# I need some tube advice. [Specifically on 12ax7/5751 and 6080/6as7]



## philodox

Hi all,

 I need some advice on tubes for an amp that I will be having built soon. I will need two 12ax7 or 5751 and four 6080 or 6as7. I was able to find some good information [see Life with 12AX7s (5751s) - Part 3] about 12ax7/5751 selection, but haven't found too much on the 6080/6as7 other than tuberollers thread on rolling the Wheatfield HA-2. I don't know much about these tubes, but understand that the 6080 is a military variant of the 6as7? Those 6as7's sure look nice though. 
	

	
	
		
		

		
			





 I think I would like to stick with the 5751's over the 12ax7 as they are 70% gain and have lower noise.

 My short list right now for each tube is...

 5751 [12ax7 variant]
 1. Sylvania gold brand black plate triple mica or government contract gold labels
 2. GE 5751-WA black plate triple mica
 3. Sylvania grey plate gold pin triple mica
 4. Sovtek 5751, gold pins [don't know much about it, but parts connexion stocks them new]

 6080 / 6as7
 1. Jan RCA 6080WC
 2. Sovtek 6AS7G
 3. Sylvania gold brand 6080 [parts connexion has 4 NOS, and I think this might be a winner]
 4. GE 6080 [they also have 4 of these NOS]

 Can anyone offer any further recommendations or confirmations on what I have found so far?


----------



## Ori

Most internet smarties will tell you to go for a 3-mica since their construction is to mil std and are supposedly less microphonic. Similar myth about the black plate. It can radiate more heat, but is it a real issue with small-signal tubes?
 The GE "5-star" 2-mica grey-plate 5751 outperforms the 3-mica by a good margin, based on listening tests in a "super-PAS3" line stage. I also agree very much with the lower gain in a line level amp. Most amps apply negative feedback, so more gain means more NFB. The 5751 offers a better solution, by lowering gain and noise a little bit over a standard 12AX7. It's the right tube for this application.

 The 6AS7G/6080 is a big power tube, optimized for voltage regulation. As a result, it has low gain and low output impedance, making it very suitable for OTL amps. If that's what you're planning, then a word of caution.
 Many OTL amps stretch the limits of this tube. The Russian variants with the ceramic base would last longer than NOS and the sonic differences are minimal for a low-gain stage.
 On the other hand, there are plenty of NOS 6AS7 tubes for a few dollars
 a piece, so perhaps tube life is not such a major issue.

 I would save my money for quality copper foil or teflon caps rather than spend it on "gold brand" tubes.


----------



## aerius

I'm not a fan of the 12at/au/ax7 family of tubes. I find that they tend to impose their sound on the rest of the circuit instead of just getting out of the way and simply amplifying the signal and driving the output tubes. 

 I had a test amp which I built up to test input tubes, and with the 12a*7 tubes in place, I could swap output tubes at will and it would always sound like the 12a*7 tube. With a 6SN7 on the input I could more easily distinguish the differences between output tubes, the input stage got out of the way and stopped colouring everything. With the ECC40, it was even more transparent.

 Since sockets for the ECC40 are damn near impossible to find, I'd go with the 12SN7, 6SN7, or 6CG7 as the input tube. Not as much gain as the 12AX7, but way better sounding IMO.


----------



## philodox

Ori - Thanks for the info on those GE tubes, good to know. 
	

	
	
		
		

		
		
	


	




 The amp is an OTL design, and thanks for the tips on those tubes as well... I think I will go with the Sovteks to start as they are cheap, readily available, and atma-sphere uses them so they must not suck. 
	

	
	
		
		

		
		
	


	




 -----

 Hey Eaphan,

 That is why I've been thinking about the 5751's. They have 70% of the gain and a lot less noise [check out that link from my first post]. I'm hoping that will give a similar effect as your ECC40's without having to make changes to the circuit or tube sockets.

 - Jay


----------



## aerius

Quote:


  Originally Posted by *philodox* 
_That is why I've been thinking about the 5751's. They have 70% of the gain and a lot less noise [check out that link from my first post]. I'm hoping that will give a similar effect as your ECC40's without having to make changes to the circuit or tube sockets._

 

It's still in the same _family_ of tubes, and I did get to try one out on my test amp (Raytheon tube). Yes it has less gain & noise than the 12ax7 (noise doesn't matter that much with the K340), but the general sound is the same. Has better midrange and it's warmer & smoother, but it still puts its sound signature over every 6sn7 output tube I've tried, it doesn't get completely out of the way. Sound quality wise, I'd say it's about the same as a mid-level 6/12sn7 or 6cg7. The ECC40 or the top level 6sn7's (Sylvania, RCA) I've tried are still noticeably better even in my simple test rig. Got a bunch of tubes off the surplus store and just ran them through one by one.


----------



## Ori

The 12A?7 and similar suffer from common design practices. They were developed as low-cost solutions for consumer applications and the "recommended" circuits were biased for economy, not supreme performance. Sadly, many current designs (including very expensive preamps!) follow the same principles and compromise performance.
 A 12AX7 biased at 1 mA is quite a poor performer when it comes to dynamic range in a driver stage. That's the case with driving the grid of a 6AS7. That is the reason why many "new" amps drive a DHT with a power pentode. A bit dumb for sonic reasons IMO, but much prefered to anemic driver stage.
 Unless you have a distortion analyzer to find the sweet spot, my suggestion is that you bias your driver at the highest level possible, within the allowable power dissipation of the tube.
 In that regard, the black plates might have a slight advantage. Still, a 5751 at a low plate voltage (as in OTL amps) can run much more than 1 mA safely. You might find that other tubes in the family are more appropriate, if you don't really need much gain. I've had excellent results with GB-5814A (a 12AU7 variant) in line level applications, biased at 5 mA. While I said to save your money when it comes to "fancy" tubes, this particular one (the 2-mica variant!) delivers outstanding sonics in driver applications.
 But that might be a whole new project...


----------



## Blooze

Quote:


  Originally Posted by *aerius* 
_Since sockets for the ECC40 are damn near impossible to find, I'd go with the 12SN7, 6SN7, or 6CG7 as the input tube. Not as much gain as the 12AX7, but way better sounding IMO._

 

Sorry for cutting in here, but was following along and was curious have you ever tried the 12SX7GT's? I've got 6 NOS pieces of these and was wondering if they were comparable to the 12SN7's. Got a stack of the 6080's, too.


----------



## Guild

The best 12Ax7 variant that I've owned is a tie: Sylvania 5751 GB (gold pin) and the Amperex PQ 7316. I can't decide between the two, but I'd say the amperex is more consistent throught the hertz spectrum (in other words, less of the characteristic tuby sound). Both of these beat out the GE JG-5751-WA in terms of soundstage and realness: just more alive and less dull. But the GE is still pretty good and much cheaper.


