the trace that pin 5 did originally.








Step 7

This is where we reinsert the pin that we took out in step 2. Put it in the hole for pin 6 and bend the tab towards the center of the socket as shown in the pic. To avoid excess play make sure it is fully seated before you bend the tab.








Step 8

Craft a little wire jumper similar to the one in the pic. This will be used in step 9.








 Step 9

For the final step, run the little jumper from pin six over to the trace that pin 7 originally connected to as shown.








Comments

This mod is fairly easy to do if you've worked with jumpers before. It's a little tedious to get them to fit right, but just take your time crafting them. If you plan on doing this mod keep in mind that you'll _have_ to disconnect the heater in the 6N1 up front or you will burn up the transformer. The heater winding feeding the 6Z4 and the 6N1 is only rated for 600mA and since the 6X4 draws 600mA by itself, there's no way you can have the 6N1 connected too. You could probably leave the orange LEDs in the front window connected since they draw so little power.

Please be careful about keeping your work clean since these pins carry HV. Needless to say, *[size=small]I assume no responsibility or liability for damage to you, your amp, or your other equipment by doing this modification. If you are not confident in your soldering ability PLEASE don't attempt this mod.[/size]*



Results

So far everything is running smoothly. I've been listening to the amp for about two hours and the transformer isn't overly warm, but I'll keep a close eye on its temp.

At this early stage, I would say that the amp does indeed sound better with the 6X4. The sound seems a bit more crisp and overall the sound seems a bit more detailed, but I want to give it some more time before I judge it.


Stock 6Z4 vs. Tung-Sol JTL-6X4








PS with 6X4


----------



## Chops

Well... Looks like I'll be planning to do another mod in the near future. 
	

	
	
		
		

		
		
	


	




 BTW, nice macro shots!


----------



## dcheming

Well I ran the amp for 7 hours last night and the transformer stayed at the same operating temp. So far so good. 
	

	
	
		
		

		
		
	


	




 I measured the heater voltage going to the 6X4 and it was about 7.12VAC. This isn't quite as high as the 7.42VAC that was feeding the audio tubes, but I still want to fix it also. Getting the heaters in the three audio tubes and in the new rectifier to the proper 6.3VAC is the next mod on my list.


----------



## Chops

Since we'll both be lowering the voltage on these heaters and NOT the grids, then the sound should pretty much remain the same. Am I correct?

 Also, do you have anymore thoughts on the sound of the new 6X4?

 I just remembered something kinda important... I think I might need a 6X4 for myself before I do this socket rewire mod that I was planning on doing this weekend.


----------



## dcheming

"_Since we'll both be lowering the voltage on these heaters and NOT the grids, then the sound should pretty much remain the same. Am I correct?_"

 I'm not exactly sure how the sound is affected by going from too high of a heater voltage to normal. I didn't really notice anything dropping down to the 6.63VAC that I'm using now for the audio tubes, but you'll just have to critically listen to yours when you do it to see.

 Speaking of dropping voltage, the 6X4 seems to drop more voltage than the 6Z4 did since the B+ now measures around 195VDC instead of around 220VDC. Since the HV was too high due to the 110V transformer anyways, this will probably be for the best. I really need to find out what B+ this amp is _supposed_ to have and go from there.


 "_I just remembered something kinda important... I think I might need a 6X4 for myself before I do this socket rewire mod that I was planning on doing this weekend._"

 LOL! I was actually thinking last night how easy it would be to do this mod and forget this fact. 
	

	
	
		
		

		
		
	


	




 That would be a bummer for sure since your amp would down until you got a 6X4.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_"

 Speaking of dropping voltage, the 6X4 seems to drop more voltage than the 6Z4 did since the B+ now measures around 195VDC instead of around 220VDC. Since the HV was too high due to the 110V transformer anyways, this will probably be for the best. I really need to find out what B+ this amp is supposed to have and go from there.


 LOL! I was actually thinking last night how easy it would be to do this mod and forget this fact. 
	

	
	
		
		

		
			





 That would be a bummer for sure since your amp would down until you got a 6X4._

 

I would imagine if the voltage isn't dropping to the point of not being able to properly operate the tube and/or cleanly drive the headphones, then it should be fine. 

 Just as a joke (since I wouldn't waist the time to ever go back to the 6Z4) you could install a DPDT switch on the back panel allowing switching back and forth between the 6Z4 and 6X4.


----------



## dcheming

Just finished working on the heater voltage correction. All those resistors came in handy for testing. I was able to get both heater circuits pretty close to 6.30VAC.


Rectifier Tube

 The new 6X4 had 7.22VAC for its heater voltage being directly fed by the transformer.

 Here's the progression I tested:

 1.0Ω = 6.63VAC
 1.2Ω = 6.52VAC
 1.5Ω = 6.36VAC

 According to calculation the ideal resistance is 1.58Ω, which would give 6.30VAC.

 The closest resistor I had was 1.5Ω, which dropped it to 6.36VAC.

 So in my amp the best I'd be able to do here would be a 1Ω 2W + 0.47Ω 2W + 0.10Ω 2W all in series giving 1.57Ω, which would drop it to 6.31VAC.



Audio Tubes

 The three audio tubes had 7.42VAC for their heater voltage being directly fed by the transformer.

 Here's the progression I tested:

 1.0Ω = 6.63VAC
 1.2Ω = 6.47VAC
 1.3Ω = 6.40VAC 
 1.39Ω = 6.35VAC

 According to calculation the ideal resistance is 1.43Ω, which would give 6.30VAC.

 The closest resistance I could do was 1Ω 5W + 0.39Ω 5W in series giving 1.39Ω, which dropped it to 6.35VAC.

 So in my amp the best I'd be able to do here would be three 0.47Ω 2W in series giving 1.41Ω, which would drop it to 6.31VAC.


----------



## Miller-8

What does a high heater voltage do? Do the tubes burn out faster?


----------



## Miller-8

Thinking of getting a 6N3P-DR NOS tube, wondering if it's worth going for the cryo version?


----------



## zer061zer0

Hi derek, mind teaching me how i can measure the voltage. What do i need.?

  Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Just finished working on the heater voltage correction. All those resistors came in handy for testing. I was able to get both heater circuits pretty close to 6.30VAC.


Rectifier Tube

 The new 6X4 had 7.22VAC for its heater voltage being directly fed by the transformer.

 Here's the progression I tested:

 1.0Ω = 6.63VAC
 1.2Ω = 6.52VAC
 1.5Ω = 6.36VAC

 According to calculation the ideal resistance is 1.58Ω, which would give 6.30VAC.

 The closest resistor I had was 1.5Ω, which dropped it to 6.36VAC.

 So in my amp the best I'd be able to do here would be a 1Ω 2W + 0.47Ω 2W + 0.10Ω 2W all in series giving 1.57Ω, which would drop it to 6.31VAC.



Audio Tubes

 The three audio tubes had 7.42VAC for their heater voltage being directly fed by the transformer.

 Here's the progression I tested:

 1.0Ω = 6.63VAC
 1.2Ω = 6.47VAC
 1.3Ω = 6.40VAC 
 1.39Ω = 6.35VAC

 According to calculation the ideal resistance is 1.43Ω, which would give 6.30VAC.

 The closest resistance I could do was 1Ω 5W + 0.39Ω 5W in series giving 1.39Ω, which dropped it to 6.35VAC.

 So in my amp the best I'd be able to do here would be three 0.47Ω 2W in series giving 1.41Ω, which would drop it to 6.31VAC._


----------



## Chops

Quote:


 What does a high heater voltage do? Do the tubes burn out faster? 
 

The heater runs hotter, shortening the life of the heater, which in turn shortens the life of the tube itself. Just like putting too much voltage into a light bulb.


  Quote:


 Thinking of getting a 6N3P-DR NOS tube, wondering if it's worth going for the cryo version? 
 

I have very little faith, if none at all with "cryo" anything. As far as I'm concerned, it's just another gimmick to suck more money out of people.


----------



## zer061zer0

Did some changes and rewiring and I have got the cover up as well, should not be opening it up again until my upcoming CK2III and SOHA is completed...
	

	
	
		
		

		
		
	


	




 (Not unless DErek tempt me with the 6x4 and voltage adjusting mods 
	

	
	
		
		

		
		
	


	




 )

 The default tubes for testing and the amp 










 The tubes configuration for my amp















 Some glowing pictures. . .















 The additional inputs so that my girlfriend can play with it

 The Wiring




 The input 





 Her ipod and the amp


----------



## dcheming (Aug 26, 2017)

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif
_Hi derek, mind teaching me how i can measure the voltage. What do i need.?_

 

Do you have a multimeter? All you have to do is turn on the amp and let it warm up for a few minutes. The easiest place to measure the heater voltage of the audio tubes is the 6N3 socket. You'll be using pins 1 and 9 which have the big space between them. Just set your multimeter to read voltage, select the AC option, and use the test probes to touch the exposed metal of pins 1 and 9 in between the socket and the PCB.







 Since you have the 220V version your heater voltage might be fine. What voltage do you use in your country anyways?

 Your new wiring job looks a lot better. I like how you used a 1/4" TRS jack for the aux input to make it easier for connecting things like iPods. So how are those massive output caps sounding?


----------



## dcheming

For the rectifier's 1.5Ω 10W resistor I just mounted it to the side of the PS:













 I'll probably mount both of the power resistors underneath the transformer later for a cleaner look. I wanted to try them out for a few weeks first before I went to the trouble to do so.

 Last night I also swapped out the two 82KΩ 2W bleeder resistors for a pair of 160KΩ 2W ones.


----------



## zer061zer0

We use some 240v over here, nice pictures, i will try to measure to later on perhaps after te festive over here. 

 I am glad that the wiring looks decent, previously i was trying out the last time on the input thingy. I also changed the wires to some silver plated wires, I am not so gd with the cat5e ones. Hope i do get better along the way.

 Gonna pause on it for a while before going into CK2III and the SOHA...
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Do you have a multimeter? All you have to do is turn on the amp and let it warm up for a few minutes. The easiest place to measure the heater voltage of the audio tubes is the 6N3 socket. You'll be using pins 1 and 9 which have the big space between them. Just set your multimeter to read voltage, select the AC option, and use the test probes to touch the exposed metal of pins 1 and 9 in between the socket and the PCB.







 Since you have the 220V version your heater voltage might be fine. What voltage do you use in your country anyways?

 Your new wiring job looks a lot better. I like how you used a 1/4" TRS jack for the aux input to make it easier for connecting things like iPods. So how are those massive output caps sounding? 
	

	
	
		
		

		
		
	


	


_


----------



## dcheming

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_We use some 240v over here_

 

So since your transformer is rated for 220V, but you're feeding it 240V you'll have the same problem with high heater voltage that we have here unfortunately.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_So since your transformer is rated for 220V, but you're feeding it 240V you'll have the same problem with high heater voltage that we have here unfortunately._

 

Do you think it might be a little worse since they're 20V higher where we're only 10V higher? Maybe pushing close to 8V to those heaters?


----------



## zer061zer0

I dun quite understand how we can connect the resistors in line to the PS and to the heater pins, Derek can you show me another pictures.


----------



## dcheming

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_I dun quite understand how we can connect the resistors in line to the PS and to the heater pins, Derek can you show me another pictures._

 

So the yellow twisted pair coming from the transformer is the heater winding for the 6Z4 and 6N1 up front. The green twisted pair coming from the transformer is the heater winding that feeds the three audio tubes. 

 By cutting one wire in the twisted pair and inserting the resistor there we are basically putting the resistor in series with the the heaters. I would start with the audio tubes first. So what is the voltage of your audio tube heaters?


----------



## dcheming (Aug 26, 2017)

Here's how I did the resistors for the audio tubes. I soldered about 6" of new twisted green wire to the audio board to give me more slack to play with. For the rectifier tube I de-soldered one of the yellow wires from the PS PCB, soldered about 2" of green wire in it's place and then soldered it and the yellow wire to the 1.5Ω resistor.








I'll be able to calculate what size you need for this resistor based on what your heater voltage is. Let me know if you want some close up pics of the rectifier area.


----------



## Chops

I just measured mine, and it's at 7.58V. A little more than your amp, Derek. 
	

	
	
		
		

		
			





 So what do you think, maybe a 1.5 ohm resistor?


 As for measuring the heater voltage on the 6Z4, can I just place the probes on the resistors' (R3,4) terminals closest to the socket to get a reading? 







 As usual, thanks!


----------



## dcheming

If your audio tubes draw around the same current as mine do then a 1.5Ω resistor should drop the 7.58VAC to around 6.4VAC according to calculation and a 1.63Ω would be ideal in your amp.

 Yeah measuring the side of those resistors closest to the socket is perfect.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_If your audio tubes draw around the same current as mine do then a 1.5Ω resistor should drop the 7.58VAC to around 6.4VAC according to calculation and a 1.63Ω would be ideal in your amp.

 Yeah measuring the side of those resistors closest to the socket is perfect._

 

Cool. So 1.63 ohms it is. 

 I'l check the 6Z4 heater voltage tomorrow and post the results. I don't want to go probing around in there now. As tired as I am, I might fry the fool thing, or myself! LOL

 BTW, I sent you a PM.

 Thanks again!


----------



## dcheming

Try the 1.5Ω first and record what the voltage drops to. Using that we can calculate the ideal resistor for your tubes in your amp instead of using the numbers I got from mine. You'll have to do this for the rectifier anyways since the voltage test data I have is for the 6X4.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Try the 1.5Ω first and record what the voltage drops to. Using that we can calculate the ideal resistor for your tubes in your amp instead of using the numbers I got from mine. You'll have to do this for the rectifier anyways since the voltage test data I have is for the 6X4._

 

That reminds me... I _still_ need to get a 6X4. 
	

	
	
		
		

		
		
	


	




 Also, I just measured the 6Z4 heater and it was up at 7.09V. Just like we spoke about last night in the PM, I measured at the resistor terminals closest to the 6Z4 socket.


----------



## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_I just measured mine, and it's at 7.58V. A little more than your amp, Derek._

 

I wonder if this is due to variations in the transformer windings or because of the different tubes being used.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_I wonder if this is due to variations in the transformer windings or because of the different tubes being used._

 

I don't know. Maybe it's because of the different tubes, or maybe all of the new resistors? 
	

	
	
		
		

		
		
	


	




 BTW, I just measured the 6Z4 again for the heck of it. It's now down to 7.03V instead of 7.09V this morning. I'm sure that's probably because there's more draw on the lines now since everybody's awake and using more juice than this morning.

 A regulated PS would solve that little problem, right?


----------



## dcheming

"_or maybe all of the new resistors?_"

 All the new resistor values are the same as stock so it couldn't be them.


 "_BTW, I just measured the 6Z4 again for the heck of it. It's now down to 7.03V instead of 7.09V this morning. I'm sure that's probably because there's more draw on the lines now since everybody's awake and using more juice than this morning._"

 That would make sense. It would be interesting to record the voltage of the wall socket at a few different times throughout the day to see how much it changes.


 "_A regulated PS would solve that little problem, right?_"

 Yeah, but you'd have to get a new transformer with higher voltage heater windings to accommodate the voltage drop of regulators.


----------



## Chops

_"All the new resistor values are the same as stock so it couldn't be them."_

 Yes, the new resistors are the same values, but they're at much tighter 1% tolerances. If a lot of the stock resistors were well above or below their rating, that could change voltages, correct? 


_"That would make sense. It would be interesting to record the voltage of the wall socket at a few different times throughout the day to see how much it changes."_

 With my Panamax MAX1000 in my home theater system, I have seen it's meter range anywhere between 106V all the way up to 129V. 
	

	
	
		
		

		
		
	


	





_"Yeah, but you'd have to get a new transformer with higher voltage heater windings to accommodate the voltage drop of regulators."_

 True, and it would probably cost a pretty penny as well. I don't know though. It might be worth it if I decide to go to a new case and all the other things I mentioned before in our PM about wanting to do.


----------



## Chops

Just installed the 1Ω - 10W resistor into the heater line for the audio tubes.

 The voltage dropped from 7.58V to 6.63V. Certainly better than before!

 Naturally, I took a picture.


----------



## regal

If you replaced all those resistors why not make it an Optimized Morgan Jones Amplifier so it can work with low impedance cans ?


----------



## dcheming

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_If you replaced all those resistors why not make it an Optimized Morgan Jones Amplifier so it can work with low impedance cans ?_

 

The Optimized Morgan Jones Amplifier uses three 6DJ8/6922 tubes and the 708B has two 6DJ8/6922 and a 6N3, so it's not just a simple matter of swapping resistors unfortunately. Also the 6N3 has a different pinout than the 6DJ8/6922 and a few months back I was considering building an adapter to use one in the 6N3 position along with changing the required resistors, but decided against it. Lately I've been thinking about doing it though since I'm planning on ordering all the resistors very soon anyways. I'm kind of on the fence on doing this mod, but nice 6DJ8/6922 sure are a lot easier to find so I just might do it after all.


----------



## Miller-8

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Just installed the 1Ω - 10W resistor into the heater line for the audio tubes.

 The voltage dropped from 7.58V to 6.63V. Certainly better than before!_

 

So was the high voltage due to the original resistor operating outside its temperature range?