----------



## aerius

Quote:


  Originally Posted by *Blooze* 
_Sorry for cutting in here, but was following along and was curious have you ever tried the 12SX7GT's? I've got 6 NOS pieces of these and was wondering if they were comparable to the 12SN7's. Got a stack of the 6080's, too._

 

Wouldn't know, haven't tried them out yet. I saw a few 6sx7's (6 volt version of the tube) in the local surplus shop but they looked heavily used so I decided not to buy them.

 Back on topic. Another tube which I've heard good things about is the ECC99 which is in current production. It's said to have much lower distortion than the 12a*7 family, can handle a lot more power, and has much better drive capabilities. Also pretty cheap at $13 a pop. I might have to try one out some day.


----------



## philodox

Interesting, the question is then... why did KG choose to use the 12ax7 in his design?

 Hmmm, it seems I may have to give this some more thought.


----------



## Ori

Quote:


  Originally Posted by *Guild* 
_Sylvania 5751 GB (gold pin) and the Amperex PQ 7316... beat out the GE JG-5751-WA_

 

The Sylvania and Amperex might be related, as is the Philips. They are nice tubes but in ceratin applications the dual-mica 5-star GE performs better IME.
 And dump those triple-mica crap while you still can...


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_Interesting, the question is then... why did KG choose to use the 12ax7 in his design?_

 

Many expensive commercial products still use the 12AX7 and 6DJ8 because they are available from new production. There might be better tubes available today, but not having vintage equipment with these means there is not much demand and that in return means an uncertain future.
 As a DIYer you may stock enough of your fave tube to last you a lifetime, so take advantage of what's out there. There are plenty of NOS 5687 and 6CG7 and the likes, in addition to the "super tubes" from Eastern Europe. They are all a step up from the 12AX7 in line stage applications, but only if you optimize the circuit for them.
 Drop them into a 12AX7 circuit and they might give you a pretty sorry performance. Tube "rolling" may look great, but as usual - there is no free lunch...


----------



## aerius

Quote:


  Originally Posted by *philodox* 
_Interesting, the question is then... why did KG choose to use the 12ax7 in his design?_

 

If I had to take a guess, I'd say it's an very common tube which is in current production, pretty cheap, and very easy to find.

 BTW, distortion specs of signal tubes. The ECC82 is the 12au7 which is in the 12a*7 family.


----------



## sacd lover

Quote:


  Originally Posted by *Blooze* 
_Sorry for cutting in here, but was following along and was curious have you ever tried the 12SX7GT's? I've got 6 NOS pieces of these and was wondering if they were comparable to the 12SN7's. Got a stack of the 6080's, too._

 


 The 12sx7gt was apparently a military upgrade to the 12sn7. Reports varied when I researched the tube.... but the 12sx7 seems to be a better matched, highly tested, less microphonic 12sn7. All the 12sx7gt's I have seen were RCA types and have the same plate structures as normal 12sn7's.

 I agree with aerius that the 12 series tubes are not especially good sounding. I much prefer the 6cg7 or 6sn7gt as well. Anymore, I avoid equipment with the 12 series or 6dj8 variants. I used this tube for years in various preamps and I never liked the sound and they can go noisy in an instant. 
	

	
	
		
		

		
		
	


	




 Ori is definitely correct, from my understanding, that the 12 series tubes were tubes made for low cost consumer consumption.... not high performance. Part of the reason this tube is used so much is that these tubes are easy to design around and are still in current production.

 The 6as7/6080 is not an especially reliable or well made tube. The tube was used mainly for voltage regulation.... and thus.... low noise, low microphonics and good section matching were not priorities. I used this tube in the EC HD300 and the Woo 3 and I had a lot of trouble with noise and hum and L/R balance. This is another tube like the 12 series I find to stamp a sameness on everything that colors the sound to much. I have stopped considering equipment that uses this tube myself.... even though I have a drawer full.

 The nos 6as7/ 6080's have a history of opening like a fuse and blowing up if pushed hard. The Russian 6as7 is much more durable and reliable.


----------



## philodox

*sigh*

 Well this is making things more diffucult. Now I have reason to believe that both tubes may be a poor choice.

 I do appreciate all the comments though guys... it just looks like I might have to either completely redesign the circuit or look for a new one. Oh well, it is the only way to learn I suppose.


----------



## djbnh

Quote:


  Originally Posted by *philodox* 
_Hi all,

 I need some advice on tubes for an amp that I will be having built soon. I will need two 12ax7 or 5751...I think I would like to stick with the 5751's over the 12ax7 as they are 70% gain and have lower noise.

 My short list right now for each tube is...

 5751 [12ax7 variant]
 1. Sylvania gold brand black plate triple mica or government contract gold labels
 2. GE 5751-WA black plate triple mica
 3. Sylvania grey plate gold pin triple mica
 4. Sovtek 5751, gold pins [don't know much about it, but parts connexion stocks them new]

 Can anyone offer any further recommendations or confirmations on what I have found so far?_

 

I can offer recommendations, and what I feel is an important caveat with the caveat coming first. IMHO, the same tube (say the tube name is "HocusPocus 5751 triple mica blackplate", or HP5751tmb) may sound different depending on the application and related gear. For example, the HP5751 tmb may work just right and take you to audio nirvana in a certain preamp, but not be as wonderful when used in a cdp or other unit or even another preamp. Albert Porter of AudioGon and audio component photography fame discusses this point succinctly (FYI - if you get a chance, I recommend perusing Mr. Porter's home system.)

 The above being said, I absolutely love the Sylvania Gold Label triple mica black plates / government contract gold labels. I use a pair of Sylvania Gold Label JHS 5751WA tmb in my Jolida JD-100 to great effect; to read about my tube rolling adventures with the JD-100, see this link and scroll down the page to all the tube sections. Thus, I reiterate the Sylvania Gold Label JHS 5751WA works wonderfully for me in the JD-100, so well that I had to purchase a back-up pair of Sylvania Yellow Label JHS 5751WA tmb tubes while prices were still reasonable (I find they are no way close to reasonable at the moment, which makes me wonder if now is a good time to seel those back-up tubes!) I note regardless of the tube has the Gold or Yellow label, they're the same tube.

 I also successfully used the GE 5751-WA tmb tubes, and was quite happy with them in the JD-100, so happy that I even purchased a back-up apir of these tubes, too. Then I had to go and try the Sylvanias discussed above... 
	

	
	
		
		

		
			





 I feel one could be well satisfied with the GE tubes, and even though I parted with one pair of the GE 5751s, I'm holding on to the other pair for, well, did I say they're nice tubes? Regarding GE 5 Star tmb - I found the sound from the 5 Stars sound markedly inferior (downright bad, I returned the tubes to the dealer) to all the other 5751 tubes I rolled, including a bargain pair of year 1967, NOS GE JAN 5751 blackplates ($4 - found in an antique store in town, complete in the original boxes!) . 