----------



## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_So was the high voltage due to the original resistor operating outside its temperature range?_

 

There was no resistor originally. The PS is designed to work off of 110V, but over here in the US, the live voltage is 120V. Since the PS in non-regulated, the voltages throughout the amp are running a little higher than they should be, hence the stock heater voltages running at 7.58V.

 The 1Ω - 10W resistor was added to drop the heater voltage down near the nominal operating voltage of the tubes, which is 6.31V.


----------



## Chops

Here's something I'm kind of thinking about doing as far as a new chassis is concerned. I just whipped it up in AutoCAD 2002. The actual dimentions will probably be something like 17"W x 4"H x 10-12"D. Of course, nothing is written in stone yet. 






 If I can find them, I'd really like the input selector and volume knob to be solid wood, about 2" diameter. The mini-toggle would be for a defeatable crossfeed circuit. The power button would be one of those nice all-metal ones that has the blue lit ring around them (unless I can get them in a different color). The 3 LEDs for the input selector and crossfeed switch would be those small orange ones that came in the 708B window unit. The entire chassis would be aluminum with a dual layered front panel. The middle section of the front panel would be a piece of brushed gold anodized aluminum mounted behind the main brushed clear anodized one so it would be recessed. It would kind of look like the front panel of Sonic Frontiers equipment! 






 I would have both the selector switch and volume pot all the way towards the back of the chassis, right in front of the 3 RCA inputs. The audio PCB would be rotated 180 degree from stock and centered right between the rods going to the selector switch and vol pot. I'll have a copper divider panel 6" in from the left side of the chassis to conceal the entire PS section on that side. The whole center section of the chassis would be completely open to the very small crossfeed circuit and a bank of large output capacitors (up to 494uF of Mundorf M-Lytic HV caps per channel). Of course, I would finally upgrade all of the wiring durring this time.

 Also, using this chassis would allow me the room to eventually move up to a better amplifier circuit design as well as a much better PS transformer and circuit.

 So what do you all think?


----------



## dcheming (Aug 26, 2017)

Looks nice.

 Are you planing on getting someone to make the front panel for you? That size of a chassis will give you plenty of room to work with and as you said, you'll have the room to upgrade to a better circuit in the future. Good idea about putting the pot and the selector switch in the back to keep the signal wire as short as possible. Here's a place that has some nice wood knobs BTW:

http://www.vt4c.com/shop/program/mai...2&cat_id=6#389


----------



## dcheming

Guess I'll post some pics of what I'm planning as well:













 I'm trying to find something nicer to use for a transformer cover. I found a nice brushed stainless steel jar that would work, but it's 5" diameter which is a bit to big. The shortened camping cup in the above pics is 4" diameter and is the perfect size, but would have to be painted and I'd much rather use something SS. To give a cleaner look I'm going to fabricate a new top panel out of aluminum to cover the vent slots and the socket mounting screws. I cut the bottom flange of the cover off last night so it can now be removed from above instead of having to slide on from the back.


----------



## zer061zer0

In case anyone is interested, I am looking to sell my modded Xiangsheng 708B
 Check out the link in my signature.


----------



## mrdon

This entire thread has been a very interesting read, but these modifications lead me to make a comment...with all due respect why didn't you all just build a Morgan Jones? You can purchase inexpensive pcb's for the MJ from China and it would have saved you a great deal of time, effort, and money.


----------



## dcheming

Quote:


  Originally Posted by *mrdon* /img/forum/go_quote.gif 
_This entire thread has been a very interesting read, but these modifications lead me to make a comment...with all due respect why didn't you all just build a Morgan Jones? You can purchase inexpensive pcb's for the MJ from China and it would have saved you a great deal of time, effort, and money._

 

The 708B is the first tube amp that I've owned and I knew that it would probably need a lot work to get it to its best, but that's exactly why I bought it. If I was just looking for a good tube amp then building a MJ would've been the way to go as you said, but my intent was to further my electronics knowledge through doing extensive mods to a cheap commercial product. The amount that I've learned about tube circuits while doing research for the mods was worth the price of the amp alone to me. 

 I've decided not to do the case mod or the MJ mod to the 708B as I'd much rather save those efforts and money for other amp projects. While I'm pretty happy with how this amp is now, I'll probably end up selling it eventually to fund bigger and better projects, which will be fully DIY by the way. So no offense taken and hopefully you see were I'm coming from.


----------



## regal

Quote:


  Originally Posted by *mrdon* /img/forum/go_quote.gif 
_This entire thread has been a very interesting read, but these modifications lead me to make a comment...with all due respect why didn't you all just build a Morgan Jones? You can purchase inexpensive pcb's for the MJ from China and it would have saved you a great deal of time, effort, and money._

 


 My thoughts exactly except I thought the MJ had to be point to point because no one made PCB's, where are you finding the PCB's ?


----------



## Chops

I finally got in my Westinghouse 6X4 rectifier tube today. So I got the old soldering iron heated up and started on the process written up by *dcheming* on page 2 of this thread. Thanks Derek!

 Everything went super smoothly and I was done in about a half hour or so. The amp's been up and running for nearly an hour now with no problems, and the heater voltage is at 6.92V. Just as a reference, the stock 6Z4 heater ran at 7.09V.

 Now for some pictures... naturally. 
	

	
	
		
		

		
		
	


	




 First off, I wanted to make note of this, as it was (and still is) a pleasant surprise to me as I'm sure it will be for others here on the forum as well. The Westinghouse 6X4 rectifier tube is slightly SHORTER than the stock Chinese 6Z4 tube! This means that people CAN do this mod and still put the lid of the amp back on without a problem! 
	

	
	
		
		

		
		
	


	










 Here's a pic of the unmodded socket. 







 Here's both the pins desoldered, rebent, and the one pin moved over one space. Also, the corner of that one trace has been scraped away.







 One of the jumpers. I only took one picture since these things are very unexciting. 







 This is the first jumper installed.







 And the second jumper.







 And one last pic of the new 6X4 glowing away.







 As of yet, I really can't pin-point the difference in sound I'm hearing, other than it seems as if the amp is "stronger" sounding. It's hard to describe. 
	

	
	
		
		

		
		
	


	




 It definately didn't hurt the sound at all, that's for sure. I'll get back to the sound in a day or so when I've had a little more time to listen to the amp with several different CDs.

 How about you Derek? Any new views on the sound of your amp since the 6X4 mod?


----------



## mrdon

For those who are modding the rectifier to 6x4, I have tried a number of NOS 6x4's in my MJ and a NOS Mullard 6x4 is absolutely the best. Highly recommended and long lasting.


----------



## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_As of yet, I really can't pin-point the difference in sound I'm hearing, other than it seems as if the amp is "stronger" sounding. It's hard to describe. 
	

	
	
		
		

		
		
	


	




 It definately didn't hurt the sound at all, that's for sure. I'll get back to the sound in a day or so when I've had a little more time to listen to the amp with several different CDs.

 How about you Derek? Any new views on the sound of your amp since the 6X4 mod?_

 

As you said it's kind of hard to pin-point what the difference is, which is why I haven't really said much about it yet. I agree that the amp does sound a bit stronger now with the 6X4 and listening to music such as Thievery Corporation and Björk makes me feel that it generally sounds more "open" as well. There also seems to be a bit more snap to the sound now. I'm looking forward to installing those Vishay-Telefunken SF4007 diodes as well, hopefully they'll help a tiny bit more.

 I finally got a replacement 6N3 by the way so I finally now have all nice tubes under the hood. I have all the PRP and MILLS resistors as well as 4 gold plated sockets coming from PC and I can't wait to get them installed. 

 I'm glad the mod went smoothly for you. That's quite a surprise that your 6X4 is actually shorter than the 6Z4 considering how tall mine is. I should be able to make mine fit under the case too, but it will be pretty close. On a side note I haven't had any problems with the transformer overheating even with daily use for multiple hours, but I still feel it every now and then just to be sure.


----------



## dcheming

Quote:


  Originally Posted by *mrdon* /img/forum/go_quote.gif 
_For those who are modding the rectifier to 6x4, I have tried a number of NOS 6x4's in my MJ and a NOS Mullard 6x4 is absolutely the best. Highly recommended and long lasting._

 

I'll have to get a NOS Mullard 6X4 to try then. Thanks for the recommendation.


----------



## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_I'll get back to the sound in a day or so when I've had a little more time to listen to the amp with several different CDs._

 

So do you have any more impressions of the 6X4 Charles? How's that new Music Hall CDP working out for you now that you've had it for a while?


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_So do you have any more impressions of the 6X4 Charles? How's that new Music Hall CDP working out for you now that you've had it for a while?_

 

Honestly, I haven't listened to the amp since I did that last mod. I've been busy and running about too much lately to be able to sit down and relax. Sucks really, but that's how it goes sometimes.


----------



## dcheming (Aug 26, 2017)

My PC order came in today and I just finished installing all the new goodies. First I put in the new sockets for the audio tubes. I went with the two piece ceramic ones with gold plated cup style contacts. The solder tabs on these were quite a bit wider than the stock ones so I had to use a 5/64" drill to enlarge the holes slightly in the PCB. I also had to bend the tabs inward a bit to get the proper hole spacing. These new sockets grip the pins TIGHTLY! 
	

	
	
		
		

		
		
	


	
















Next I swapped out all of the resistors using mostly PRP metal films. I wanted to use a low noise NIWW type for the 6N3 anode load resistor, but I had trouble finding 33K in a 5W so I ended up using a pair of 12W MILLS instead.













I also got a pair of RIKEN 10R 2W for the output resistors to match the RIKEN input resistors.








Finally I added the Vishay-Telefunken SF4007 diodes and the PRP resistors in the power supply.


----------



## caromire

Hello, I can rolling the 6N1 for the 12AT7?

 Thanks


----------



## dcheming

Quote:


  Originally Posted by *caromire* /img/forum/go_quote.gif 
_Hello, I can rolling the 6N1 for the 12AT7?

 Thanks_

 

The 12AT7 has the same two pins for it's heaters as the 6N1, but it's designed to run off 12 volts instead of 6.3 volts like the 6N1. So in order to use a 12AT7 in the place of the 6N1 you'd have to change the heater supply wiring at the socket a bit. Since the 6N1 is just there to look pretty and is not even in the audio circuit I don't think it would be worth the effort to do the conversion though. Just stick with the cheapest 6N1 you can find for the window tube.


----------



## dcheming (Aug 26, 2017)

I was intending just to simply swap the little jumper with a scrap lead from the 2W RIKEN resistors tonight, but due to boredom ended up three hours later with an auxiliary TRS jack with metallized output caps as well. I've always wanted to see how metallized would compare to electrolytic when used as output caps and now it will be easy to switch between the two. The biggest films caps I had laying around were some no-name 6.8uF 250V metallized mylar. I know they're pretty small value, but I have 300Ω impedance headphones so still get pretty good bass response. Depending on the sonic results I might go with some much bigger metallized caps later. I'll post my impressions in a few days.


----------



## zer061zer0

Derek, U are one serious dude in this modding. . . any impression of it. 

 hows the ck2III going? I got a frd to help me in the end as I was too busy with sch..


----------



## dcheming

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_Derek, U are one serious dude in this modding. . ._

 

Thanks. 
	

	
	
		
		

		
		
	


	




 Two days ago I added a pair of 2.15uF mylar caps to each 6.8uF to bring it up to a total of 11.1uF per channel. According to calculation this should give my 300Ω impedance headphones a -3dB rolloff point of around 48Hz, compared to around 2.65Hz with the ELNA electrolytic caps. 

 Impressions: The 'lytics have deeper and punchier bass, but the mylar caps sound better everywhere else IMO. They still go pretty deep in the bass though and what is there is quite a bit tighter than with the 'lytics. The midrange is noticeably clearer with the mylar caps too. The high end is more crisp and the general presentation has more "air" to it. I'd really love to try a nice metallized polypropylene next, maybe something like a Mundorf 100uF M-Cap bypassed with a small 0.10uF M-Cap ZN, which would give a -3dB point of 5.3Hz. I don't think those bad-boys would fit in the XS though. 
	

	
	
		
		

		
		
	


	




 Besides I'd much rather save the buying the good stuff for the upcoming Cavalli Lovell Futterman anyways.


  Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_hows the ck2III going? I got a frd to help me in the end as I was too busy with sch.._

 

I haven't gotten around to ordering the parts kit yet due to truck related repair expenses. Hopefully in the next few weeks I'll be able to the ball rolling on it though.


----------



## Chops

I've been doing a little more tweaking to my amp lately, also with the output caps. 

 For now, I am replacing the Mundorf M-Lytic 100+100uF caps with a pair of Samwha 1500uF -385V Photo Flash caps. I also replaced the cloth-covered 18awg solid core wire with multi-strand 18awg tinned copper wire. As a result, I had to slightly enlarge the holes on the PCB to get the stranded wire to fit. Not a big deal at all. 

 I've had the amp powered up since 9pm last night. I have noticed that the left channel cap is reading 114v DC where the right channel cap is reading 118v DC. I'm hoping that this will equalize eventually once they have a good amount of hours on them. I plan on leaving the amp on straight until Wednesday morning which should give me just about 83 constant hours of burn-in.

 As for the sound, I noticed right away (and to my surprise) that these new caps are much more open and airy sounding. The soundstage is simply huge! I am noticing a bunch of fine details in the music that I never heard before, such as faint shimmers of a cymbal or high-hat, the textured sound of human flesh tapping on hand drums and tamborines, the gritty, grainy sound of someone's throat as they sing, very fine and detailed sounds that were burried in the background, and a lot more ambience around the musicians and recording venues. There's just a bunch more detail in every part of the sound. 

 The only thing so far that is lacking a little (and this is probably because the caps are brand new) is the lowest bass. As I recall, this was also true with the Mundorfs when they were new. In all actuallity, these new 1500uF caps should be providing more bass output than the Mundies, and I'll just have to wait and see what happens after this first round of burn-in time. 

 Even with the slightly less bass output, the rest of the sound is so good at this point, I really don't mind the leaness that much. It's so good in fact, I'm not even sure if I'm going to put those Mundies back in. 

 I'm guessing the reason I'm getting all of this new-found detail and openness is due to the phase distortion being pushed right out of the audible range, well below 20Hz (probably around 15Hz or so at 62Ω). I'll definately keep you updated on how things turn out with burn-in.

 Anyway, I just know all of you are dying for some pictures, so here they are! 
	

	
	
		
		

		
		
	


	




 Test fit...











 Enlarging the holes with a very fine-tipped file...






 The completed wire and cap upgrade...


----------



## pwrusr

Wow what a thread you have here dude! 

 I'm very interested in hearing your thoughs after you get done with the burn-in.


----------



## Senad

Guys,

 Based on your experience, how would you compare reasonably modded 708B (new NOS tubes, signal wiring, ALPS stereo pot, Input/Output Resistors and new inter-stage coupling and hp output caps) to let say ….

 Darkvoice 336i
 Little Dot 2++ (or any other variations of Little Dot)
 Cayin HA-1A (I know, this is completely unfair considering the price difference)


----------



## dcheming

Quote:


  Originally Posted by *Senad* /img/forum/go_quote.gif 
_Guys,

 Based on your experience, how would you compare reasonably modded 708B (new NOS tubes, signal wiring, ALPS stereo pot, Input/Output Resistors and new inter-stage coupling and hp output caps) to let say ….

 Darkvoice 336i
 Little Dot 2++ (or any other variations of Little Dot)
 Cayin HA-1A (I know, this is completely unfair considering the price difference)_

 

Unfortunately my 708B is the only amp that I have experience with so I can't give any useful comments. However, in a few weeks I'm planing on loaning my modded amp to a well-known head-fier to be included in his massive amp review. That will provide you with the info that you seek.


----------



## Senad

That is great. I am looking forward to the review. I plunged in myself since I posted the question and ordered one for myself. It should be here in another week. I am also about to order parts for the majority of the mods and I have couple of upgrade tube sets on the way (NIB JAN 5670W, Amperex A-Frame 6DJ8, and 6N3P-EB). 
	

	
	
		
		

		
		
	


	




 Where did you get your knob for ALPS? I know that the primary concern is the sound quality, but the one from Radio Shack that everybody used is just plain butt ugly! The original knob looks rather decent, how come nobody tried to reuse is (I am sure it would require some retooling but still)?

 Could someone tell me what is the diameter of the original knob? I found a very nice and reasonably inexpensive knob (KILO International JD90) but I am not sure if it is big enough. Shaft socket seems to be a good match:
http://www.kilointernational.com/kno...jd_series.html


----------



## dcheming

I'm also looking forward to seeing how it compares to some of the other amps now. I got that knob from Rat-Shack too, PN: 274-407. The original knob is made out of plastic and felt pretty cheap. It was made for a splined shaft and the ALPS I got had a smooth shaft so I couldn't use it. I like the contrast of a black knob against brushed aluminum much better anyways. I threw that vol knob away so I don't know how big it is, but the circular pocket on the front panel is around 1.302" (~33mm) diameter on my amp for reference, so that JD90 would be too small to fill it up fully.