 I've tried a bunch of 12AX7s, including the Sovtek LPS, Amperex, and a number of Telefunkens, in the JD-100; none of them, to me with my gear, was overly impressive and I sometimes thought, "That sound sucks". Then again, when those same Telefunken 12AX7s are humming in their native environment, my vintage Fisher 800-B receiver, they are making some sweet music. Which brings us back to my initial caveat in this thread...

 Hope your tube ride will be ultimately successful, but I'm predicting there may be some trial and error. That being said, take good care of those NOS tube boxes in case you need to resell the tubes down the road. Another point is that I swear by the application of Walker SST on the tube pins, but that's another thread, or perhaps contact by PM.

 Happy listening.


----------



## NotoriousBIG_PJ

Quote:


  Originally Posted by *philodox* 
_*sigh*

 Well this is making things more diffucult. Now I have reason to believe that both tubes may be a poor choce.

 I do appreciate all the comments though guys... it just looks like I might have to either completely redesign the circuit or look for a new one. Oh well, it is the only way to learn I suppose. 
	

	
	
		
		

		
		
	


	


_

 

Zanth is in love with his melos that uses 12au7's and I'm over joyed with my amp, which gets its tone more from the 12au7 tubes then the el34 tubes. If its a gilmore amp, then its likely to have a lot of feedback, and that means most tubes will sound pretty similar. My amp used to have feedback and now it doesn't so I have some experience in that respect.

 My beef with the 12au7 tubes is that the new production ones all suck, and a lot of the nice old tubes are expensive and microphonic.

 Biggie.


----------



## Ori

Quote:


  Originally Posted by *NotoriousBIG_PJ* 
_My beef with the 12au7 tubes is that the new production ones all suck, and a lot of the nice old tubes are expensive and microphonic._

 

You gotta try the Sylvania "Gold-brand" GB-5814A!
 Attention to detail: 2-mica only!!! With gold pins only!!!
 It's expensive - but the tone...


----------



## philodox

Hi all,

 I asked Kevin what he thought and he gave the following advice: Quote:


  Originally Posted by *Kevin Gilmore* 
_Since you are going to be hand wiring the thing
 you can build it with any front end tube you want.
 All would have similar biasing characteristics, the
 most you would need to change is one resistor.
 So 12a?7, 6dj8, 6sn7, 12sn7... All work fine.

 As far as output tubes the 6as7,6080,6336 really
 is it for low impedance output tubes. Even the
 worst of the worst beats the crap out of the 5781.
 And they are cheap.

 The nos 6336 military tubes from the 1960's are
 absolutely the best._

 

Due to the poor availability of good quality tube sockets for the 6336 tubes, I doubt I will go that way. Unless anyone has any suggestions on where to get some?


----------



## drarthurwells

Quote:


  Originally Posted by *philodox* 
_Ori - Thanks for the info on those GE tubes, good to know. 
	

	
	
		
		

		
		
	


	




 The amp is an OTL design, and thanks for the tips on those tubes as well... I think I will go with the Sovteks to start as they are cheap, readily available, and atma-sphere uses them so they must not suck. 
	

	
	
		
		

		
		
	


	




 - Jay_

 


 Art: My Bada is a hybrid with three 6NS7 (or 6H8C and 6N8P which are the same) tubes plus MOSFETS in the output.

 It is amazing how much tubes affect the sound quality - you would think the MOSFETS would level the tube playing field but it absolutely does not.

 I have used Shugang, Namking, Russian, and Electro Harmonix Gold in all combinations as the single input and output tubes. All combinations are colored or distorted to some extent except for one - one Shugang 6N8P as the input tube and two Electro Harmonix Gold as the output tubes. The voice/instrument timbre is perfectly natural and realistic with this one combo. If I try 6N8P all around the sound is too "hard" - too solid state in quality (with slightly more timbral body than SS though). If I try Electro Harmonix Gold 6NS7 all around the sound is too tube-like - sweet and liquid but to the point of coloration.

 So, one Shugang 6N8P as the input tube and two Electro Harmonix Gold as the output tubes is perfect. Now if I put two Russian tubes in place of the two Electro Harmonix Gold, I lose my natural timbre and pick up some distortion. I can make this even worse by using other combinations.

 Part of this is the revealing and neutral nature of themy E5 CDP and the Sony SA5000 headphones - they detect subtle differences in amplification as well as in the recording qulaity of the CD.

 Bottom line: Tubes are critical, singly or in various combinations. Also, I would advise against the Russian tubes, except for the Electro Harmonix Gold (in certain combinations with others and in certain apllications).
 The tubes that originally come with a new amp are likely not he best choice, but the best choice can be found only by experimentation with many different tubes and tube combinations. I was lucky to hit on the best combo for my amp early - I simply don't think it could possibly be improved. It certainly emerges way above the other combinations, including the tubes supplied by the manufacturer.


----------



## philodox

Ok, I think I've managed to digest all of the information that has been given to me. Thanks again guys.

 The conclusions I've made are as follows:

*Input Tubes*

 5751 is out: They are a good substitute for 12AX7, but why put them in there if designing a new circuit.

 assorted tubes out: 6SN7 seems to be the superior tube out of all the input tube recommendations.

 6SN7 is out: Though it is basically the best all around dual triode out there, all accounts say that the 12SX7GT's beat out any 6SN7 tube.

 So 12SX7 is in... looks like there are two options that I can find, RCA and GE. People seem to like the RCA's and I can't seem to find any info on the GE tubes of this type. I do have a good supplier for lightly used GE 12SX7's though, so if anyone has any experience with them please let me know.

*Output Tubes*

 6080/6AS7G/6336/A1834/ECC230 are really the only options since we need a low impedance tube for this OTL design.

 After some reading and, a recommendation from Kevin Gilmore, and looking at the specs of the 6336 tubes they seem to be a natural choice. They are quite pricey, but I can always put a 6080 or 6AS7 in there to begin with as the servo should be able to handle any of these tubes.

 I found a supplier that has NOS/NIB TungSol, Raytheon and Cetron. I also found a build log for an amp where the builder suggests the RCA tubes over the Raytheon and Cetron due to lower distortion and higher power output. I can't seem to find the RCA's anywhere, but Kevin Gilmore has used the TungSol's and finds them to be excellent.

 Any comments or further criticism are welcome.


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_6080/6AS7G/6336/A1834/ECC230 are really the only options since we need a low impedance tube for this OTL design_

 

That would be true for 8 or 16 ohm loads. Most headphones don't need the ultimate efficiency.
 Consider the filament requirements of these beasts and you can tell they are not ideal for the task... 
	