----------



## Senad

I finally found a source for a good size knob. It is somewhat on the expensive side ($12 + $5 S&H) but reasonable considering they are machined out of solid chunk of aluminum and great looking. The diameter is 1.4" (36.2mm) what should be a good fit (slightly bigger than the original knob):

http://www.goldpt.com/prices.html

 Btw, they also make/sell very impressive stepped pots.


----------



## dcheming

Those are some nice looking knobs. It's a shame the K1.4 is a bit too big and the K1.1 is a bit too small. I looked around some and I only found a few 30mm aluminum knobs, but no 33mm ones. The best fit would be the KB-BE-32 from VT4C Studio, but it's made out of wood though. Man if I had a metal lathe here I'd just make you a custom one.


----------



## Senad

Why do you think that a larger knob is a problem? I understand that the knob would not fit inside the original recess, but if you mount it so it just slightly clears the flat surface of the enclosure, it would look good. Am I missing something?


----------



## dcheming

Nah, I didn't mean it'd be a problem going with the larger knob, since it'll completely cover up the pocket anyways as you said. I just meant that it's a shame you can't get one the exact size that the pocket was milled out for.


----------



## jamato8

I didn't read through all of the mod but I hope the output caps were increased in size as the knee of the .22 would hold down the bass and there is no need for 1K resistors on the input but 200 to 400 ohm would work fine. The higher input resistor will curtail the highs some. I would use Riken Ohm as much as the tants. I have used many tants from the better tants (before they were no longer made in Japan and were nonmagnetic) to the later and not as good Audio Note tants and the Riken hold their own just fine if not better.


----------



## rockcod

What about those power supply filter caps? Are they Nippon Chemi-con KMG? Do you think changing them to Panasonic TSHA would yield better results?


----------



## dcheming

Quote:


  Originally Posted by *rockcod* /img/forum/go_quote.gif 
_What about those power supply filter caps? Are they Nippon Chemi-con KMG? Do you think changing them to Panasonic TSHA would yield better results?_

 

I'm not sure how much of a difference that would make, but I've been wanting to replace those caps too just to see. I know zer061zer0 replaced his with something else so maybe he can make a comment on this.


----------



## zer061zer0

I change and change them back i dont hear any specific differences.

 dcheming, hows the review coming along?

 i pm u some questions was wondering if u can help me out, thanks.


----------



## zer061zer0

Derek, any results from the shootout of the amps.


----------



## zer061zer0

I just gotten the stuff to remod and recase this amp. I started with the 6x4 mod yesterday and found it rather hot, is that normal. My PS board is slightly different but i am quite sure i got the connections correctly.

 Chops, Derek, do you have this problem? is it normal?


----------



## zer061zer0

Hi Guys,

 I have bought a casing to recase my XS. Right now, I hear a lot of noise and humming. Everything is out now, I have yet to place them into the new casing. I do not know what is the problem. Could some one help me out?


----------



## dcheming

Sorry zer061zer0, it's been a while since I've been here on Head-Fi. About the amp reviews, you misunderstood me. I'm not doing the shootout, I'm going to be eventually sending my amp to someone to include in his shootout.

  Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_I just gotten the stuff to remod and recase this amp. I started with the 6x4 mod yesterday and found it rather hot, is that normal. My PS board is slightly different but i am quite sure i got the connections correctly.

 Chops, Derek, do you have this problem? is it normal?_

 

So what exactly is getting hot, the tube, the transformer, or the PS board? My transformer is fairly warm, but the 6X4 definitely gets hot.


  Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_Hi Guys,

 I have bought a casing to recase my XS. Right now, I hear a lot of noise and humming. Everything is out now, I have yet to place them into the new casing. I do not know what is the problem. Could some one help me out?_

 

In my amp if I don't tie signal ground to the case then I get hum. Once you put the amp guts into the new case and tie ground to it hen it should go away.


----------



## tbonner1

Glad to see this thread come up again. The street price is going down on these amps to Little Dot level. You have better tube options with the Xiang Sheng and I am seeing the chinese recitifier tube available on some US tube websites which is comforting.


----------



## Sid and Coke

Fantastic thread, this little amp seems to have quite a following and I'm hoping to have a play with mine soon. 

 I just picked up a farly cheap s/h unit off e-bay , it arrived this morning. Initial impressions are that it has potential to sound good, although it is a little hissy at the moment. I don't kow the age or history of my unit so just for now the first things i intend to do are change all of the Valves for some newer/better ones. 
 I'm not even bothered about Boutique brand NOS items with huge price tags I just want to fit a set of brand new decent sounding valves to get me on my way, I've read that the front widow valve is purely cosmetic so wont change this one oif its not required. 

 The Volume sounds a little un-balanced and doesn't have a nice feel to it so I'll probably make changing the Volue pot to an Alps Blue Velvet 100K log my second mod after replacing my valves. 

 My Amp didn't come with a user manual and I'm 'fairly' new to valves , (although i did buy & build a World Designs KEL84 integrated amp last month which works and sounds fantastic). 
 I'm not familiar with British or American valve numbers let alone Russian and Chinese designations.

 Can some kind soul please give me some suitable part numbers for valves to go and buy, cheap but effective and easy to source preferably. 
 Any chance of a basic shopping list to get me going please.


----------



## Agnostic

More info in this thread: http://www.head-fi.org/forums/showth...191432&page=20

 The front tube of the three tubes inside the amp (not the front window, that's just for show as you said) is the gain tube and should have the biggest influence on the sound. For this you can use:
 For the 6N3 ------> 6N3P, 5670, ECC42, 2C51 and 396A

 The two behind that are the left and right channels:
 For the 6N11 -----> 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23

 As I mentioned in the other thread the NOS GE JAN 5670W can be found on ebay cheap and sounds a lot better than the stock chinese tube. With the stock chinese 6N3 I had some buzzing and hiss that completely went away with the GE.
 The Russian military NOS 6N3P-EV is also said to sound good, I have two on the way, so I can't say if I like it more than the GE JAN. I will post impressions once I get them.

 You'll find more tube suggestions in the other thread as well.


----------



## Sid and Coke

Agnostic, thanks for the speedy reply. 

 I must admit that when i first fired the amp up i was a little disappointed at the bit of hiss i was getting and also with the general dynamics of the sound. However i did also think it had enough potential to be worth spending a bit of time, effort and small amount of cash with as long as the new valves don't cost too much. . 

 If i'd known that the very front valve was just a widow display i would probably not have bought it, as it sems a little cheap to do such a thing, possibly damaging to the sound even.


----------



## Agnostic

Some people have tried taking it out completely but it doesn't seem to affect the sound in or out. Anyway I think it looks kinda nice but maybe that's because I have bad taste lol.
 Without music playing with the GE JAN I only get hiss if I open up the volume way beyong listening levels at 1 or 2 on the dial and then only very softly, have to strain to hear it over background noise. This thing really has absurd amounts of gain as I guess you've noticed.


----------



## Sid and Coke

I've open the amp up and have a mixture of valves in there. 

 VE1 is a Chinese 6N3 (probably the OEM item)
 VE2 & VE3 are Phillips ECC88 (Holland) so probably later additions. 

 The Valve in between the two BIG 450v Caps is a Chinese one I think it has 6Z4 written on it . It looks very dusty, old and tatty with a milky hue to the glass envelope. 

 Basically as i don't have any means of properly testing what i already have i would like to replace them all with new (tested ideally ) items then i can take it from there... 

 There was also a small peice of Grey coloured cardboard type stuff with hot glue on the back floating around loosely inside, where shoud this be fixed


----------



## dcheming

Quote:


  Originally Posted by *Sid and Coke* /img/forum/go_quote.gif 
_The Valve in between the two BIG 450v Caps is a Chinese one I think it has 6Z4 written on it . It looks very dusty, old and tatty with a milky hue to the glass envelope._

 

If the top part of the 6Z4 is whitish in color then it's most likely lost its vacuum and definitely needs to be replaced. Thankfully 6Z4 rectifier tubes are cheap.

  Quote:


  Originally Posted by *Sid and Coke* /img/forum/go_quote.gif 
_There was also a small peice of Grey coloured cardboard type stuff with hot glue on the back floating around loosely inside, where shoud this be fixed_

 

That used to be glued to the PCB side of the power switch as a means of short circuit protection.


----------



## Sid and Coke

I have ordered a new 6Z4 to replace my old dusty looking original.

 However, having re-read many of the 708B threads and posts it seems to me that the way to get this amp to sound good ( i don't believe the stock unit is reallly any good at all ) , is to replace just about every single component apart from the Case, Transformer, PCB's and main 450v PSU caps. 

 I had a whole load of components ready to buy via various online sources , but then took a reality check once i'd added up the cost and had another listen to both the Headphone output of my Tascam CD-RW900 CD Player and also to my RedSound micro-amp Head phone amp . Both of which sounded better to my ears than the Stock 708B. 

 I initially was disappointed with my Red Micro amp , due to lack of drive capability into my Beyer-Dynamic DT-150's I then realised that i was using the headphone output of the CDP for my failry infrequent headphone listening. I used the Line level tape outputs of my World Designs KEL-84 integrated Valve amp and finally the RedSound came alive, basically it just wasn't getting enough signal before. Being such a slimline unit it also slips onto my Hi-Fi rack without using its own shelf. 

 Great thread, glad to see you guys are having so much positive fun and results from these DIY upgrades, but i've decided that in this case i won't be spending any more time, or money on my 708B. I will concentrate my efforts on inproving my KEL-84 as there are a few good improvements and mods i have lined up for that, and I only built it last month. 

 Thanks for all your time , effort and great replies, Keep up the good work. 

 maybe next time...


----------



## dcheming

Sid and Coke,

 I wouldn't say that it's necessary to go as far as I did in mods. Much of the stuff I did towards the end only provided marginal improvements and you're right about the total cost making in not worth it in the long run. For not much more I could have bought a Ear HD+ kit from Mapletree which is a much better design, but as I said elsewhere this was more of a learning experience to understand how parts affect the sound than trying to build an awesome amp. That's also why I say it's not necessary to go nearly as far as I did. If you're planning on keeping the amp then I'd at least get a better vol pot, but I definitely understand your decision to move on. Mods are all about the maximizing sonic improvement verses the investment so it seems like you'll be much better off in the long run modding your KEL-84 instead. Happy soldering.


----------



## Sid and Coke

I am planning to sell on my 708B . As mentioned earlier I had a slightly discoloured 6Z4 rectifier tube so have replaced it with a brand new one ( just 4 days to ship HK > UK 
	

	
	
		
		

		
			





 ) 

 I then noticed that the front 6N1 'display window' tube is actually a 6N11J in my amp, obviously changed by the previous owner. 
 I have removed it for the time being and the amp works fine. 

 Would it be OK to put either a 6N11, 6N11J or a ECC88 tube in this display position- which i have going spare. 
 I don't really want to have to spend money getting a new 6N1, or waiting for it to be shipped , when it doesn't really do anything apart from suck power and look pretty.


----------



## dcheming

In the front window you could use either ECC88, 6922, or 6N11. The 6N11J that you have in your window now is one of the old Chinese output tubes that comes stock in the amp.


----------



## hnb2907

Hi folks,

 I'm a bit of a newbie to the headphones & amp world, but I've a bit of a love (or fetish) for valves. Was thinking of getting a Little Dot, but kept seeing bad news about them having design problems. The 708B seemed to get a fair bit of much better news/reviews in this respect. Was also looking for an amp that had valve rectification, which this one has.

 Having done a bit of reading around this and other forums, I've purchased a 708B from "on9mart" via ebay for £72 including airmail. It arrived very well packaged in about 4 or 5 working days, got the tracking ID from the seller. In fact, the parcel was so ornately wrapped, it was a shame to rip it open!











 Bear with me, I'm going to give a quick run down of the mods I've made...

 So, first impressions, are that it's not at all a bad bit of kit. It's well put together, and the inside is neat and tidy. Straight out of the box, I first powered it up using a Variac set to 220VAC (instead of my usual 237VAC), and checked the heather voltages were roughly 6.3V, which they were. So the first mod was definitely going to be sorting those out, as they went around 7.0-7.2V when running on 240.

 Anyway, it even then after about 4 hours burnin, it sounded better than the headphone output my Dads old QED amp or another old Technics amp. The major critiscm was an amount of AC hum in the headphones that was not loud but a little distracting in quieter music. BTW, I'm using a pair of Senn 555's,and a Technics CD SL-PG370A CD player.






*Mod 1*
 SAFETY FIRST!
 Being a bit electrically minded, I didn't fancy the idea of those really large PSU caps staying charged up. So, I've fitted a 1Mohm bleeder under each, as they're not quite in parallel with each other, owing to them being in a CRC filter network. I originally had 3W 68Kohm, but these were getting warm, and I don't like the idea of slow cooking caps, 'cus it shortens their lifetime. The 1Mohms don't get much above gently warm.

 I've also added a wire from the earth pin on the mains IEC socket to the chassis. It's a bit naughty not to have that! 
	

	
	
		
		

		
		
	


	


















*Mod 2*
 Had to be to get it to run on UK 240VAC, so that's sorting out the heater and HT voltages.

 First off, heaters. A bit of trial and error found that I needed about 1/2ohm in series with the rectifier/frontpanel valves. Not having anything suitable to hand, a length of about 20CM stranded equipment wire was giving the correct voltage drop. This is neatly folded up in some heatshrink tubing under the PSU board (which I've raised slightly on it's mounting bolts).

 The second heater circuit for the 3 valves on the amp board. I've had to change this again, as since I originally modified it, I've installed a different pre-amp valve, which changed the voltage again. Now it's 0.47 and 0.5 in series, both 5W. I had a 1ohm but that dropped the voltage too much.

 So, now on an average day with about 237+/-7VAC on the line, I get about 6.1 to 6.35V on all the heaters.





*Mod 3*
 Folks have already mentioned in the thread about the 2 high wattage resistors on the PSU board getting cooked. I've basically made roughly the same resistances up by paralleling some 3W bits together. In doing this, I've increased the resistances slightly, which helps the AC hum filtering in the CRC network. It's also had the useful effect of dropping the B+ rail back to about 185VDC now I'm running it on higher UK mains voltage.

 While the PSU board was out, I replaced the 1N4007 diodes with Vishay SF4007's, as mentioned elsewhere on the forum. Didn't notice anything obvious when these were replaced though.








*Mod 4*
 This isn't critical, but I replaced the phono sockets. The stock ones were ok, but they seems about 0.5mm too thin in diameter. So, these have been replaced with some new gold finished ones. I replaced all 4, even though I don't plan to use the pre-amp out.







*Mod 5*
 Again, not critical, but I replaced the 2off 47u electrolytics on the amp board, only because the top of one looked a bit dented.








*Mod 5*
 This is probably the biggest and most dramatic improvement I've made to date, having read up with folks' experience of tube rolling. I've replaced the Chinese 6N3 with a 1987 NOS GE JAN 5670W. Wow. Immediately the AC hum is almost none existant - the M6 motorway is 1.5miles away, and at 8pm with the window open, that's louder than the hum. This valve has really brought the sound of the amp to life. Also shortened the spring clip to suit the lower height of the 5670W.





*Mod 6*
 Replaced the potentiometer with an ALPS 100KA. I have access to a PCB machine at work which scratches copper from copper clad board. So, I knocked up a simple 2-sided board to interface the ALPS PCB pins to the phono wires.
 The ALPS needs a new hole drilling for the locating peg, and the stock knob doesn't fit. I still need to sort a new knob out 
	

	
	
		
		

		
		
	


	




 for it, of course!
 Having read other reports of changing this, I was expecting more improvement than I got. The old pot was fairly well balanced, but it did suffer from not much graduation in volume when set at about 0 to 25%. This ALPS is definitely a lot better for this.

 At this point, I thought I'd lost the plot. When I first powered up, I got a deafening hum out of my Senn's, which were quickly unplugged. It turns out that somehow the ALPS pot creates an earth loop, where the stock pot didn't. This was fixed by adding qty=10 of 100R 1/2W in parallel, wired between the earth pin on the mains socket and the earth commons on the isolated phono in's.














*Mod 7*
 As I'm not exactly a millionaire, I was going to replace the WIMA 220nF interstage coupling caps with Musicaps. For some reason, I couldn't find any reasonably priced ones in the UK (compared to the US prices). So, I've dropped a pair of SONIQS SAX220nF/630V in. Had these on a component tester at work. These were both bang on 223nF and measure an ESR of about 1/2ohm at 100hz, 1khz and 10hz.
 These too have made an improvement, the sound is certainly a little clearer than with the WIMA's.




*Mod 8*
 My CD player seems to have a LOUD output level. So I've put a 560K resistor in series with the phono in socket, which has given a lot more play on the volume control. Maybe I should have dropped a 560KA ALPS pot in there instead or had a go at reducing the amplifier gain.



*In Conclusion*
 The amp was £72, I've spent about another £40 in bits.











 That's probably as far as I'm going with the mods. It sounds excellent (to me anyway), it reproduces a wide range of live music recordings exceptionally well. Listening to the BBC CD of "Last night of the proms" is a new experience, compared with the old transistor amps at home. CD's like Norah Jones, and classical recordings and the such like have come to life, I can now hear the musicians shuffling around, not just the music
	

	
	
		
		

		
		
	


	




 My advice, go for it, and do a bit of tweaking - especially replacing the 6n3 and sorting the heaters if you're not on 110V or 220V! 
	

	
	
		
		

		
		
	


	





 Cheers,
 Chris.


----------



## Agnostic

If you can find one try a NOS Sylvania 5670. I had a used one first but found a NOS quite cheap and I think it's even better than the NOS GE JAN 5670W that I tried as well.

 I'm very happy with the synergy it has with my HD580 now, really detailed, lush, rich and textured sound.


----------



## hnb2907

Quote:


  Originally Posted by *Agnostic* /img/forum/go_quote.gif 
_If you can find one try a NOS Sylvania 5670. I had a used one first but found a NOS quite cheap and I think it's even better than the NOS GE JAN 5670W that I tried as well.

 I'm very happy with the synergy it has with my HD580 now, really detailed, lush, rich and textured sound._

 

Thanks! What about replacing the 6N11's? Is that likely to make much difference?

 Cheers,
 Chris.


----------



## Agnostic

I tried replacing the 6N3 with a russian 6N3P-EV(EB) but they increased the buzz and didn't improve the sound. Also tried replacing the 6n11's with two Sylvania JAN 6DJ8 but one channel was much louder than the other. Anyway they didn't seem to have a very big effect on the sound signature.

 There's more info on the Xiang Sheng in this thread by the way if you hadn't seen it already:
http://www.head-fi.org/forums/showth...ang+Sheng+708B

 The Russian 6n23p-eb(ev) is supposed to be a good replacement for the 6n11 I read but my experience with the Sylvania 6DJ8 made me feel sound quality would not improve greatly from replacing the 6n11's.


----------



## hnb2907

BUMP - sorry! Added some pics for your enjoyment.... 
	

	
	
		
		

		
		
	


	




 Cheers,
 Chris.


----------



## dcheming

hnb2907,

 Welcome aboard, thanks for adding your mods and pics to this thread. 
	

	
	
		
		

		
		
	


	




 It's a shame that all the units seem to only have 110/220V transformers. Your measured heater voltages of 7.0-7.2V while on 240V are pretty much how mine where when on 120V.

 I also was a bit surprised to see that this amp is not grounded. From what I gather they did this as a cheap way to prevent ground loop issues. I was hesitant to add a ground wire since I didn't know what effect it would have on the amp. Next time I fire up my soldering iron I'll properly ground it for piece of mind.

 Correcting the B+ voltage is something I never got around to. Last time I checked it was at around 215V. I have to drive 80miles to get suitable resistors so it's been put it off long enough to have been forgotten about.

 The stock phono sockets on my amp where undersized as well. New standard gold plated jacks are so cheap anyways I would of replaced them regardless.

 The two electrolytic caps on my amp and on a few others where dented also. I wonder where Xiang Sheng gets their high-quality components. 
	

	
	
		
		

		
		
	


	




 From my limited tube rolling experience, changing the 6N3 tube seems to make more difference than the 6N11's, but I'd still recommend upgrading the 6N11's as well.

 That's a nice little PCB for the pot. Definitely easier than soldering directly to the pins. My old stock pot was pretty bad on the balance issue. Also it seems like it was a linear pot which is why it would get loud super fast. Concerning the deafening hum when you plugged in your Senns, that's what I was worried about with grounding the chassis. I'll have to try it with the resister or maybe a ground loop breaker.

 That sucks about the high prices on the Musicaps. At least SONIQS SAX are polypropylene. I want to step up to a film and foil cap eventually, either PP or polystyrene.

 I don't think it's necessarily that your CD player has a loud output. The problem is that the gain of the XS is extremely high. There really isn't an easy way to lower the gain of this amp aside from building a potential divider on the input.

 It looks like they used different signal wire in your amp. The stock stuff in mine had white insulation.


----------



## hnb2907

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_hnb2907,

 Welcome aboard, thanks for adding your mods and pics to this thread. 
	

	
	
		
		

		
		
	


	


_

 

Thanks, no problem!
  Quote:


 It's a shame that all the units seem to only have 110/220V transformers. Your measured heater voltages of 7.0-7.2V while on 240V are pretty much how mine where when on 120V. 
 

I did ask the seller about this, and got the reply I expected - that it'll run on 240, which to be fair, it does. 
	

	
	
		
		

		
		
	


	




 I used it for the first few weeks on a variac set at 220V, until I got the voltages back.
  Quote:


 Correcting the B+ voltage is something I never got around to. Last time I checked it was at around 215V. I have to drive 80miles to get suitable resistors so it's been put it off long enough to have been forgotten about. 
 

Doing electronics as a hobby in the UK has become so expensive recently. My local store has just closed after about 80years of service, they were great for finding all sorts of goodies. We have a Maplins, but that's turned into a toyshop. So, I'm left with the alternative of finding bits on ebay, or lashing out at least £30 per order with RS, Farnell or Rapid, often for a handful of resistors. 
	

	
	
		
		

		
		
	


	



  Quote:


 From my limited tube rolling experience, changing the 6N3 tube seems to make more difference than the 6N11's, but I'd still recommend upgrading the 6N11's as well. 
 

...maybe I'll try that too?
  Quote:


 That's a nice little PCB for the pot. Definitely easier than soldering directly to the pins. My old stock pot was pretty bad on the balance issue. Also it seems like it was a linear pot which is why it would get loud super fast. Concerning the deafening hum when you plugged in your Senns, that's what I was worried about with grounding the chassis. I'll have to try it with the resister or maybe a ground loop breaker. 
 

The PCB machine is a useful bit of kit, but it can be a PITA to use!
  Quote:


 It looks like they used different signal wire in your amp. The stock stuff in mine had white insulation. 
 

I might change the signal wiring, and I'm also tempted to replace the PCB with the mains switch and headphone jack. For some reason I'm not too happy about them both being on the same PCB. 

 Something else I've been investigating with another project is putting a 120ohm NTC themister in series with the mains feed to the transformer. This should reduce the inital inrush current until the thermister is at running temperature. It'll need to be mounted in a ceramic terminal block, as it will probably run at about 110C. The only downside is that it will drop a few volts, so may need the heater/HT voltages sorting again.

 I've now got a fluted 28mm aluminium knob to replace the plastic one, and finally fitted the cover back on it! 
	

	
	
		
		

		
		
	


	




 Cheers,
 Chris.


----------



## zer061zer0

Looking at this page really brings back some fond memories, so looking forward to listening to my XS as I havent been home for quite a while. 

 I think the 6dj8 does have an impact on the sound, I uses the 6x4 with a slyvannia 5670(6n3) and 6dj8(6n11) and i believe that there is a sharper edge given to the music (if memory serves me correctly). although the sharpness was slowly tone down over time.


----------



## zanash

I've just got mine out again after getting some 6n3's off fleabay ...much better than the original!

 Added a stepped attenuator too ...

 mine has the white signal cable .....[screened silverplated copper in ptfe ...rather bright and thin sounding if used as an IC ime]

 I shall be doing a silver in ptfe rewire over the weekend ....


----------



## obuckley

I was getting a lot of distortion and way too much gain from my 708B, so did an Internet search to see what might be out there and found this thread.

 I have now done most of the mods recommended on this thread and am generally pleased with the results, although I have what I think is a better way of doing some parts of the process, although I also have a problem with (UK) 50 Hz mains cycle hum.

 Hopefully these are not related 
	

	
	
		
		

		
		
	


	




 My main difference with the orthodoxy on this thread is that you can get transformers with multi-tapped primaries fairly easily over here. So in order to adjust the working voltage of the pre-amp from 220v to 240v (same applies to get US 120v to 110v) all you need to do is fit in a small (20 - 30VA) transformer, wire the input mains between the 240v and 0v connection and use the 220v tap and the 0v to feed the existing toroidal transformer in the pre-amp. There is comfortably enough space in the case to house such a transformer at the back next to the mains input. 
Attachment 10482
 You need to rotate the PSU board 90 degrees (caps to the rear works best) and by slicing off a small portion of the rear of the audio board and jumpering a couple of severed connections, that can still be fitted on 3 of its 4 original mounting screws.
Attachment 10488
 This avoids the need to adjust the heater voltages for the valves with resistors and gets the main DC voltage to the right level.

 I am trying to post some photos, but with a 25kB limit, I am having trouble; cosmetically my fully modified version looks pretty similar to the original, it just has better components more or less throughout. 
Attachment 10486
 I have a much more detailed resolution version of the photo if anyone has an idea how or where to post it.
 Apart from the extra transformer, another practical measure was to stick my soldering iron into the rear of the original volume control knob until the plastic of the mounting hole got a bit soggy, then rammed it firmly onto the ALPS pot and let the plastic re-set. You need to keep it a) straight and b) central, but really this worked like a charm and avoids the need to use nasty Radio Shack knobs.
Attachment 10485
 All the distortion has gone and the gain is now manageable. I can just about listen to the pre-amp output at full volume (it is still quite loud); on headphones, full volume is perfectly acceptable without risking inner ear damage.

 I added an Earth (Ground) connection from the front panel and the case body to the Earth pin of the mains input (somebody on the thread did something similar). However, I ended up with very loud 50Hz mains hum and had to install an earth-lift switch at the rear to optionally connect or disconnect this. 
Attachment 10487
 Even with the earth pin of the mains input totally disconnected from anything, I still get a fairly noticeable mains hum. I have not done anything to specifically Earth the ALPS Blue Velvet pot (I am not sure I know how I would do that, but I noticed some other posters had trouble in this area).

 Can anybody advise me how to eliminate this hum.


----------



## obuckley

Some better resolution photos here:
http://cid-e175340f8fc0f7f7.spaces.l...F8FC0F7F7!108/


----------



## zanash

This is very interesting ........as I mentioned I've a small amount of hum on the right channel [left is silent] ....what appears to have made a considerable improvement was just changing the 6n3. The vol pot has made little or no difference to the hum level.

 Not certain I like the extra transformer idea...20va seems rather small and could compromise the current capacity of the psu ....not that it won't work its just my experience that when you fit bigger tranny's things always sound better.


----------



## zer061zer0

i used to have some hums after i XS recase project, but its gone after i added a earth and also short the gnd pin on the volume pot, not sure if it works for you but hope it helps.


----------



## Ideal

Quote:


  Originally Posted by *Agnostic* /img/forum/go_quote.gif 
_I tried replacing the 6N3 with a russian 6N3P-EV(EB) but they increased the buzz and didn't improve the sound. Also tried replacing the 6n11's with two Sylvania JAN 6DJ8 but one channel was much louder than the other. Anyway they didn't seem to have a very big effect on the sound signature.
 ....
 The Russian 6n23p-eb(ev) is supposed to be a good replacement for the 6n11 I read but my experience with the Sylvania 6DJ8 made me feel sound quality would not improve greatly from replacing the 6n11's._

 

You couldn't be more wrong!
 The 6N11's/6922's do a lot for the sound image, I won't say what I've replaced mine with but I can tell you this, it sure isn't Sylvania.
 I like open and dimensional sound, Sylvania and RCA have a closed boxy claustrophobic type sound.

 As for the 6N3J, I use the stock one, as I have been unable to find a suitable replacement. You just balance it out by choosing other E88CC/6922/6N11's.

 When I tried LM Ericsson 396A instead of 6N3J all I got was buzz, just like you experienced.
 I think the 6N3J may be "primed" at it's max operating limit, and that probably exceeds 396A/5670.


----------



## mojolo

would you mind telling us which 6DJ8/6922/7308 tubes you are using?

 i have some Amperex Holland Orange Globe Halo O Getters and Philips Holland A-Frames ecc88's/d6j8's. have you any experience with these? (i just ordered the xiang sheng and it is on the way)

 For the 6N3, did you try any other versions such as a 6n3p-ev or Sylvania/GE/Raytheon/RCA 5670/2c51 tubes?

 I think the date code is also important for these as post 1960's Sylvania 5670 are not supposed to be that great while 1950's Sylvania's (possibly early 60's) are supposed to be better. Same goes for GE and RCA. of course it all depends on how well they specifically match to the xiang sheng.


----------



## lordmozilla

Hey,

 I have followed and read your thread and have been very impressed with your modding, and have ended up buying a 708B in order to have a play with headphone amps.

 I have been looking at parts to buy and have decided to go with an alps blue velvet pot, but you mention the mundorf ZN caps and would like to get them. However i can only find 250V 0.22uF versions. Or i can get the mundorf supreme caps rated 1200V. 

 I dont know many audiophile electionic shops but there is this one amplifier valve kits, HIFI pre-amplifiers, speaker kits, upgrade components in the UK, feel free to direct me to another, but they do not seem to stock caps that do not cost too much and have the right rating.

 Also for resistors, they stock the Kiwame in 2W but would you recomend getting 0.5W resistors such as the Shinkoh. Are the kiwame good resistors for this use?

 These are the changes i was going to do now, and maybe get to tube rolling later on, but i have had a play with it stock and would like to try it differently now.

 Thanks for all the real clear pictures and instructions,
 Brendan


----------



## zer061zer0

a little update on my XS, which i have recased, not the best of works, but i am very comfortable with the uniform look given to my amps


----------



## lordmozilla

Hey,

 Just for reference i have the UK version of the 708B

 Nearly finished modding my 708B. I replaced teh front 6n3 with a 6n3p-IV cryo treated (the cryo bit was cheap so  ), changed all the audio resistors with kiwame 2W resistors and changed the phono plugs (i didnt think i would but they are really bad). Also changed to pot to an alps blue velvet. Managed to keep the stock volume knob by just mashing up the knob with a screwdriver to make it a tight fit.

 Removed the wima caps for the headphone path - not the preamp with LCR 630V 022uF caps.

 Also i removed the whole front window assembly as i just didnt see the point.

 All that needs doing is replacing the two driver tubes with a pair of matched NOS 6N1P-Vi tubes and adding two 3pDt switches in order to switch between inputs and another to remove the noise from having a preamp plugged in when listening to headphones.

 I'm kinda having trouble sourcing these so i'll have to look a little harder. I'm also going to to try to replace the headphone jack but thats for a little later.

 also changed the power cord with a tagan EMI shielded cord - the stock power cord really deserved to die.

 As for the audio change from stock to modded i dont know, i'm amazed every time I plug my headphones into this amp so i really can't say lol. I'll post some pictures later on if i remember.


----------



## dcheming

Brendan,

 Glad to have you on board. 
	

	
	
		
		

		
		
	


	




 Sorry for being so late, I must have missed this thread since I don't come around here as much as I used to.

 "_but you mention the mundorf ZN caps and would like to get them. However i can only find 250V 0.22uF versions. Or i can get the mundorf supreme caps rated 1200V._"

 I wanted to go with the ZN series because they are film 'n foils, but their long size would necessitate mounting them vertically. Doing this will expose them directly to heat from the nearby tubes, especially since the ZN caps are black IIRC. This is why I went with the Auricaps since they are very short and compact so they can be easily mounted horizontally. The Mundorf Supreme caps are even longer, but at least they are white so they wouldn't absorb as much radiant heat. Of course you could also just bend the leads underneath like how I mounted the Dayton film 'n foils on my amp. BTW, Parts Connexion carries 630VDC ZN caps in 0.22uF.


 Please do post some pics if you get a chance.

 zer061zer0,

 I like the smoked lexan cover. I'm considering doing something similar when I get around to building a new cover for mine, but with glass. Have you had any problems with overheating in that case? You could always drill some holes in the case under the PCBs to aid ventilation if so.


----------



## Senad

Could somebody tell if it is possible to remount the original knob on alps blue velvet with some tweaking (I am aware that it doesn't fit)?


----------



## dcheming

Quote:


  Originally Posted by *Senad* /img/forum/go_quote.gif 
_Could somebody tell if it is possible to remount the original knob on alps blue velvet with some tweaking (I am aware that it doesn't fit)?_

 

The original knob is designed to be used on a vol pot with a splined shaft so if you found a RK27 with such a shaft it might fit. Your other option would be to fill in the webbing on the inside of the knob with epoxy or better yet JB Weld, then drill and tap a small hole for a setscrew. If you don't have the tools to do this I could do if you as long as you paid for shipping.


----------



## lordmozilla

thansk for the response, i've gone with some LCR caps that i found, they are ok. I just got my two 6n1p's in the post and man do they make such a difference, the bass is real deep now and much more present, the difference is unbelievable. Compared to swapping the 6n3 they seem to be much more worth it. I'm really surprised because I really expected there to be nothing noticeable as with the 6n3, but the difference is definitely notable.

 I've also checked my heater voltage, and my 6n1p was at 6.2V. Now my voltmeter is a cheap old thing, but seems ok to me. I guess power must be quite low where i am pluging it into. I have noticed though that my board difffers in a few places from yours, especially round the PSU, where the resistors are much beefier than the ones in the pictures of the stock ones you posted.

 I want to replace the output caps, but i have no idea what to use, can you make any recommendations on capacitance? I'll be using a variety of caps but wouldn't want the bass to dissapear. The stock are 120uF, so maybe just stick with that? What would you recommend from parts connexion?

 also what is your opinion on changing the diodes? worth it? Also the two capacitors C9 and C8, do you think there is much point in changing them?