	
	
		
		

		
		
	


	




 Consider a 12B4A, a linear triode with 1K plate resistance, or a 7233 voltage regulator, which might work well with enough feedback.
 If you are starting from scratch, I'd look at a single 6EW7 per channel. Triode 1 is similar to a 6SN7 and triode 2 has plate resistance of 800 ohm.
 Best part is that they are not used in popular amps, so you can probably "steal" them for pennies on the dollar... 
	

	
	
		
		

		
		
	


	



 Of course, if you intend to go with zero feedback then the final tube must have extremely low plate resitance. In that case, the 6336A, 6AS7G or 7233 are your very limited options. I suggest that you do not design parallel tubes without feedback. The whole idea of zero feedback is to maximize coherency. Parallel tubes will "ruin" the good virtues of the topology, unless you match them dynamically to the extreme, which is not very practical.
 No free lunch!


----------



## philodox

Excuse my ignorance, but what do you mean by parallel tubes? There is one 6336 for the left chanel and one for the right, is that what you mean? Or the fact that there is a 12sx7gt on the input and the 6336a's on the output?

 I doubt it will be zero feedback [KG said he will likely add some feedback to the design], but if so are you saying the tubes must be matched very closely for there to be a benifit?


----------



## philodox

By the way, I don't think I've mentioned it yet in this thread, but the amp I am building is the Kevin Gilmore Bamaslama.

Schematic [shows both channels]

 I plan on starting off with a single ended amp for testing and parts selection, but hope to eventually build a balanced version of this amp.


----------



## sacd lover

Quote:


  Originally Posted by *philodox* 
_Ok, I think I've managed to digest all of the information that has been given to me. Thanks again guys.

 The conclusions I've made are as follows:

*Input Tubes*

 5751 is out: They are a good substitute for 12AX7, but why put them in there if designing a new circuit.

 assorted tubes out: 6SN7 seems to be the superior tube out of all the input tube recommendations.

 6SN7 is out: Though it is basically the best all around dual triode out there, all accounts say that the 12SX7GT's beat out any 6SN7 tube.

 So 12SX7 is in... looks like there are two options that I can find, RCA and GE. People seem to like the RCA's and I can't seem to find any info on the GE tubes of this type. I do have a good supplier for lightly used GE 12SX7's though, so if anyone has any experience with them please let me know.


 Any comments or further criticism are welcome. 
	

	
	
		
		

		
		
	


	


_

 



 I too have read those accounts that the 12sx7's are better than any 6sn7 and I now know thats bull. Out of curiosity ..... I bought two 12sx7's .... one RCA gray glass and one GE clear glass .... to try out as gain tubes in my mpx3's. The RCA tube didnt sound any better/ different than the typical RCA 12sn7gt gray glass. The GE sounds like the JAN GE 12sn7gt I have .... which is not bad .... but nowhere near as good as the better 6/12sn7's.

 So.... already the 12sx7 is no better sounding than the same type 12sn7 and has almost no variety. Of the two 12sx7's, the RCA is a better sounding tube than the GE but has a very distinctive character thats warmer, slower and darker sounding than any other 6/12sn7 I am aware of. This RCA tube has a love it or hate it type of sound. The GE is bass shy with good mids but sounds sort of dull, smeared and closed in at the top .... and is generally ranked as the poorest sounding 6/12sn7gt. If you dont like the sound of these two tubes you are screwed. 
	

	
	
		
		

		
		
	


	




 The 12sn7 has every type you would find as a 6sn7; Ken Rad black or clear glass/ National Union black glass/ three types of Tung Sol/ several types of Sylvania/ Hytron tall and short bottles/ Raytheon/ Dumont Labs/ three types of RCA including the gray glass like the RCA 12sx7, and finally, the GE like the 12sx7 above. I have every one of these tubes and I have used them all extensively and the 12sn7 has several advantages. The 12sn7 tube is every bit as good or better sounding, less expensive ($6-12 vs the $22-30) .... easier to find and the 12sn7 gives you far more variety to fine tune your setup. I personally prefer the Ken Rads, two of the Tung Sols, the N. Union, the tall bottle Hytrons and the tall bottle Sylvanias to either 12sx7. Furthermore, while the 12sn7 is identical to the 6sn7, except for the heater voltage, they cost pennies on the dollar compared to the 6sn7. 
	

	
	
		
		

		
		
	


	




 12sx7 out ..... 12sn7gt in!!!


----------



## philodox

Thanks Earl... now I'm confused again when I thought everything was becoming clear. 
	

	
	
		
		

		
		
	


	




 I wonder why everyone goes on about the 12SX7GT's being better than all the 6SN7's in that case?


----------



## sacd lover

Quote:


  Originally Posted by *philodox* 
_Thanks Earl... now I'm confused again when I thought everything was becoming clear. 
	

	
	
		
		

		
		
	


	




 I wonder why everyone goes on about the 12SX7GT's being better than all the 6SN7's in that case? 
	

	
	
		
		

		
		
	


	


_

 


 Maybe because they have never listened to them and just repeated one persons opinon. 
	

	
	
		
		

		
		
	


	




 Maybe they compared them to some 6sn7gtb's? 
	

	
	
		
		

		
		
	


	




 I love the 14n7 if you could find some good loctal sockets. There is no variety as all these loctal tubes are almost exclusively sylvania tubes. The 14n7 is either a sylvania tall bottle 6-12sn7gt/ vt-231 type .... or a short bottle gt type .... with a loctal base instead of an octal base. This tube has good bass, beautiful transparent mids and the most open spacious highs of any 6sn7 variant I have tried. Oh yeah ... and I have paid between $2.25-$3 for them.


----------



## philodox

Quote:


  Originally Posted by *sacd lover* 
_Maybe because they have never listened to them and just repeated one persons opinon._

 

Yeah, those hype machines can get rolling quickly can't they? 
	

	
	
		
		

		
		
	


	




 I do think that the rolling opportunities would be better with the 12SN7s though, and I could always pick up a pair of 12SX7s to try out as they are interchangable. 
	

	
	
		
		

		
		
	


	




 I'm thinking that those 14n7's might have similar 'lack of choice' problems, but I'll look into them. During the testing stage maybe we can hook up one of these loctal sockets and give them a go.

 Thanks,

 Jay


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_By the way, I don't think I've mentioned it yet in this thread, but the amp I am building is the Kevin Gilmore Bamaslama_

 

Looking at the ciruit, I'm going to risk it and play a backseat driver...
 Based on the Svetlana 6AS7 curves, the current source (or sink) at the bottom is biased at 115V and 110mA. That puts this section at 12.5 watts power dissipation, right at the limit of 13 watts. Unfortunately it puts the top section way over the recommended limit. Not good!

 The choice of +-150V for the supply seems a bit off for a headphone amp. We don't need a high voltage swing on the output and the 6AS7 remains quite linear at much lower voltages than calculated.