 I think i'm going to order some mundorf caps to replace the LCR's and use the LCR's on the output. 

 Cheers, pics will be up tmrw hopefully.

 sorry for all the questions but i am new to audio modding, but i'm loving it allready!

 Brendan


----------



## dcheming

"_I've also checked my heater voltage, and my 6n1p was at 6.2V. Now my voltmeter is a cheap old thing, but seems ok to me. I guess power must be quite low where i am pluging it into. I have noticed though that my board difffers in a few places from yours, especially round the PSU, where the resistors are much beefier than the ones in the pictures of the stock ones you posted._"

 Guess you lucked out on the heater voltage issue. The layout of the 220V version is definitely a bit different, but mostly in the PS board and in where the B+ comes into the audio board. Also every time I see the insides of one of these amps the parts are visibly different. Bargain bin level components for sure.


 "_I want to replace the output caps, but i have no idea what to use, can you make any recommendations on capacitance? I'll be using a variety of caps but wouldn't want the bass to dissapear. The stock are 120uF, so maybe just stick with that? What would you recommend from parts connexion?_"

 This is a tuff call since one wants to go big uF for good bass response, which invariably means electrolytic, but one also wants the highest SQ for this important component, which means big and expensive film caps. A while back I almost ordered a pair of 100uF Mundorf M-Caps during the 15% sale at Parts Connexion but changed my mind at the last minute. I decided that enough money has been spent on my 708B in the name of experimentation so I'm saving the good stuff for when I build a Bijou amp. So for quite a few months now I've just been using a 7.5uF Solen as an output cap. Sure there's some loss of deep bass but for the time being I feel it's worth the trade off for clarity until I build the Bijou. So it really depends on whether you want to go 'lytic or film here.


 "_also what is your opinion on changing the diodes? worth it? Also the two capacitors C9 and C8, do you think there is much point in changing them?_"

 For $0.35 each I definitely think it's worth it to replace the diodes with something like the ones I used. The contribution to better SQ that they make is probably very small, but just having a quality component there makes me feel better about safety. Which is also why I replaced C8 and C9 with something better. Piece of mind for a few bucks is worth it to me.


 "_sorry for all the questions but i am new to audio modding, but i'm loving it allready!_"

 Yeah there's definitely something truly enthralling about modifying audio equipment IMO.


----------



## lordmozilla

hey,

 thanks for the response. I do have another question :

 I'm guetting some small cracking on one channel. weird because its never happened since this morning and i haven't changed anythign last night when it was fine. the other channel actually does crack a tiny bit too. i'm gonna swap the tubes to see if they are the cause.

 Cheers

 edit 1 : its not the 6n1p


----------



## dcheming

Quote:


  Originally Posted by *lordmozilla* /img/forum/go_quote.gif 
_hey,

 thanks for the response. I do have another question :

 I'm guetting some small cracking on one channel. weird because its never happened since this morning and i haven't changed anythign last night when it was fine. the other channel actually does crack a tiny bit too. i'm gonna swap the tubes to see if they are the cause.

 Cheers

 edit 1 : its not the 6n1p_

 


 Did you try swapping out the 6N3 also?


----------



## lordmozilla

no i'm just at work at the moment, i'll try again when i get bak home. Only had time ot swap the 6n1.

 i'm kinda hoping its the 6x4 (or 6z4 i never know which one it is!)


----------



## lordmozilla

The noise problems has nothing to do with the tubes unless its the 6z4 which i can't test. Maybe it's the pot? see the question below :

 What are the side effects of removing C101 and C201 ? I don't really understand

 another question is i get some noise after 13-14 o'clock which only gets worst. Could this have something to do with the new alps pot? How can it be grounded? does that mean a connection from the screw on the outer case to the ground?

 Someone mentioned grounding the pot to the phono output and grounding that to the case with 10x100ohm resistors in parralel. ? Should i just do that and use 1 x 10ohm resistor thats 5W ?


----------



## zer061zer0

Check the solder points to and out of the volume pot and the wires to the RCA


----------



## aesthetik

Quote:


 More Tube Rolling:
 I decided to go out on my own this time and try some tubes that interested me. One of which others have tried on a "stock" 708B, but never on a modded one. This is the NOS GE JAN 5670. This one in particular is from 1968. The reason no one liked this tube is because it created too much hum or noise into the signal. Again, this was with all stock 708B amps. However, I put this 5670 tube in my amp and it is totally, completely and 100% dead silent, even at max volume and everywhere in between.

 I also want to try some different output tubes as well. I've heard that the Amperex-Japan 6922/E88CC tubes were actually made by Matsu****a, which are very VERY good tubes.

 Here they are...





 Out with the NOS RCA 6DJ8 output tubes and NOS Russian Military 6N3P EB driver tube...


 And in with the new (NOS) GE JAN 5670 & Amperex-Japan 6922/E88CC tubes.


 As for the sound, I feel that they have a better "grip" on the music than the UK made RCA 6DJ8's and Russian 6N3P EB that drarthurwells recommended. They DO have a wider soundstage, the dynamics are more dynamic, bass seems to be a bit tighter and definately deeper, the midrange is full and natural, and the treble is still smooth, crisp and extended.

 And a close-up of them in action. 
 

Hi all,

 Hope no 1 minds the bump.

 I just got mine yesterday, paid about 110€ all together with shipping. May be a bit expensive for an American but in Europe it's a pretty good price.

 OK, I'm just looking for a stupidly simple clarification on the tubes.

 Mine has 5 tubes. 4 that are the same size as only what I can call a 12ax7 (I only know that from playing guitars for 22 years.)

 Two are in a row of three and say 6n. (circuit board VE2 and VE3)

 1 of that row of 3 says nothing on the tube. (VE1)

 1 is the 1 in the view window. (also no number printed on it)

 A 5th is a different type all together and is much thinner.(6z4)

 Could someone clarify which to change and with what?

 I know there is some difference preference sound wise, and I'd be for hearing all the different likes and dislikes, but I need it dumbed down to which and how many are to be changed and which can stay.

 Thanks in advance.


----------



## aesthetik

ok, think I'm going to answer my own question here just want confirmation.

 Found a clearer post in the thread.

  Quote:


 The front tube of the three tubes inside the amp (not the front window, that's just for show as you said) is the gain tube and should have the biggest influence on the sound. For this you can use:
 For the 6N3 ------> 6N3P, 5670, ECC42, 2C51 and 396A

 The two behind that are the left and right channels:
 For the 6N11 -----> 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23 
 


 Just want to be sure....

 1. Window tube is just a dummy?

 2. Smaller tube is rectifier, no need to change for sound quality?

 3. Front tube can be replaced with 6N3P, 5670, ECC42, 2C51 and 396A?

 4. and back 2 tubes with 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23?

 5.wrap up, only the 3 in a row tubes need to be changed for sound quality?


----------



## dcheming

Sorry I missed this.

 1. The tube in the window is indeed only for show and can be removed completely without affecting the functionality of the amp.

 2. The taller, thinner tube is indeed the 6Z4 rectifier. I believe there are only a few different brand 6Z4 tubes made and they're all the same basic quality level. So even if you did want to roll this I don't think it'd be worth it. Spend your money on the other three tubes which _will_ make a difference.

 3 & 4. I've only replaced the two 6N11's with some 6DJ8's so I can't directly comment on this. You have to be a bit careful because even if the pinout is the same between two different types the heaters might draw too much current for the power transformer to handle or the operating points might be different enough between them to cause issues. So some tubes on that list are closer substitutes than others. One of the sources that I use for checking this is this site.

 5. Pretty much. This is where I'd focus on to get the most bang for your buck.

  Quote:


  Originally Posted by *aesthetik* /img/forum/go_quote.gif 
_ok, think I'm going to answer my own question here just want confirmation.

 Found a clearer post in the thread.




 Just want to be sure....

 1. Window tube is just a dummy?

 2. Smaller tube is rectifier, no need to change for sound quality?

 3. Front tube can be replaced with 6N3P, 5670, ECC42, 2C51 and 396A?

 4. and back 2 tubes with 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23?

 5.wrap up, only the 3 in a row tubes need to be changed for sound quality?_


----------



## Chops

How about this... It's been almost exactly a year since I was last on here!

 Shortly after my last post here, my computer got slammed hard with a virus, thus taking out my hard drives and my website that I was hosting. Then shortly after that, I decided it was time for a new job. So since then I have been working my tail off with this new job, am a manager in training and have much better pay and benefits. 

 Also since my last post, my amp has remained the same and has been sitting for the entire time unused. I also sold my AKG K701's! However, tonight I just decided to fire up the ole' 708B and use it with my trusty Sony MDR-7506's. Those 1500uF caps certainly did take away a good chunk of the bass, so if I keep listening to this amp again, I'm going to put those Mundorf caps back in.

 In the meantime when not working, I have been focusing all of my energy, time and mostly money into my 2-channel/7.1 home theater system. I have been spending some mad cash on this system, especailly lately. As it is, I have a pair of speakers on order right now waiting to be shipped. 

 Anyway, I have a new website up. Actually it's more of a gallery than anything, but I'm no longer hosting it on my PC like before. Pics load much faster and the layout looks great. It's: Zenfolio | CharlesT Photography

 I think I'm going to be sticking around again for a while. There's some really decent people here who I lost complete contact with over the past year. So there you have it!

 Since I can't post without leaving at least one pic, here's a pic of the speakers I'm getting. They are the Onix X-Statik from AV123.com. They're a hybrid sealed/open baffle setup. I'll give more info in my next post!


----------



## Chops

Well I just finished uploading the 708B pictures to my site then reinserting them into this thread in all my various posts. I also reinstalled the Mundorf output caps into my amp. They really are much better sounding than the 1500uF caps I had in there.

 I hope to keep this thread going as it is chalk full of great information. It's simply amazing how far you can take one of these amps. A true sleeper!


----------



## Chops

Another update...

 I've only had this amp back up and running for a few days now and I've already done another mod to it... Kinda.

 I had a couple of Solen 51uF caps sitting around that I used in a loudspeaker project a short while back. They were used to absorb any turn-on thump from an amp to a pair of compression horn drivers (as they were connected directly to the amp and were crossed over via a digital x-over). However, there was never any turn-on thump, but just in case there was.

 I decided to try them as bypass caps on my Mundorf's. I just have them connected will alligator clips for the moment until I can figure out how to install them. I think it may be time to think about getting a new chassis to transfer everything over to as I'm growing out of the current 708B chassis.

 Initail impressions are that these Solen caps remove a bit of grain from the upper midrange and treble and also thicken up the bass just a tad bit. This is more the way a tube amp is supposed to sound. I also notice that there's more inner detail to be had now as well a little more spaciousness. BTW, these Solen 51uF caps are even larger than the Mundorf's!! 
	

	
	
		
		

		
		
	


	




 What is it with me and huge caps?!?! 
	

	
	
		
		

		
		
	


	




 However, this is also listening through my Sony MDR-7506's which can tend to be a bit bright and a shade thin in the midrange, dare I say a little harsh even. Maybe I should look into getting another pair of better cans again, but nothing overly expensive this time around, and something easier to power as well.

 Anyway, as usual, I have pictures! Imagine that! LOL So here we go...


----------



## jrhill

Hello All,
 This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see.

 In Oz, we have 240v, 50Hz and this gives high volts (B+) of +215 on pcb, heaters of 7.3v ac, and the current thru the input valve (6N3, etc) is about 2.7mA, and 10.5 mA thru the 6N11s.

 Reducing the B+ to 195v via p/s resistors (1k2 for the 1k0 and 2k2 for the 2k0) and also added 1R0 in heater line=6.3vac.

 Some resistor changes are 34.5kR for 33k, 230R for 220R (on 6N11) and the rest of the resistors were simply changed to good quality Beyschlag M/oxides.

 All the other upgrades completed too, including replacing the o/p wire and the crappy socket.

 Perhaps this may be useful.

 ... jh


----------



## glitch39

@jrHill - this is useful

 A lot of tube amp enthusiasts also have a tendency to roll tubes and tinker a bit. The 708B is very simple, has lots of space to work with and parts are *relatively* reasonable. By that I mean the knowledge learned from the tweaking is invaluable.

 My 708B has been a reliable component for a while now.


----------



## Chops

Quote:


  Originally Posted by *jrhill* /img/forum/go_quote.gif 
_Hello All,
 This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see.

 In Oz, we have 240v, 50Hz and this gives high volts (B+) of +215 on pcb, heaters of 7.3v ac, and the current thru the input valve (6N3, etc) is about 2.7mA, and 10.5 mA thru the 6N11s.

 Reducing the B+ to 195v via p/s resistors (1k2 for the 1k0 and 2k2 for the 2k0) and also added 1R0 in heater line=6.3vac.

 Some resistor changes are 34.5kR for 33k, 230R for 220R (on 6N11) and the rest of the resistors were simply changed to good quality Beyschlag M/oxides.

 All the other upgrades completed too, including replacing the o/p wire and the crappy socket.

 Perhaps this may be useful.

 ... jh_

 

I think most of us hear in the states have done this mod as well considering we are 120V/60Hz, 10V higher than the 708B p/s. As for the rest of the resistors, every single one in my amp has been replaced.


----------



## jrhill

Yeah, following on from your work and others in this thread is like getting first hand instructions and made the upgrade quite straightforward - many thanks to you and all the others for writing up your work ... jh


----------



## Chops

Quote:


  Originally Posted by *jrhill* /img/forum/go_quote.gif 
_Yeah, following on from your work and others in this thread is like getting first hand instructions and made the upgrade quite straightforward - many thanks to you and all the others for writing up your work ... jh_

 

I'm glad to hear that we were able to help in some way. It is much easier to perform mods to something when you have good instructions and plenty of good pictures to go by. I certainly wouldn't have been able to do any of this stuff without Derek's (*dcheming*) help throughout it all. You ask that guy a question and he tells you everything you need to know in great detail!

 As if you couldn't tell, I'm trying to bring this thread back to life. I am getting back into headphones again and just bought a pair of HD600's (as seen in my Sig below). In fact, years ago, I started with a pair of Sennheiser HD545's. A couple months later I moved up to HD580's, then a couple months later the HD600's. That was over 5 years ago! Then a little over a year ago, I got into headphones again and bought the AKG K701's. And now I'm going back to the HD600's. Go figure... I think I might be mental! LOL


----------



## Cloth Ears

Quote:


  Originally Posted by *jrhill* /img/forum/go_quote.gif 
_Hello All,
 This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see._

 

Hi Head-fi'ers,

 mine is the 708B that James is modding (also my first post, over from DIYaudio & AudioKarma). I'm going along tomorrow to have a listen and discuss, so I'll take the camera and hopefully have a couple of shots to document the progress.


----------



## Chops

Quote:


  Originally Posted by *Cloth Ears* /img/forum/go_quote.gif 
_Hi Head-fi'ers,

 mine is the 708B that James is modding (also my first post, over from DIYaudio & AudioKarma). I'm going along tomorrow to have a listen and discuss, so I'll take the camera and hopefully have a couple of shots to document the progress._

 

Welcome Cloth Ears! I knew that name sounded familair. I know you from DIY Audio. Looking forward to seeing some pics and hearing what you have to say about the new mods/sound.


  Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Maybe I should look into getting another pair of better cans again, but nothing overly expensive this time around, and something easier to power as well._

 

Well, so much for keeping within my new headphone requirements. I just ordered a pair of HD600's yesterday! LOL


----------



## Cloth Ears

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Well, so much for keeping within my new headphone requirements. I just ordered a pair of HD600's yesterday! LOL 
	

	
	
		
		

		
		
	


	


_

 

Ain't that always the way...?

 I read somewhere on this site (before I'd become a real person and signed up) that someone wasn't sure what the voltage to the audio board should be. So I asked at the source, and received this response:
  Quote:


  Originally Posted by *xshn* 
_Dear sir: 708 transformer output alternating voltage is 215V, arrives at the enlargement part d C operation voltage should about 190V._


----------



## Chops

Quote:


  Originally Posted by *Cloth Ears* /img/forum/go_quote.gif 
_Ain't that always the way...?

 I read somewhere on this site (before I'd become a real person and signed up) that someone wasn't sure what the voltage to the audio board should be. So I asked at the source, and received this response:_

 

Yeah, you'll have to insert a resistor in the heater curcuit to lower the voltage. Mine was 7.58v and after I installed a 1 ohm - 10 watt resistor, it dropped down to 6.63v. That's much better and much closer to the 6.30v that it should be.


----------



## dcheming

Wow, quite a bit happened since I was last here.

 It's good to have you back Charles. 
	

	
	
		
		

		
			





 Bummer about the computer virus causing so much trouble. Congrats on the new job though. I've also been making plans to put together a nice two channel system since I'll be moving to San Francisco this summer. Hopefully we'll find a place big enough to have a proper setup.

 Those Onix X-Statik look interesting. Open baffle speakers have some serious benefits, but bass depth and weight is usually a problem. So the hybrid approach makes a lot of sense. I've been thinking about something like the GR Research OB-7 kit, but I'd really want to hear one first before spending that much on parts. I'm also considering some Magnepan 1.6s, but again I would need to spend some more time with them first.

 Since you sold your 701s and have some HD600s on the way you'll be able to use a smaller uF cap on the outputs. Have you tried those Solen 51uF caps purely as output caps? Even though Solens aren't really that great I still much prefer them to the ELNA e-caps. Of course your Mundorf e-caps are likely much better than the ELNAs I was using so YMMV.

*Cloth Ears* and *jrhill*, please feel free to add as many pics and impressions/thoughts/ideas to this thread as you want. I'd like this thread to as diverse a source of info as possible. That and I just enjoy seeing other DIYers work.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Wow, quite a bit happened since I was last here.

 It's good to have you back Charles. 
	

	
	
		
		

		
		
	


	




 Bummer about the computer virus causing so much trouble. Congrats on the new job though. I've also been making plans to put together a nice two channel system since I'll be moving to San Francisco this summer. Hopefully we'll find a place big enough to have a proper setup.

 Those Onix X-Statik look interesting. Open baffle speakers have some serious benefits, but bass depth and weight is usually a problem. So the hybrid approach makes a lot of sense. I've been thinking about something like the GR Research OB-7 kit, but I'd really want to hear one first before spending that much on parts. I'm also considering some Magnepan 1.6s, but again I would need to spend some more time with them first.

 Since you sold your 701s and have some HD600s on the way you'll be able to use a smaller uF cap on the outputs. Have you tried those Solen 51uF caps purely as output caps? Even though Solens aren't really that great I still much prefer them to the ELNA e-caps. Of course your Mundorf e-caps are likely much better than the ELNAs I was using so YMMV.

*Cloth Ears* and *jrhill*, please feel free to add as many pics and impressions/thoughts/ideas to this thread as you want. I'd like this thread to as diverse a source of info as possible. That and I just enjoy seeing other DIYers work. 
	

	
	
		
		

		
		
	


	


_

 

Just to get this out of the way first... 
	

	
	
		
		

		
		
	


	










 You're back!! LOL 
	

	
	
		
		

		
		
	


	




 Yeah, the whole PC virus bit sucked as I didn't only lose my own site, but also a client's! But he was fading out his business anyway so it didn't both him too much.

 The X-Statik's are more than just interesting, they are phenomenal! I just got them in yesterday and have had them going non-stop since 3:30pm yesterday. I was just blasting the hell out of them for the first time a few minutes ago with a pipe organ CD. All I can say is WOW! Definately more than my money's worth on these babies!

 I was originally going to get those OB-7 from GR (Danny Richie), which also helped design and DID design the crossovers in the X-Statik. The reason I didn't is because I suck at wood work. I mean, I can do it, but it looks like b u t t when I'm done with it. 

 I can tell you this, don't think about those OB-7's. JUST BUY THEM AND BUILD THEM! Trust me on this. They use 5.25" woofers where mine uses 6.5" woofers, and his uses a ribbon tweeter where mine uses a 1" soft dome. But the major thing is that the OB-7 uses 4 woofers in the sealed enclosure and uses premium caps in the crossover networks. I only have 2 woofers in the sealed enclosure and decent caps. Plus with the OB-7, you can upgrade the drivers for a small fee and have more output, lower distorsion, lower bass and higher power handling. I've had Maggies before and they don't stand a chance against these X-Statik's. The OB-7's are most likely even better!!

 BTW, I have the matching X-Voce center channel coming as well. Just ordered it today.

 Yep, I have the HD600's coming. As far as the amp though, I think I'm going to leave it as is until I make the move to a new chassis, which I am going to do. Then I'll have it to where it's easy to switch out caps, kind of like what you did in your NAD cdp.

 And to the new guys on the block, please do post pics and comments. Every little bit helps!

 Now for just a little bit of off topic eye-candy to motivate Derek!


----------



## Cloth Ears

A question from a "tube-newb" (I'm copy-writing that, hands off...
	

	
	
		
		

		
		
	


	




). Someone mentioned that the 6Z4's out there were all the same (or close to it). A quick search found this




 which looks like the original and this




 which looks rather different. One by RCA and one by Westinghouse. Different, or the same in different shape (leopard changing its spots, maybe?).

 I took some photos (and have a schematic that I'm going to transcribe) from last night - although I think James has already started making changes based on Morgan Jones' Mini amplifier circuit. But I probably won't be able to get to them until the weekend. Definitely work in progress.


----------



## Chops

There's two different types of 6Z4's out there. The ones you pulled up are the American versions. The Chinese version is the one in our amps. Two totally different tube types, however they might be designed the same internally. I guess if you changed out the tube socket, it might work, but don't quote me on that.

 Derek would be the one to ask.


----------



## glitch39

the american 6z4's are tall. so even if you change out the socket, it won't fit the case. the chinese 6z4 is almost like the size of a 12ax7


----------



## Chops

Quote:


  Originally Posted by *glitch39* /img/forum/go_quote.gif 
_the american 6z4's are tall. so even if you change out the socket, it won't fit the case. the chinese 6z4 is almost like the size of a 12ax7_

 

Well, for one thing, the American 6Z4 is an entirely different tube which also happens to be a full size tube. IF, you wanted to convert over to this tube (if you even can), you would have to relocate it anyway and use a new chassis as the 708B chassis has no room for it, let alone it being taller than the chassis.


----------



## Cloth Ears

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Well, for one thing, the American 6Z4 is an entirely different tube which also happens to be a full size tube. IF, you wanted to convert over to this tube (if you even can), you would have to relocate it anyway and use a new chassis as the 708B chassis has no room for it, let alone it being taller than the chassis._

 

Thanks, Chops - pictures can fool you (the RCA looked the same size). 

 I've managed to take the following pictures of the work in progress, but I'd have to leave it to James to explain exactly what's been done. Please excuse the crappy photography...




 James circuit diagram of what's currently been done.




 One picture from above.




 Another picture from above.




 And a picture showing the (temporary) set-up for testing headphone jacks and volume controls.

 Since these were taken on Thursday night he's looking at changing it to be closer to the Morgan James (with feedback) amp posted by Chu Moy (I hope I spelled that right!) somewhere on the WWW. This 708B circuit seems to resemble it in a lot of ways - which probably accounts for why it sounds so good for so little (imagine a $200 Earmax Pro).


----------



## Chops

Quote:


  Originally Posted by *Cloth Ears* /img/forum/go_quote.gif 
_Thanks, Chops - pictures can fool you (the RCA looked the same size). 

 I've managed to take the following pictures of the work in progress, but I'd have to leave it to James to explain exactly what's been done. Please excuse the crappy photography...

 James circuit diagram of what's currently been done.

 One picture from above.

 Another picture from above.

 And a picture showing the (temporary) set-up for testing headphone jacks and volume controls.

 Since these were taken on Thursday night he's looking at changing it to be closer to the Morgan James (with feedback) amp posted by Chu Moy (I hope I spelled that right!) somewhere on the WWW. This 708B circuit seems to resemble it in a lot of ways - which probably accounts for why it sounds so good for so little (imagine a $200 Earmax Pro)._

 


 Sorry for the delay, been busy with work and doing research on my new vehicle (swapped cars with one of my bro's just for the heck of it!). Looking to do lots of mods to it... Imagine that! LOL

 Anyway, you amp looks pretty nice, though I'm not too sure what's up with all the duct tape everywhere. 
	

	
	
		
		

		
		
	


	




 Aussie's electrical tape? 
	

	
	
		
		

		
		
	


	








 BTW, my new cans came in a few days ago and I've had them running the entire time. They have 80 hrs on them so far and are starting to really open up. Man, I forgot how good these 600's sounded!







 And just for fun, here's another shot of my amp with the 51uF Solen's along with the 100+100uF Mundie's.


----------



## Cloth Ears

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Anyway, you amp looks pretty nice, though I'm not too sure what's up with all the duct tape everywhere. 
	

	
	
		
		

		
		
	


	




 Aussie's electrical tape? 
	

	
	
		
		

		
		
	


	







 BTW, my new cans came in a few days ago and I've had them running the entire time. They have 80 hrs on them so far and are starting to really open up. Man, I forgot how good these 600's sounded!_

 

The duct tape is there for 2 reasons. First, electrical safety - all the 240v wires have been covered pending sealing them up later. Second, to hold the caps (and other stuff in place). Eventually it'll come off and be replaced by heat-shrink or proper mounts.

 Still waiting on 2 pairs of 6DJ8's from different places, a GE JAN 5670 and a triplet of Chinese military 6Z4's (to see if we can get any better sounding). And some other Sangamo, Sprague and PIO (Russian) 0.22uF caps for trying out in the 3 places where they're used. And 3 different volume pots, a cheap one, an ALPS blue and a 24 stop stepped attenuator.

 We may have to redo the baord before we finish...

 Pictures and the new circuit when we get closer to finalising it.


----------



## dcheming

So yeah, as Chops said there's the full-size 6Z4 as well as the mini 6Z4 that's used in the XS. I have no idea why they share the same number. As was mentioned there's no way to fit such a large tube inside the case. Also I doubt the power transformer could handle the heater current since it can barely handle the 6X4 I put in, which draws slightly more than the stock mini 6Z4.

 I'm interested to hear your impressions on the three different volume pots. I was considering rigging up a switch setup to compare pots with but never got around to it.

 Chops, what kind of car did you borrow? I've done this before too with cars and bikes of friends. Such a great way to experience new machines for free.


----------



## Cloth Ears

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_So yeah, as Chops said there's the full-size 6Z4 as well as the mini 6Z4 that's used in the XS.

 ...

 I'm interested to hear your impressions on the three different volume pots. I was considering rigging up a switch setup to compare pots with but never got around to it._

 

Well, I know these 6Z4's are definitely the mini ones, as they're coming from a guy in Shanghai and they've got 7 small pins. And hopefully aren't cloudy like the one in my XS.

 My impressions of the pot probably won't be much - I'll let James do that. I mostly worry about what it sounds like when it's being turned up or down. The one we've currently got in there is apparently quite expensive, but it makes a bit of a rushing noise (sort of a combination of a hiss and the sound of the ocean far away) when being turned up or down. Bit it sound quite good otherwise. I wouldn't expect too much of the pots I'm getting - nothing over $8US!!!


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Chops, what kind of car did you borrow? I've done this before too with cars and bikes of friends. Such a great way to experience new machines for free. 
	

	
	
		
		

		
		
	


	


_

 

Hey Derek,

 Well at first, I was just borrowing it, for a day or two. But now, we have officially switched. We swopped my '02 Miata for his '02 Blazer. 

 Just in the past week alone, I have pulled the roof rack, properly reinstalled and realigned the headlights, realigned the hood, shaved all the emblems, washed it and detailed it inside and out as well as the engine compartment, and even managed to burn up the vacuum cleaner while vacuuming it out!

 Near future plans include getting the windows tinted, dropping it 4" in the front and 3" in the rear, full tune-up, new rotor and cap, coil, plugs and wires, all Amsoil full synth oil, manual transmission fluid, rearend fluid, brake & steering fluid, custom chip, headers, hi-flo cat, 3" single exhaust (super quiet as well), upsizing the rear tires from 235/70-15 to 275/60-15 and eventually a posi-rearend and a supercharger.











 And going from this...





 To this...


----------



## nikongod

Did anyone else's 708B come with a linear pot in the volume spot? 

 Mine did.

 I have big plans for this little amp.


----------



## Random Murderer

Quote:


  Originally Posted by *nikongod* /img/forum/go_quote.gif 
_Did anyone else's 708B come with a linear pot in the volume spot? 

 Mine did.

 I have big plans for this little amp._

 

how do you tell?
 mine just says "B100K" on it...


----------



## nikongod

Quote:


  Originally Posted by *Random Murderer* /img/forum/go_quote.gif 
_how do you tell?
 mine just says "B100K" on it..._

 

REALLY weird, mine has a 50k ohm pot. 

 anyways, you measure from the input of the RCA to the input on the amp board. 

 As always observe HV safety.

 With the volume set to min, it should read 100k.

 If you set the knob to the middle of its travel it should be more than 70k for a 100k-ohm AUDIO pot. If it is 50k, its linear.

 Linear pots also have a very non-linear volume scale. They only offer a few degrees of usefull range, with the rest offering little increase in volume for lots of spin...

 This just means that when I have money again I have to get one of the Chinese steppers. maybe i can rig a shunt in the mean-time (yuck)

 edited/added:
 I added a 7.5k ohm shunt resistor between the output and ground. Its better than it was, but still leaves a little to be desired. Its just a band-aid until I can get some cash for a Chinese stepper.


----------



## Random Murderer

Quote:


  Originally Posted by *nikongod* /img/forum/go_quote.gif 
_REALLY weird, mine has a 50k ohm pot. 

 anyways, you measure from the input of the RCA to the input on the amp board. 

 As always observe HV safety.

 With the volume set to min, it should read 100k.

 If you set the knob to the middle of its travel it should be more than 70k for a 100k-ohm AUDIO pot. If it is 50k, its linear.

 Linear pots also have a very non-linear volume scale. They only offer a few degrees of usefull range, with the rest offering little increase in volume for lots of spin...

 This just means that when I have money again I have to get one of the Chinese steppers. maybe i can rig a shunt in the mean-time (yuck)

 edited/added:
 I added a 7.5k ohm shunt resistor between the output and ground. Its better than it was, but still leaves a little to be desired. Its just a band-aid until I can get some cash for a Chinese stepper._

 

why not just get an alps black beauty?


----------



## nikongod

Quote:


  Originally Posted by *Random Murderer* /img/forum/go_quote.gif 
_why not just get an alps black beauty?_

 

from where?

 familygate only has linear rk40's. Michael Percy has audio RK40's but only mono, and by the time he responds to my first email, i could have had mikhail do it 
	

	
	
		
		

		
			





 I may just use a blue velvet. Im on the fence regarding whether I want better channel balance, or more precise control. I suppose the superior channel balance of a stepper could be negated by a flaw in one of my tubes, and an amp with this much gain does better with more control at the lower volume levels.


----------



## dcheming

I just dug up my original stock pot from the parts bin and it's also labeled as B100K and measures at ~100kΩ. At the halfway point it measures ~50kΩ so it is indeed linear.


----------



## hnb2907

Quote:


  Originally Posted by *jrhill* /img/forum/go_quote.gif 
_Hello All,
 In Oz, we have 240v, 50Hz and this gives high volts (B+) of +215 on pcb, heaters of 7.3v ac, and the current thru the input valve (6N3, etc) is about 2.7mA, and 10.5 mA thru the 6N11s.
 ... jh_

 

Hi folks,

 It's a long time since I posted about my mods on here... Not had chance to do anything else with my amp since.

 I just bought a 500VA autotransformer with 240V and 220V taps. Nominally the voltage in our house (UK) is 237V@50Hz, and usually between 231..243V when I check it, depending on the load in our house and time of year.
 500VA is probably overkill, but I've got a couple of other 220V things I can run on it too. Check out Electronic components distributor. Electronic equipment and electrical products from Rapid part 88-1926.

 The output of the autotransformer was bob on 220V with a 239V input this morning. 

 I'm going to take out the heater series resistors that I added for 240V operation, and have a quick play around with the HT power resistors to see how it runs on 220V. If it is ok, using the autotransformer is a load easier than messing around with some of the mods I did before! 
	

	
	
		
		

		
		
	


	




 Also didn't like the idea of 240V feeding the transformer rated at 220V for long periods of time.

 Will post an update if I ever get around to finishing the experiment! 
	

	
	
		
		

		
		
	


	





 Best wishes,
 CC.


----------



## Random Murderer

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_I just dug up my original stock pot from the parts bin and it's also labeled as B100K and measures at ~100kΩ. At the halfway point it measures ~50kΩ so it is indeed linear._

 

well then, looks like my first mod(other than possible tube rolling or a 6x4 mod) will be to fit a new pot. the blue velvet looks like a winner 'til i can find someone that sells a black beauty or an elks pot for a decent price...


----------



## Random Murderer

today i made a trip to ace hardware(it's maybe a 1/2 KM from here)because i needed a brick. yes, one single brick. while i was there i picked up the same toggle switch that derek used and installed it. i also got some nylon standoffs for the transformer, but i'm not doing that mod just yet.
 in order to install the switch, all i did was de-solder the existing push-switch from the pcb and the wires as well and then connected the wires to the new switch. i didn't need to cut the pcb to make any room or anything...
 and while i was tinkering inside the amp, i noticed that this amp had been tube rolled by the previous owner. in it are 2 RCA-Holland 6DJ8 tubes and a GE-JAN 5670.
 i'm hoping to get a 6x4 soon, most likely the westinghouse because of it's size, and also some matsu****a 6922's.
 EDIT: and all these thoughts of tube-rolling come after breaking in these tubes for ~100 hours. they _do_ sound great(though anything would, coming from a cmoy!), maybe i'll hold off...