 The supply must be well regulated, because this section of the circuit does not offer much rejection of supply noise. There are much better loads that will provide stable DC conditions and supply rejection.

 I don't particularly like the servo circuit, especially the point where the correction is injected, right in the thick of the signal path.
 The servo circuit itself requires +-15V, so that's additional hardware.
 The integrator is a bit simplistic (passive RC), but probably works anyway. I'm not sure the relay would salvage anything had a disaster strike... I hope someone is testing this with a dummy load at extreme conditions.

  Quote:


 Excuse my ignorance, but what do you mean by parallel tubes? 
 

That was a comment assuming you are starting a new design. Some people attempt to lower the output impedance by using more tubes in parallel and I just wanted to caution against that if your goal is the ultimate sound.

  Quote:


 hope to eventually build a balanced version of this amp. 
 

And the reason is?!

  Quote:


 I found a supplier that has NOS/NIB TungSol, Raytheon and Cetron. I also found a build log for an amp where the builder suggests the RCA tubes over the Raytheon and Cetron 
 

The 6336A tube was very specialized, so most NOS are re-badged Tungsols. I had a bunch of them and they all looked the same to the last detail... Not sure about the Cetron, which is a newer manufacturing.


----------



## NotoriousBIG_PJ

If theres one thing I've learned, its that almost all of the tube discussion is based on personal preferences, not on hard facts. Experimenting for your self is best.

 Biggie.


----------



## philodox

Wow Ori, thanks for all of the advice on the design. I can't comment much myself as I am very new to this, but I will bring up those points with my 'teacher' and with the designer to see what they think. Though I am building this amp I will be getting quite a bit of assistance from another head-fi'er, looser101, especially at the beginning. Quote:


  Originally Posted by *Ori* 
_And the reason is?!_

 

Well, I want a beast of an amp that will dish out anything I ask of it. My main headphones are the AKG K340's, which are a very difficult load to drive properly and I think they deserve a balanced amp... Quote:


  Originally Posted by *Ori* 
_The 6336A tube was very specialized, so most NOS are re-badged Tungsols. I had a bunch of them and they all looked the same to the last detail... Not sure about the Cetron, which is a newer manufacturing._

 

Good to know... I missed out on an ebay auction for a pair of them at $12 yesterday afternoon. 
	

	
	
		
		

		
			





 Quote:


  Originally Posted by *NotoriousBIG_PJ* 
_If theres one thing I've learned, its that almost all of the tube discussion is based on personal preferences, not on hard facts. Experimenting for your self is best._

 

Thanks Jamie... I think most things audio are like that in the end.


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_I will be getting quite a bit of assistance from another head-fi'er, looser101_

 

Quite sad that you seek help from a self-declared loser... Eh, looser... 
	

	
	
		
		

		
		
	


	



  Quote:


 My main headphones are the AKG K340's, which are a very difficult load to drive properly 
 

 I wonder why. The AKG specs say 400 ohm and around 5V max for a sane drive, which translates to 14V p-p swing and about 35mA p-p current. You don't need more drive than a single 6AS7 can deliver in this circuit.
 I can understand a small transistor or opamp unit "choking" with this load, given the voltage limitations they usually have, but not a tube circuit. The 400 ohm load is a _dream come true_ for an OTL designer... 
	

	
	
		
		

		
		
	


	



 This topology is not inherently balanced. Merely attaching two amps to a single load does not assure good sonics, especially with zero feedback. Any mismatch of characteristics would translate to distortion, which would offset the cancellation of even harmonics by the bridged circuit. It's possible that you won't even like how it sounds, since you are really moving away from a *single-ended* topology to a *push-pull*/bridged. 
	

	
	
		
		

		
		
	


	



 You'd also need a truly balanced source, not a _kludge_ for one.
 As I always say, there is no free lunch... Just keep it simple!


----------



## philodox

Quote:


  Originally Posted by *Ori* 
_Quite sad that you seek help from a self-declared loser... Eh, looser..._

 

Well, I met Renato at the last Hamilton meet and he is a very nice guy... knows his stuff too, so I feel that i am in good hands. 
	

	
	
		
		

		
		
	


	




 Quote:


  Originally Posted by *Ori* 
_I wonder why. The AKG specs say 400 ohm and around 5V max for a sane drive, which translates to 14V p-p swing and about 35mA p-p current. You don't need more drive than a single 6AS7 can deliver in this circuit. I can understand a small transistor or opamp unit "choking" with this load, given the voltage limitations they usually have, but not a tube circuit. The 400 ohm load is a dream come true for an OTL designer...

 This topology is not inherently balanced. Merely attaching two amps to a single load does not assure good sonics, especially with zero feedback. Any mismatch of characteristics would translate to distortion, which would offset the cancellation of even harmonics by the bridged circuit.

 You'd also need a truly balanced source, not a *kludge* for one._

 

Kludge?

 Anyways, we are building it unbalanced first to test for parts selection and if I am happy with it at that point we will go no further.

 I used to own a Dynahi, which drove the K340 quite well. I decided to go balanced with my new tube amp mainly so that it could be an end it all sort of affair...

 How would a topology be 'inherently balanced'? My understandind was that a fully balanced amp amounted to doubling everything including the powersupply. How would a mismatch translate to distortion? I thought that one of the benefits of a balanced design is to minimize the impact of any mismatch. 
	

	
	
		
		

		
		
	


	




 As to why the K340 is such a hard load to drive, I can't answer that. It is though. 
	

	
	
		
		

		
		
	


	




 To address some of your earlier comments: Quote:


  Originally Posted by *Ori* 
_Based on the Svetlana 6AS7 curves, the current source (or sink) at the bottom is biased at 115V and 110mA. That puts this section at 12.5 watts power dissipation, right at the limit of 13 watts. Unfortunately it puts the top section way over the recommended limit. Not good!_

 

More like a little over the limit. You can lower the rails to +/-125, but these tubes have a cutoff voltage around 40 volts... so you really do need rails that high. Quote:


  Originally Posted by *Ori* 
_The supply must be well regulated, because this section of the circuit does not offer much rejection of supply noise. There are much better loads that will provide stable DC conditions and supply rejection._

 

All of Kevin Gilmore's power supplies to date have been regulated, so this should not be a problem. Quote:


  Originally Posted by *Ori* 
_I don't particularly like the servo circuit, especially the point where the correction is injected, right in the thick of the signal path._

 

Where else would you suggest that the servo inject DC? Quote:


  Originally Posted by *Ori* 
_The integrator is a bit simplistic (passive RC), but probably works anyway. I'm not sure the relay would salvage anything had a disaster strike... I hope someone is testing this with a dummy load at extreme conditions._

 

The integrator should be fine as it has a low frequency cutoff of about .05hz. The safety circuit was used by Kevin Gilmore with another designers permission and does it's job just fine.