----------



## Random Murderer

So today i did a bit of resistor work. I added one 2w 511k resistor in parallel with each filter cap as bleeders and replaced the 10Ω headphone output resistors with a pair of hand-matched(by me) 1% 10Ω Claro resistors.
 I had also bought 3 2MΩ resistors(for a total of 1 2MΩ and one 1MΩ for the power supply) only to find that most of the resistance and capacitance values in my amp are different than all of yours...

 while in there, i also removed all of those little spring things that were holding the tubes down and the jumpers that held the springs to the boards. i think it looks better this way, and the tubes definitely didn't need those springs to stay in place... oh, and i replaced the jumper in the right channel path with a lead from one of the resistors i used...
 EDIT: just added the standoffs to the power supply, i don't know if it's affecting temperatures or not, but it definitely looks a heck of a lot better.


----------



## Random Murderer

alright, i ended up disconnecting the leads from the window tube so it just sits there with the led's shining on it...
 today, i received my vintage(~1950) westinghouse-USA 6x4. it's actually a bit taller than the stock tube, and even though i lowered the power supply pcb as low as it will go without touching the case, i still can't put the cover back on...
 suggestions?


----------



## nikongod

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_If you replaced all those resistors why not make it an Optimized Morgan Jones Amplifier so it can work with low impedance cans ?_

 

on a scale of 0 to 15, with 15 being much more than 0, you are 92/7

 the stock 708B does well for low impedance headphones, but why not change it to a really solid design when its guts are all out....

 I posted about the volume knob thing. In an effort to save my thinning wallet, I have not yet replaced the rigged solution. im not even sure that i will. that would put me over budget.

 I changed the cathode resistor value on the input tube to drop the plate voltage into the range necessary for the output stage. I bypassed this resistor with an electrolytic cap, as described in the headwize article.

 I copied the output stage from the optimized MJ article at headwize as closely as my resistors of choice would allow. (rn65 vishay, which are too large. paralleling 2 rn60 gives a better fit....)

 I increased the output caps from 200uf/150V (i think, im not digging in the garbage...) to 330uf/250V because of the output stage, there is still ample basssss.

 I changed the 2K ohm resistor on the power supply board to a 1.8K to get the B+ voltage a LITTLE higher. i should have gotten 1.6K, its still only hitting 210V.

 I will post photos later, im crunked and its late.

 edited:
 Its now early in the morning, and I took some pictures.
 I am no longer feeling the effects of the alcohol, but my camera always is...


----------



## nikongod

I added adjustable feedback the other week.

 This evening I replaced the resistor/cap on the cathode of the input/gain triode with a single RED LED.

 Led cathode bias makes me hard in the pants.


----------



## Random Murderer

Quote:


  Originally Posted by *nikongod* /img/forum/go_quote.gif 
_I added adjustable feedback the other week.

 This evening I replaced the resistor/cap on the cathode of the input/gain triode with a single RED LED.

 Led cathode bias makes me hard in the pants.




_

 

what does the led add or subtract in terms of the sound, rather than the caps/resistors?


----------



## nikongod

The LED on the cathode of the triode rather than the resistor & cap helps to achieve the theoretical ideal of Rk=0ohms across a wider frequency range. This is good because the bass performance will NEVER suffer with this setup, and distortions in general decrease. 

 Scope jockeys will put on a smile 2 miles wide when a $0.01 LED (yes, one American penny when bought in bulk) in that spot outperforms a resistor/cap pair many times more expensive. This is not to say the sound is inherently better, there are a bunch of people who prefer the sound of a resistor/cap.

 I have seen some posts by people who don’t like the sound of an LED with such little DC current going through it. Its about 2.6mA with a red LED. The recommendation I saw was “run the LED at rated current or don’t run one.” I think it’s a littttle extreme, but we all have our sonic preferences.

 The first thing I noticed with this was the very good bass detail. Im not sure that the advantages to the midrange and topend are particularly large, and with the general distortion of the amp being so calm and nice anyways its a change from “a small but pleasant error” to “a smaller and still pleasant error.”


----------



## dcheming

I _finally_ ordered a replacement for the stock 6N3 I've been running this whole time. Nothing fancy for now, merely a JAN-5670W. I've been putting this off for so long I just wanted to get something, anything. 

 I also ended up rewiring the entire amp a while back with 20 and 18 awg PVC Neotech UP-OCC Mono-Crystal wire. This came about after a friend and I did a comparison between one of my Belden CAT6 IC's, my Belden 9258 co-ax, and a Belden Teflon co-ax IC. I didn't realize how much the CAT6 was getting in the way of the upper frequencies and air. It was slightly muddy compared to the two co-ax cables. So that motivated me to change it all out for something else (and to also order a 5670). To be honest the change wasn't as big as I was expecting, but the sound is a bit more natural so it was worth the trouble.







 BTW, all the extra slack in the gray braid is to allow for future changes in output caps.


----------



## NoPants

wow that's some good-looking wiring, do you have any thoughs as to what you might put in to replace your exisiting caps


----------



## dcheming

Quote:


  Originally Posted by *NoPants* /img/forum/go_quote.gif 
_wow that's some good-looking wiring, do you have any thoughs as to what you might put in to replace your exisiting caps_

 

Thanks, since I have higher impedance cans I was thinking about just going with a pair of 47uF Mundorf M-Caps. Not sure if I actually will though since I'm going to build a Stacker II Hybrid as soon as boards are readily available and I'd like to get all nice parts for it instead of doing more things to my XS.


----------



## sonq

I was thinking if all this modding is worth the time and cost of components - may add up to be almost same or even more than the cost you've saved by not buying from Woo or RSA. 

 Looking at the incredible workmanship in your photo, you must find much pleasure & satisfaction in modding.


----------



## nikongod

Quote:


  Originally Posted by *sonq* /img/forum/go_quote.gif 
_I was thinking if all this modding is worth the time and cost of components - may add up to be almost same or even more than the cost you've saved by not buying from Woo or RSA._

 

Make a parts list, and see what the upgrade parts cost is to build an amp like this using parts of the same "grade" as is found in a Woo, singlepower, mccalister, decaware, or RSA amp. 

 According to the "mot" rules, DIY'ers are not allowed to assign a price to hourly labor, and must sell their work purely at parts cost. If you would like to ignore that, the labor rate is $25/hour based on what I guess a solder tech who can probably solder better than you gets paid. If it takes you 2 hours to unstuff and repopulate this board you have it bad or are likely P2P'ing the amp which is not required if you use parts of similar quality as mentioned above, and should only account for $50.

 Now then we have the finished product, with a base price of $120 which is about the average used price for the amp , the upgrade parts costing about $75, and $50 of labor (which we should not even count, but I am initiating a premptive strike.) This is just a hair under $250! 

 As far as SQ is concerned, this is a very subjective matter. Try it against something from RSA at the $250 price point used LOL. Hornet M FTW! Im not sure if any of the woo amps ever sell in the $250 range, I dont think they do.


----------



## glitch39

The 708B is a true sleeper. Earlier in this thread, it was noted that this is almost a clone of the MJ amp, which is very highly regarded

 Modded, the SQ rivals the 336 and beats the figaro, both within striking price range

 Not the same league and performance level as my woo 3+, however


----------



## Stephen_Ri

For several months now I've had to apply leftward pressure to the the end of my headphone cord while it is plugged into the 708B in order to hear the left channel. The problem is not with my cord. Recently, the left channel has stopped working entirely. Any suggestions? How difficult is replacing the jack input, and what reasonable options are avail?


----------



## Random Murderer

Quote:


  Originally Posted by *Stephen_Ri* /img/forum/go_quote.gif 
_For several months now I've had to apply leftward pressure to the the end of my headphone cord while it is plugged into the 708B in order to hear the left channel. The problem is not with my cord. Recently, the left channel has stopped working entirely. Any suggestions? How difficult is replacing the jack input, and what reasonable options are avail?_

 

it might not be the jack itself, but the front aluminium plate. it seems to 'drift' forward a bit and cause the plug to not be able to be fully inserted into the jack. try fixing that first.
 the jack itself is not hard to replace, the real challenge is either finding a jack with the same pinout or an open jack that's long enough to fit through the aluminium panel.


----------



## Random Murderer

i just fell in love with my 708b all over again. i replaced the crappy wima film caps today(mine weren't the mentioned 400v 220nF but 630v 220nF, they must've run out of 400v) with some cheapy Evox 400v 220nF($.25 a piece) caps i got at a local electronics store, and even THESE are a ton better than the wimas. i highly recommend that everyone do this if you haven't already, like i said, even my cheapo caps are better than the wimas they used. the evox caps sound a bit more airy and natural, and i actually found that they calmed down the outrageous bass a bit and extended the high end a bit more.

 and just to reiterate:
 [size=medium]SPEND A DOLLAR AND REPLACE THE WIMA CAPS!!![/size]


----------



## nikongod

This evenings festivities involved replacing the plate "resistors of sound quality" on the input/gain tubes with constant current sources.

 For quite some time I have been using the stock 33k ohm resistors here with a LED cathode bias and everything was pretty good, but why not mess around with perfection? In my amp, the 33k resistors put about 105-110V on the plate of the input tube which agrees with the voltage in the headwize MJ article. As I have been researching the 6DJ8 I have come to realize that it is a fairly dinky bit of metal and decided that running the output tubes at rated power (per section!) may not be the best idea anyone has had in a while. 

 I used 10m45 CCS made a few calculations for the resistor value and got VERY close to my desired result.

 I took this opportunity to reduce the current in the gain stage by about 10% (from 2.2ma to 2ma) which reduced the plate voltage on the gain tube a few volts, and since the plate voltage is "echoed" through the output stage the current in the output stage dropped a little too! My choice to reduce current in the gain stage comes with the fear of less linearity, but my thought process says that the CCS loaded tube at a slightly less linear operating point is MORE linear than the resistor loaded gain tube.


----------



## Cloth Ears

Finally, an update to my little XS headphone amp (previously mentioned in despatches here in this post).
 Looks the same on the outside, right?





 But there's been a few changes on the inside. PS board has been mounted edgewise (and runs cooler as a result). Bleeder resistors added to the PS caps. New wiring. New headphone jack. New RCA's. ALPS volume control ($8!!). 3 new output tubes (PS are still original). Blue output caps are probably Philips. Other caps are EROMAK (gold), plain-jane Mundorf (white - very neutral, we like them), Russian PIO (green), and I'm blowed if I can remember what the black cap with the red ends is (maybe a Wima?).




 And here's an overhead shot:




 And a close-up of the changes at the front:




 And the crossfeed circuit (which we have to mount outboard, as it just doesn't seem to work properly when we tried to mount it in the chassis):





James did all the work, I just scrounged around for some of the parts. He may be able to answer any of the complex questions this might arise...

 Sound, well it's still pretty fat at the bottom end, but much better controlled and very lacking in distortion. Top end is much better then previous. The mids were always quite good and we have only minor gains there. Volume control is about 12 o'clock for 'quite loud now, as against 8 o'clock previously.

 About $100 spent on parts, of which about $50 made it into the chassis. James spent countless hours playing around with the set-up and had numerous insights into how he's going to build his own headamp.


----------



## Random Murderer

I'm thinking of adding in another 6DJ8 as a buffer, using one triode for each channel.
 Schematic as follows:








 Ideas? Comments? Suggestions?


----------



## fraserh

Hi! I've been following this thread as a lurker for some time, and went to the trouble of importing a xiang sheng 708B and all the parts needed for modding from hificollective- thing is it took so long to get here that i can't be bothered anymore and i have another pre! I've put everything on eBay as one auction so if anyone's interested in doing this project then have a look, it's going cheap at the moment. 

 Fraser


----------



## dcheming

nikongod,

 So how did the CCS affect the SQ? In the end which way do you prefer: res/cap bypass, LED, or CCS? Would you mind posting some pics of this latest addition when you get a chance.


 Cloth Ears,

 What size uF output caps are you using? I'm wondering how much deep bass I've been missing with my paltry 7.6 uF caps. I'm hesitant to go back to electrolytic and big films are expensive so I've just left em alone for the time being. How do you feel about the SQ and presentation of your modded amp now that it's been a while?


 Random Murderer,

 Did you ever play around with adding a buffer stage? I've often wondered how well balanced the push/pull output stage of this amp is in the first place. I think focusing on that might help the "drive" of this amp. Now that I have my o-scope back I want to take a look and see how bad things are in this amp. Optimizing the component values will be my next step with this project.


----------



## nikongod

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_nikongod,

 So how did the CCS affect the SQ? In the end which way do you prefer: res/cap bypass, LED, or CCS? Would you mind posting some pics of this latest addition when you get a chance._

 

The CCS&led in the gain stage improved detail overall, and (sadly in the opinion of some im sure...) made the amp less "tooby". Its still not the ultra-hard sound of a "lots of global feedback" monster, but its not the typically tubey sound the amp once was.

 For the gain stage there is NO question, I prefer it with CCS plate-loading & LED bias. For the output stage, I think things are still "up in the air" as far as what to do. I originally used a 2V LED to bias but the sound did not improve as much as I had hoped. I replaced the 2V LED with 1.7V which improved the sound, but im now running AT the rated power of the tubes.

 My gut-instinct is that the 2V LED is not as good for this purpose as the 1.7V one. I cant test, but I assume that the 2V led has a higher AC impedance than the 1.7V, which could actually over-ride the whole reason I used it! The solution would be to use 2*1.7V LEDs for 3.4V OR rebuild the gain-stage (again) to get a different output voltage which allows me to use 1*1.7V LED. There is no way a 3.4V bias voltage will allow enough current with a 6dj8, so Im going to rebuild the gain-stage when I become motivated. Motivating me is the hardest part... 

 OOH, I re-adjusted the plate resistors in the output stage to HIGHER than what the "optimization" calculations would sugjest on the hope that output impedance comes down a few ohms. I dont need all the current or voltage that it can swing with the lower values.

  Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Did you ever play around with adding a buffer stage?_

 

The output of the amp is a buffer.
  Quote:


 I've often wondered how well balanced the push/pull output stage of this amp is in the first place. I think focusing on that might help the "drive" of this amp. Now that I have my o-scope back I want to take a look and see how bad things are in this amp. Optimizing the component values will be my next step with this project. 
 

Check out the head-wize MorganJones article.
 Regal mentioned the possibility of "just" dropping the values in from that, which was the inspiration and source of the values i used. It requires some alterations to the gain stage cathode resistor value, but its totally doable.


----------



## Random Murderer

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Random Murderer,

 Did you ever play around with adding a buffer stage? I've often wondered how well balanced the push/pull output stage of this amp is in the first place. I think focusing on that might help the "drive" of this amp. Now that I have my o-scope back I want to take a look and see how bad things are in this amp. Optimizing the component values will be my next step with this project._

 

the amp already has a buffer stage, but i was contemplating putting in another buffer for the hell of it.
 never got around to it. instead, i decided i'm happy with the amp the way it is(6x4, 2x 6dj8, jan 5670w, led biasing, full resistor replacement, custom crossfeed on the phone output), but i may cap-roll it.


----------



## satshanti

I'll revive this thread and see how many of its previous contributors are still out there. 
	

	
	
		
		

		
		
	


	




 About 6 months ago I ordered a Xiang Sheng 708B on eBay and have since then upgraded the power cord and its 3 main tubes to military grade Russian ones, which have all improved the sound substantially. I had two other DIY tube amps, but sold them, because the XS was so much better.

 Now, I was looking into options for a further upgrade, and in stead of looking for a different amp, I remembered this thread and have read it completely. I have now decided I will give these mods a try. To that effect, I first needed to refresh my soldering skills. I haven't touched an iron for 25 years, the last time I was still a teenager, so I ordered one on Ebay, got it today and will start practicing a bit before I do anything serious with it. 

 I didn't want to ask any really silly newbie questions, so I already did a lot of research on my own on the internet, but I don't know a whole lot (actually very little) about electronics and circuits. 
	

	
	
		
		

		
		
	


	




 I have looked into the subject of audiophile components, like capacitors and resistors, but before I decide on my budget and place an order, I have some questions, and I was hoping some of you are willing to help me out with the questions I didn't manage to find an answer to online:

*1)* I have read somewhere that some components can be damaged during the soldering, and I also read that it's better to do it fast and hot, than less hot and too slow. I'm afraid that if I order some pretty costly components I inadvertedly damage them through my lack of experience in soldering. Can you give me some tips, or things I particularly have to observe?

*2)* I have read that the two small input capacitors can be removed altogether if the source output DC is low enough. Now, I've read a bit about the dangers of DC offset, but how do I measure this? I have also obtained a multimeter on eBay, so I'll be able to measure, but how and where, and should all equipment be connected and on, or even playing? I have no idea.

*3)* Having read in this thread that some of you discovered that certain component values were different from the ones listed at the start of this thread, I decided to check that and discovered the following discrepancies:





> main filter bypass caps are 22 uF in stead of 33 uF
> the coupling and preamp out caps are rated at 250V in stead of 400V
> The power supply resistors are placed differently and have completely different values: R1 has 400R in stead of 1K and R2 has 15K in stead of 2K, which leads me to my next question


Are these values different because of the 220V power in stead of the 110V? Or could it be that the manufacturer changed these values to give the tubes a lower voltage, as some of you have been trying to do?

 I know some of you have put in higher value resistors here in order to lower the B+ voltage. What about these totally different values I have in my model? And some of you also mentioned taking the additional measure of inserting a very low value resistor in front of one of the tubes to lower the tube voltage. Where is this tube voltage measured, and where would I need to insert this resistor? I have 233V power myself, If I would leave this as is, would this too high voltage influence the sound, or just the life expectancy of the tubes?

 I know these are a lot of questions, but I'll be grateful if anybody can help me along a bit.
 I'm kind of feeling like a kid again and can hardly wait to get started.


----------



## Random Murderer

1) capacitors and resistors are pretty hardy in regards to soldering. the sensitive components are usually ICs and/or transistors.
 2) to measure DC offset, turn on your source and amp, let them warm up, and play sound/music through them. set your multimeter to DC and put the positive on the input and the negative on the ground. ideally, the value will be 0, but it is rarely so, so we use input caps to filter out the DC.
 3) your different values could be a result of the 220V mains, but it's likely you just have a different revision. my amp is the 110V version and my values are different from everyone else.


----------



## zer061zer0

Well I am still here and I am thinking of suing the head amp as a preamp.
 have gotten some holand musicaps for the job, let's just see when's the time to do so.


----------



## Random Murderer

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_Well I am still here and I am thinking of suing the head amp as a preamp.
 have gotten some holand musicaps for the job, let's just see when's the time to do so._

 

i have mine running as an amp and a preamp. as a preamp, it made my crappy speakers sound worlds better.


----------



## Stephen_Ri

Quote:


  Originally Posted by *Random Murderer* /img/forum/go_quote.gif 
_the jack itself is not hard to replace, the real challenge is either finding a jack with the same pinout or an open jack that's long enough to fit through the aluminium panel._

 

Aren't the only relevant pins R, L, and G? The stock jack has several others, but they seem to serve only to secure the jack to the circuit board. I ask because I soldered a jack I had around from an ancient sony CD player, and now I'm not getting amplified sound from either channel.


----------



## zer061zer0

Quote:


  Originally Posted by *Stephen_Ri* /img/forum/go_quote.gif 
_Aren't the only relevant pins R, L, and G? The stock jack has several others, but they seem to serve only to secure the jack to the circuit board. I ask because I soldered a jack I had around from an ancient sony CD player, and now I'm not getting amplified sound from either channel._

 

I might have sething for you from my stach of spares. Let me see what i can find during my "operation" this weekend to put in the musicaps for the preamp out.

 Random,

 Yeah i have experience the same thing too with some vishay output caps, and i going for the musicaps now to see what mmore i can achieve. currently it adds depth to my t-amp, and my tube amp as a preamp. this isverify by my gf who happened to have better ears.

 Can i check if anyone knows what am i adjusting if i were to alter the values for the input caps.


----------



## zer061zer0

I have changed in the musicaps however now I am experiencing some hums on the left side when the volume knob is at zero. As I turn up the volume knob, the hum gets louder even when nothing is playing. Any help anyone?


----------



## zer061zer0

Anyway I have managed to solve the problem. It was a faulty tube, the 5670.
 I am replacingit with a tungsol 2c51 which works very well.


----------



## satshanti

After having owned the Xiang Sheng 708B for about a year and after having read this thread, I decided to have a go at modding it. I had already done the easy parts almost at the start, exchanging the power cord for a Lapp Ölflex, and rolling the signal tubes to military grade Russian ones. Until I did my final mods I was actually really happy with the amp, and at that time I couldn't imagine how it could possibly sound better (little did I know ! 
	

	
	
		
		

		
			





).

 I must first say that I had not had a soldering iron in my hands for 25 years up until two months ago, when I slowly started to recable some of my headphones, just to practice and warm up for the amp project. I read this whole thread and made a plan of what I would want to change and how much money to spend. This took me a while, but then I placed my order at PartsConnection, where I ordered most parts, with the exception of the hookup wire and coupling caps, which I ordered at the Hong Kong-based DIY Hifi Supply.

 I removed or replaced everything except all wiring connected to the transformer, the mains switch, headphone socket, two main filter caps to which I added a bleeder resistor. I removed the display tube with LEDs, the input caps and the complete pre-amp stage to make place for the new bigger caps.

 This is what I used as replacements: 

Black Gate VK 150uF output caps: I decided not to use film caps here, because they would either be too big and expensive or of too low value for my taste. I like my bass clean and I didn't want to introduce any phase shifts into the audible spectrum.

Solen 22uF MKP filter bypass caps: I have read here that these are not that convincing in the signal path, but are excellent in a power supply. They barely fitted in the space vacated by the showcase tube.

Obbligato Premium Gold 0.22uF coupling caps: They should provide excellent value for money. See this review and comparison. Can be had on DIY Hifi Supply website, but they also sell through the auction site.

 I replaced the potmeter with an Audio Note that is supposedly even a little better than the Alps Blue. I replaced the diodes with Vishay's. I replaced almost all resistors with the exception of the two large ones at the main filter caps. I used the better ones for the crucial locations: Caddock MK132 at the input, Mills MRA-5 wirewounds at the output, with the rest a mixture of Kiwame, PRP and Takman. I replaced the RCA connectors, and used Wonder Solder to fix everything in place. For the signal wire I used solid high purity soft-annealed silver wire from Hong Kong. 

 While I was at it, I managed to auction a pair of Amperex Bugle Boys, which I used to replace the pair of Russian tubes. I thought, now that I was going to burn in for hundreds of hours for the Black Gates, I might as well get some better tubes as well. The only thing that's missing is some Bendix or Tung Sol 2C51, but that will come. I have auctioned a cheap RCA 5670 in the mean time, which should also be an improvement over the Russian one.

 Here are the before and after pics:









 I was a little afraid I would inadvertedly destroy my valuable amp, but it worked out fine in the end. I forgot to ground the pot, which created a hum, but besides that it was great fun and very rewarding. I started to burn her in, and couldn't resist listening to it after about 20 hours. If I wouldn't have had previous experience with burn-in, I might have wept at that point. It sounded awful. I did the same yesterday at 40 hours and it already sounds much better, not as smooth and coherent as it was, but with much more detail and musicality. I'm sure the rough edges will disappear during the rest of the burn in. I'll keep you posted.


----------



## Arlekiin

Nice 
	

	
	
		
		

		
		
	


	




 I am eyeing one XS aswell and if everything favours me ill be getting one in a week or so. Now I am sure that I want to mod aswell. Probably should start with caps and tubes. Does anyone know if there is a place in London somewhere where I could buy electronics parts? Yeah sure I could order them but it would be comfortable to go and get them quick. 
 Hmm as I posess full HD video camera I should probably make a video log which shows the modding process. I could combine my passion for tinkering and soldering with my proffessional fetish for design and video production 
	

	
	
		
		

		
		
	


	




. But hey who knows how it goes. 
 So if anyone can point me to the right direction as from where can I get replacement tubes and stuff..

 Regards,


----------



## zer061zer0

My humming problem seems to be back and tracng it seems to show that the tubes are not the culprit. 

 Could it be the tranny? Anyone any advice?


----------



## Random Murderer

could be multiple things.
 it _could_ be the transformer, but since the humming is intermittent i would bet against it. my bet is that the humming isn't even coming from the amp, rather something around it or the mains circuit it's on.
 do you have any magnetic or wireless devices in close proximity to your amp? what else is on that particular circuit? something like a fan or a tv can cause interference.
 also make sure that your signal cables(rca or whatever they may be) aren't getting interference from anything, either.


----------



## Chops

Once again reviving an old thread that shouldn't ever die...
   
  First off, MERRY CHRISTMAS!!!
   
  It's been a very long time since I've visited this thread (or the site really). There's phases I go through. One minute I'm into headphones, the next I'm into cars, the next I'm into my 2-ch analog set-up, then I'm into car audio, etc, etc... It's a very viscous cycle and usually ends up costing me many hard earned $$$ in the process. A lot has changed since I was last here, and I do mean a lot. I have moved, I have a new car, I have all new stereo equipment and speakers, I have a bunch of new kitty cats, and most importantly, I have a new, wonderful girlfriend that I've been with for a year and a half now! She's very techy and nerdy like me, so she easily copes with my many expensive obsessions.
   
  Anyway, the reason for getting this thread back up and running is that I have had my 708B sitting collecting dust for the past 2+ years, totally unused at my parents' house. I finally decided to go get it and bring it home with me. My old Technics SU-8099 integrated amp that I have been using for my dedicated 2-ch analog system died recently. I've been using my little KingRex Pre-amp in the meantime which sounds great but lacks many inputs. For the amplification end of the system, I revived an old Baldwin electronic organ tube amp that's been modified to run in stereo. I recapped it and rolled all the tubes in it and this sucker sounds better than any of the amps I've had in the past, running a quad of coin base Russian 6L6's in P-P. Since I have three turntables in my system, and no way to play them, I just bought an Accuphase C-200 preamp that I'm going to use. Being that it's 30 years old, I'm going to start recapping it soon once I get a parts list worked up. It's a killer of a preamp that still pulls a premium of close to $1k today, and it sold new for $1,800 30 years ago! It also surpasses many new preamps of today costing upwards of $5k!!
   
  My point to this whole post is that the planning of recapping the Accuphase made me think about the Xiang Sheng 708B and all the mods and recapping I did with it, which is why I decided to go pick it up from my parents house and try it out again. I brought it home just a few days ago. I dusted it inside and out, did a visual check to make sure everything was still okay with it, reseated all the tubes a couple times, plugged my old iPhone 3G that is now reduced to an iPod Touch as I have a new iPhone 4. I plugged the power cord into it, plugged in my old and trusted Sony MDR7506 cans and fired it up. Once the tubes got up to temp I could smell the smell of dust cooking. LOL  As expected, the amp still works flawlessly and sounds excellent. This morning I just plugged in my newly acquired AT ATH-A700 cans which have only 50 hours or so on them. These cans work well with the amp and sound more refined than the Sony's. They also offer up a fuller, more forward midrange. Like someone else said a few posts back, I fell in love with my 708B all over again! And another plus, my girlfriend likes it a lot too. She thinks it's cool that it's a tube amp and never heard of using a dedicated amp for headphones! Haha

   
  I hope some of you guys are still around with these amps. If you are, post up a few new shots of your Xiang Sheng 708B and a small list of the mods done!
   
  A few recent pics of mine taken with a camera app on my iPhone 4...


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## Chops

Nobody?!...


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## zer061zer0

Well, I am still around. My xiangsheng have been restore to its original state except for all the changed parts. Was working in China in the last 2years plus and managed to locate the manufacturer and restore it back to its original look with the tube in front.
   
  Since then I have left it at my mum's place, will be getting it back for some good old fun once I settler into my new place.


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## Chops

Quote: 





zer061zer0 said:


> Well, I am still around. My xiangsheng have been restore to its original state except for all the changed parts. Was working in China in the last 2years plus and managed to locate the manufacturer and restore it back to its original look with the tube in front.
> 
> Since then I have left it at my mum's place, will be getting it back for some good old fun once I settler into my new place.


 


  Just curious as to why you restored it to its original state?...


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## danzal

I've just bought one of these, but the gain is crazy high. A few degrees and it becomes too loud to listen to. What can I do to decrease the gain? I plan on fitting a different volume pot (Alps blue), so is there anything I can do there to improve things, such as buy a different rated pot (I don't know the value of the stock potentiometer), or perhaps bung in a couple of different rating resistors (if so which ones should I change)?
   
  Also, can anyone recommend some cheapish but decent caps that I could consider using for a recap (as per the OP)? I don't really want to spend ~£10 per capacitor at this stage! I've already added a pair of bleeder resistors over the filter caps, and will look at some other resistors for the rest of the amp too.
   
  Finally, the 'show valve' in the window - what needs to be done in order to remove this? Is it as simple as simple de-soldering it, or does anything need putting in its place (i.e. will the amp work witout it and without any additional components?


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## danzal

Nobody?
   
  I've gone back to my trusty Shanling PH-100 for the time being, and will work on the 708B as and when I can.


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## adrianjones

Hi all,
   
  In answer to the above, yes the vol pot greatly affects gain and is pretty poor.
   
  My 708B stopped working a month or so ago.  I noticed that the output phono connectors were badly corroded! (gold plated?)  When I opened it up, the 6z4 had dropped through.  2 of the soldered terminals were broken.  I just re-soldered them, removed the wire tube clips(the cause), and fitted quality phonos.  All working again!
   
  Upon reading this brilliant thread, I was inspired!  
   
  Here are the mods that I have made so far.
   
  1-Changed the 6n3 for a russian 6n3p-  more gain, clarity, detail.
  2-Changed input wiring to ofc/ silver plated- again, more clarity, detail, much improved treble.
  3-Changed 2 rear valves to russian 6n23p- at first very bright, but after 2 hours running became very smooth, with typical old school valve sound.  Fab mid range and treble.
  4-Changed vol pot to Audio Note(100k), ofc/ silver wired output, replaced output resistors with Kiwame.  Now sounds very smooth, detailed, holographic.  Treble is all there, but clean.  There was some distortion in mids/highs prior to this part of the upgrade, now totally gone.  There was a big drop in gain with the Audio Note pot, but now more realistic.  The old pot only hit about 8 o'clock to give high volume.  I kept the original knob by using next size up drill bit to enlarge the hole size.  Had to file the front panel hole to fit.  It works great and all looks original.
  The amp just sounds amazing.  I am using a high end Alchemist DAC with Audioquest Diamond and a pair of stunning Beyer DT990 600ohm.  Each upgrade has been very easy to hear.  I am looking at changing the output caps next.


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## danzal

I'm looking at buying some caps for my 708B, namely some to replace the Wimas (C104,C204), and I've found these:
   
  http://www.ebay.co.uk/itm/Russian-PIO-Capacitor-K40Y-9-0-22-uF-400V-1-pc-or-more-/220744543407?pt=Vintage_Electronics_R2&hash=item3365666caf
   
  Would these work okay? I understand that they are PIO and not metalised mylar as per the Wimas, but not sure whether they will be good to use?


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## vrao81

Can anyone confirm that the values for R107 and 207 are 1/2W 1kohm resistors and the headphone output resistors are 10 ohm 2W?
  On page 1 the input resistors are shown as 1/2 and 1 watt (don't know which one is correct), and page 1 also shows 2 W and 1 W for the output resistors. I just want to make sure I have the right values before I order.


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## danzal

Just finished modding my 708B. Did a recap and swapped all the resistors for Kiwame/Vishay Dralorics etc.
   
  I have two problems with it now though, which is annoying. The first issue is I bought a cheapo version of a Neutrik locking headphone socket, and it's crap. The socket allows the 6.35mm plug to wobble about and it loses connection. The second issue is that I have a low frequency humming on the right channel. It's not volume dependant, and starts almost as soon as the amp is powered on. It's intrusively loud, so I need to have a fiddle to see what the issue might be.
   
  I removed the preamp output caps (to make room on the PCB) and resistors R9 and R10, but I'm pretty sure that's nothing to do with the hum as it's only coming through one channel.
   
  @ vrao81 - all the resistors you mention in your post I used 2W versions of in my build.


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## danzal

Now fixed the hum - was a severed track on the PCB which caused a ground issue with one of the valves (as far as I can see). Sounding quite fantastic so far...


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## zer061zer0

Good to see that this thread is still around after all these years. I still love my XS. It's a gift during my student's days and it still brings back some good memories. 

I suggest trying the cap change. It gives the greatest benefit I would say. And maybe also the output resistor to a carbon film one. 

Have fun.


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## pulkass

I have two. One was tweaked by my hi fi guru lampizator
lampizator.eu/AMPLIFIERS/PRE/Mini%20pre/Subminiature_pre.html


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## Chops

pulkass said:


> I have two. One was tweaked by my hi fi guru lampizator
> lampizator.eu/AMPLIFIERS/PRE/Mini%20pre/Subminiature_pre.html


 
  
 That really has nothing to do with the 708B as the only things that remained was the chassis and power supply section. All of the important parts that make it an amplifier was completely gone. All you have there is a preamp, that's it. It's pretty much pointless in this thread.
  
  
  
 And on that note, here's my old 708B amp that I just pulled out of mothballs today. Hooked it up, fired it up and... Still works! Still sounds pretty darn good as well, even with these rather demanding headphones of mine.


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## TinyCappo

So picked one of these up very cheaply and I'm looking to have a play. It looks stock internally but as you can imagine with some of the original posts being from 2007 we are well into necroposting territory. Frankly I'm amazed some images that are still linked work. 
  
 Are any of the modders still about?


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## dcheming

tinycappo said:


> So picked one of these up very cheaply and I'm looking to have a play. It looks stock internally but as you can imagine with some of the original posts being from 2007 we are well into necroposting territory. Frankly I'm amazed some images that are still linked work.
> 
> Are any of the modders still about?


 
  
 I still have all the original pictures on a CD-ROM in storage somewhere. Someday I'd like to re-upload them to this thread as attachments instead of hosted images.


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## dcheming

Today I actually found the CD that had all the old pics on it and I'm beginning the long process of adding them to my gallery and editing in new links. Even though this is a legacy thread, having it back to 100% will help me sleep a little better at night, lol.


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