 I'm not sure what alternatives there are for an OTL tube design that has balls... would you suggest that we use a single supply, throw away the servo and tack on a crappy sounding electrolytic output cap!?!?


----------



## philodox

BTW... I can't say for sure as I have a *very* limited understanding when it comes to this, but I think you might be using the terms push pull and bridged out of context. If I decide to go balanced, I will be feeding this amp a balanced signal and it will be outputing a balanced signal to headphones that have been recabled to accept it. From everything I've read on single ended vs push pull designs it seems to be referencing an output transformer. Since this is an OTL design do these terms still apply?

 Also, to avoid confusion, I wanted to point out that in my previous post when I mentioned 'single-ended' I meant 'unbalanced'... sorry if that made things unclear. 
	

	
	
		
		

		
		
	


	




 [Edit: I'd also like to restate that the above schematic is for both channels and would be effectively doubled if we went to a balanced design.]


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_Kludge?_

 

Oh, maybe that's a strong word... What I meant is that many so-called balanced components merely add an inverting amp to achieve the balanced output. That's where _more is less_. 
	

	
	
		
		

		
		
	


	



  Quote:


 How would a topology be 'inherently balanced'? 
 

Differential circuits or any sort of cross-coupling, like ARC gear. The idea is that each phase on the output is affected by both phases on the input, leading to more symmetry (if done right).
  Quote:


 How would a mismatch translate to distortion? 
 

Any mismatch leads to distortion. Asymmetry in gain of the two halves of a balanced circuit would result in a "chopped" sine wave on the output when you feed a pure sine wave at the input. That's harmonic distortion! 
	

	
	
		
		

		
		
	


	



  Quote:


 As to why the K340 is such a hard load to drive, I can't answer that. 
 

The high impedance requires a high voltage swing to achieve decent output levels. That would be an impossible task for an amp that was optimized to run on a low voltage supply, so I can see why such a statement would emerge. 
	

	
	
		
		

		
		
	


	




 However, there is also a possibility that the load is very reactive. No data on that in the short technical spec. 
  Quote:


 You can lower the rails to +/-125, but these tubes have a cutoff voltage around 40 volts.... 
 

Excellent... You really need each tube to "swing" about 7-10 volts towards the rail to drive the K340 to insane levels. I was thinking more of 100V supply, but you have to consider linearity without feedback.
  Quote:


 Where else would you suggest that the servo inject DC? 
 

There are many options, but all of them require a change of the circuit. The "natural" place is the current source. KG has done that with his solid-state amps, I think.
  Quote:


 The safety circuit was used by Kevin Gilmore with another designers permission and does it's job just fine. 
 

A proven design - excellent! 
	

	
	
		
		

		
		
	


	



  Quote:


 would you suggest that we use a single supply, throw away the servo and tack on a crappy sounding electrolytic output cap!?!? 
 

Not if the servo does not interfere as much with the signal path. Otherwise you might as well move to an output cap. With 400 ohm load you need only 22uF for 20 Hz response. Not even an electrolytic... 
	

	
	
		
		

		
		
	


	



  Quote:


 From everything I've read on single ended vs push pull designs it seems to be referencing an output transformer. Since this is an OTL design do these terms still apply? 
 

Absolutely so. Single-ended means that there is only one (effective, including parallel) amplifying device in the stage, which generates both the "high" and "low" swings of the sine wave.
 Any circuit that employs two (or groups of) devices which contribute to different parts of the signal is by definition not single-ended. Most of these toplogies follow the push-pull concept and a bridged circuit is not a whole a lot different from a push-pull circuit in that respect. It is extremely amusing to see people bridging SE amps. Why bother - just go with a good push-pull... 
	

	
	
		
		

		
		
	


	



 I think you can sense where I'm standing from pure sonic point of view. When in doubt, simplify it - and there is nothing simpler than the basic single-ended circuit!
 The schematics of this KG amp are indeed single-ended topology, so it's a great starting point. The low output power makes it very practical and the high impedance of your headphones make an OTL not only possible but perhaps optimal. No complaints about the concept, just that it can be improved with minor tuning.


----------



## looser101

Quote:


  Originally Posted by *Ori* 
_Based on the Svetlana 6AS7 curves, the current source (or sink) at the bottom is biased at 115V and 110mA. That puts this section at 12.5 watts power dissipation, right at the limit of 13 watts. Unfortunately it puts the top section way over the recommended limit. Not good!_

 

Ori,

 If the bottom tube is the current source (sink) at 110mA and the top tube is in series with it, does that not imply that the same 110mA is flowing through the top tube? Could you clarify as to why the top tube is over the recommended limit?

 Thanks,

 - The Loser...LOL


----------



## Ori

Quote:


  Originally Posted by *looser101* 
_same 110mA is flowing through the top tube? Could you clarify as to why the top tube is over the recommended limit?_

 

Assuming the servo works, the output is at ground potantial. The top tube sees the positive supply so its Vpk=150V. The bottom tube sees the negative supply less the bias voltage drop on the cathode resistor, so it only sees Vpk=115V.
 The top (half) tube simply exceeds the 13W Pd,max spec.


----------



## looser101

Quote:


  Originally Posted by *Ori* 
_Assuming the servo works, the output is at ground potantial. The top tube sees the positive supply so its Vpk=150V. The bottom tube sees the negative supply less the bias voltage drop on the cathode resistor, so it only sees Vpk=115V.
 The top (half) tube simply exceeds the 13W Pd,max spec._

 

Makes perfect sense. Do you think this is a serious design issue or will it only affect tube life?

 Thanks,

 - Renato


----------



## philodox

Quote:


  Originally Posted by *Ori* 
_Oh, maybe that's a strong word... What I meant is that many so-called balanced components merely add an inverting amp to achieve the balanced output. That's where more is less._

 

You are correct that it is quite rare to find a truely balanced source. Mine is not. I had planned on either trying out Garbz' balanced to unbalanced convertor [transformer based] or installing a ZapFilter in order to get closer to or achieve a true balanced source. Quote:


  Originally Posted by *Ori* 
_Any mismatch leads to distortion. Asymmetry in gain of the two halves of a balanced circuit would result in a "chopped" sine wave on the output when you feed a pure sine wave at the input. That's harmonic distortion!_

 

Hmmm, I can't come up with a good cross argument, but I think this is wrong. There was a very technical discussion on here recently about balanced configuration and I'm pretty sure that type of distortion is only possible if you are listening at INSANE volumes... even more so with a balanced amp. Like I said though, the thread was way over my head so who knows. Quote:


  Originally Posted by *Ori* 
_The high impedance requires a high voltage swing to achieve decent output levels. That would be an impossible task for an amp that was optimized to run on a low voltage supply, so I can see why such a statement would emerge. However, there is also a possibility that the load is very reactive. No data on that in the short technical spec._

 

Quite possible as it has a very non standard design, with its electret, passive diaphragms and integrated circuit... Quote:


  Originally Posted by *Ori* 
_Excellent... You really need each tube to "swing" about 7-10 volts towards the rail to drive the K340 to insane levels. I was thinking more of 100V supply, but you have to consider linearity without feedback._

 

Would we be better off with a tube like the 6336?

 6336 - 30 watt @ 400ma per plate, 6.3V @ 4.75A Filament, mu=2.7
 6336A - 6.3V @ 5A Filament

 6AS7G [for comparison] - 6.3V @ 2.5A Filament, Pmax=13 watts per triode section, 26 watts total, Mu=2

 Or even just the 6080's which are ruggerdized 6AS7's? Quote:


  Originally Posted by *Ori* 
_Not if the servo does not interfere as much with the signal path. Otherwise you might as well move to an output cap. With 400 ohm load you need only 22uF for 20 Hz response. Not even an electrolytic..._

 

Interesting... I might do this at first to make a simple version that we can do parts selection on, but eventually I would like this amp to be able to drive headphones other than my K340's. 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *Ori* 
_Any circuit that employs two (or groups of) devices which contribute to different parts of the signal is by definition not single-ended. Most of these toplogies follow the push-pull concept and a bridged circuit is not a whole a lot different from a push-pull circuit in that respect. It is extremely amusing to see people bridging SE amps. Why bother - just go with a good push-pull...
 The schematics of this KG amp are indeed single-ended topology, so it's a great starting point. The low output power makes it very practical and the high impedance of your headphones make an OTL not only possible but perhaps optimal. No complaints about the concept, just that it can be improved with minor tuning._

 

I am not talking about bridging single ended the design, which you are right would turn it to a push pull circuit. I am talking about creating a balanced single ended design.

 I think this explains what I'm getting at:

 single ended - one tube, one side does signal, the other side does ground
 push pull - two tubes paralleled for more current driving the signal the other side is ground
 balanced - two tubes, one side of the first tube does signal, the other side of the first tube does inverted signal, the other tube does ground

 Ok, after re-reading that, it doesnt make complete sense, but you see what I am getting at. I am not bridging the amp, I am balancing it. Therefore it is still a single ended topology. Not push pull.


----------



## philodox

Better description of balanced [thanks to looser101 for major clarification on this]:

 Balanced output comes from the source which rather than sending just signal (+ground), sends the signal and an inverted signal.

 balanced input + > amp input > amp output > transducer +
 balanced input - > amp input > amp output > transducer -

 note 1: balanced outputs driven by balanced inputs
 note 2: other transducer driven by identical setup as above

 Each side of the transducer is being driven by a seperate feed. Since the transducer is being driven by opposite symmetrical voltages [twice the voltage swing], any sound anomalies get cancelled out. 

 With push pull, if I understand correctly, one tube will handle the top part of the signal and the other will handle the bottom part of the signal while still referenced to ground... the whole signal is then passed to one side of the transducer. A mismatch here would be worse than in a single ended circuit.


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_There was a very technical discussion on here recently about balanced configuration and I'm pretty sure that type of distortion is only possible if you are listening at INSANE volumes..._

 

Distortion only depends on the "shape" of the waveform. It's a very basic theory that shows that any asymmetry translates to a series of harmonics of the fundamental frequency. Balanced circuits do have advantages in rejecting common mode noise in a transmission line and inherent rejection of the even harmonics (just like push-pull!).
 It's like anything else in audio - a compromise... For all those good virtues it costs you in sacrificing the ultimate coherency. That doesn't mean it's impossible to build a good balanced amp, but I'd resort to that approach when I need a lot of power, not with a headphone amp...
  Quote:


 Would we be better off with a tube like the 6336? 
 

The 6336 does have more power capacity (it's bigger!) but it's also wasteful. Just the filament supply alone is worth 31 watts - no green amp for sure! A 6AS7 will be fine if you drop the supply voltage a little and bias it lower. A 110mA is more than double the peak current with the K340s. You might want more current for lower impedance headphones and then the 6336 is a better choice, but at a great cost.
  Quote:


 push pull - two tubes paralleled for more current driving the signal the other side is ground
 balanced - two tubes, one side of the first tube does signal, the other side of the first tube does inverted signal, the other tube does ground 
 

I think a picture will be worth a thousand words. Not clear at all...
  Quote:


 Since the transducer is being driven by opposite symmetrical voltages [twice the voltage swing], any sound anomalies get cancelled out. 
 

Sound anomalies do not cancel out because of the balanced topology. Only even harmonics do - and that's only if the two "sides" swing in a very symmetrical manner, i.e. they are gain- and phase-equalized. Again, this is extremely basic to show. It might be a good idea for you to pick a simple circuit analysis program such as Electronic Workbench or the more professional PSPICE and play a little with such concepts.
 Bottom line is that you only need to consider the very basic elements in the topology. If there is a single active device, then the behavior of the circuit is "smoother". With more devices, you are inevitably splitting the signal, and that's a source of problems by itself. No math, just common sense!


----------



## philodox

Quote:


  Originally Posted by *Ori* 
_I think a picture will be worth a thousand words. Not clear at all..._

 

Thank you, I stated myself that it did not make that much sense. The second explination was much more clear however. I am definately not the one to be explaining this though as the discussion is already way over my head.

 Bottom line is that balancing an amp is different than bridging it and grants certain benefits that are desirable.


----------



## aerius

Quote:


  Originally Posted by *philodox* 
_Bottom line is that balancing an amp is different than bridging it and grants certain benefits that are desirable._

 

But at a cost. More complexity, more chances for mis-matches & imbalances. There is no free lunch in engineering, it's all about trade-offs and which ones you can live with.


----------



## philodox

Oh definately... and who said I was looking for a free lunch?


----------



## philodox

Ok, one last try... I did a diagram that shows everything pretty clearly I think. 
	

	
	
		
		

		
		
	


	









 So the first half shows a balanced single ended setup and the second shows an unbalanced push pull setup. You could also have a balanced push pull setup by copying the second half of the second diagram and pasting it where each output stage is on the first diagram. The same type of diagram for an unbalanced single ended setup would be very simple: [CDP] -> [Input Stage] -> [Output Stage] -> [+ on transducer]

 EDIT: Oh, and here is that balanced thread I was talking about. I still don't understand most of it, but with each read it is becoming more clear.


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_Ok, one last try... I did a diagram that shows everything pretty clearly I think._

 

A good pic! First, it does show differences between balanced and push-pull, but they are accentuated. Your drawing shows a class B or AB push-pull against a class A balanced.
 In any case, the comment about the balanced topology equalizing on both ends is only correct if the gain ratio between the two amps is fixed. I have to say that it is an advantage of the balanced topology over a push-pull. The latter would require matched gain, not just a fixed ratio, to minimize harmonic distortion.
 Now back to real life. Any mismatch from the "fixed gain ratio" would generate distortion in the balanced topology.
 I don't want to sound too negative, but the balanced and push-pull topologies do have some common characteristics, resulting from the cancellation of even harmonics. While all internally-generated harmonics are considered distortion, they are inevitable in any amp. The ratio between the different harmonics (2nd, 3rd and so on) determines very much how natural the amp would sound. That's why I said that you may not like the sound of a bridged amp, even if you loved the individual amplifiers it uses.
 Approach with caution...
 Another issue to consider with balanced amps is that the voltage swing doubles but the current is still limited to the level set by the current source/sink in the individual amps. You may need to re-think the bias of the individual stages, since then 110mA may become the limiting factor, even with high-Z headphones.
 Just more food for thought...
 I'd really like to hear your thoughts of a purist amp without the servo and with a decent 22uF/400V (safety margin for inductive load!) copper foil cap at the output.
 If you ever attempt a more complex variation on this design then you might be able to utilize DC coupling between the stages and servo on the output.
 Add a very stable fixed current source and you won't have any dependency on the final supply cap either, because the amp consumes then only DC.
 That would entail a complex design, but I can see a lot of virtues stacking in favor of at least trying it.
 Another thought about a balanced amp. If you hand match components and dial in the bias, you could end up with close to zero DC on the load without a servo. As you see, it's all pros vs. cons and no clear winner IMO - until you try them.
 More food for thought...


----------



## philodox

Quote:


  Originally Posted by *Ori* 
_A good pic! First, it does show differences between balanced and push-pull, but they are accentuated. Your drawing shows a class B or AB push-pull against a class A balanced._

 

Glad I got close. 
	

	
	
		
		

		
			





 How is the push-pull diagram not class A? Quote:


  Originally Posted by *Ori* 
_In any case, the comment about the balanced topology equalizing on both ends is only correct if the gain ratio between the two amps is fixed. I have to say that it is an advantage of the balanced topology over a push-pull. The latter would require matched gain, not just a fixed ratio, to minimize harmonic distortion.
 Now back to real life. Any mismatch from the "fixed gain ratio" would generate distortion in the balanced topology._

 

Fair enough... so we need to be careful there. I suppose this would make tube matching more important as well? That could get expensive with those 6336's. 
	

	
	
		
		

		
		
	


	




 Quote:


  Originally Posted by *Ori* 
_I don't want to sound too negative, but the balanced and push-pull topologies do have some common characteristics, resulting from the cancellation of even harmonics. While all internally-generated harmonics are considered distortion, they are inevitable in any amp. The ratio between the different harmonics (2nd, 3rd and so on) determines very much how natural the amp would sound. That's why I said that you may not like the sound of a bridged amp, even if you loved the individual amplifiers it uses._

 

I understand what you are getting at, and as I said, we are going to build it unbalanced first... who knows, we might stop there. Balanced designs obviously can work quite well in tube amps... look at the Singlepower XLR-SDS. 
	

	
	
		
		

		
		
	


	




 PS. Don't worry about sounding negative... I tend to get really puffed up and dreamy when it comes to thinking up my next audio purchase, so it is good to have someone bring me back to reality. Now that Renato has taken it upon him self to teach me how to build this stuff it can only get worse. 
	

	
	
		
		

		
		
	


	




 Quote:


  Originally Posted by *Ori* 
_Another issue to consider with balanced amps is that the voltage swing doubles but the current is still limited to the level set by the current source/sink in the individual amps. You may need to re-think the bias of the individual stages, since then 110mA may become the limiting factor, even with high-Z headphones._

 

Are we going to have a shortage of current in this design? Or is that largely dependant on the tubes? Quote:


  Originally Posted by *Ori* 
_I'd really like to hear your thoughts of a purist amp without the servo and with a decent 22uF/400V (safety margin for inductive load!) copper foil cap at the output.
 If you ever attempt a more complex variation on this design then you might be able to utilize DC coupling between the stages and servo on the output.
 Add a very stable fixed current source and you won't have any dependency on the final supply cap either, because the amp consumes then only DC.
 That would entail a complex design, but I can see a lot of virtues stacking in favor of at least trying it._

 

Well, I suppose it sounds interesting from that point of view, but there are two major flaws with this design that sort of rule it out for me. One, this will effectively make the amp only good for the K340 [or other very high impedance headphones] which limits its versatility quite a bit. And two, what happens if that cap fails? Bye bye headphones. 
	

	
	
		
		

		
		
	


	




 We will probably use a 22uf cap and a couple pots in leu of the servo and its accompanying PSU during parts selection though. I hope to try out a few different caps and resistors... and maybe different tube types. 
	

	
	
		
		

		
		
	


	




 Quote:


  Originally Posted by *Ori* 
_Another thought about a balanced amp. If you hand match components and dial in the bias, you could end up with close to zero DC on the load without a servo._

 

So it might be possible to get by without the servo or a cap? Interesting.


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_How is the push-pull diagram not class A?_

 

The two split signals are _abruptly chopped_.
  Quote:


 I suppose this would make tube matching more important 
 

I'd consider low negative feedback. That's one place where it's appropriate.
  Quote:


 Now that Renato has taken it upon him self to teach me how to build this stuff it can only get worse. 
 

Always nice to have a good _elmer_. You're right... Renato does sound like a nice guy! 
	

	
	
		
		

		
		
	


	



  Quote:


 Are we going to have a shortage of current in this design? 
 

This depends on the load (headphones) impedance and their sesitivity - or power requirements. If you double the voltage swing (as in a balanced design), then over the same 400 ohm you will have double the current swing - unless your supply (or circuit) limits the current. The amp will then clip due to current starvation.
 The problem may get worse with low impedance headphones. For example, 32 ohm headphones that require 100mW drive translates to 5V p-p and a simple division gives 158mA. A circuit biased at 110mA would only develop only 48mW at the threshold of hard clipping (i.e. rising distortion).
 You may have to go with a 6336 for such application.
  Quote:


 what happens if that cap fails? 
 

Yep, it ain't nice at all... You may still need some shutoff circuit, but not DC servo.
  Quote:


 I hope to try out a few different caps and resistors... and maybe different tube types. 
 

And that is _all the fun_ with DIY projects! I'll be looking froward to reading progress reports, even if the smoke monster gets loose... 
	

	
	
		
		

		
		
	


	




 That's part of learning!
 You noticed there are so many options and not all of them are appropriate for all types of headphones. As a DIYer, you can build a *point solution*, a design that is optimized for a specific narrow set of requirements - and do it well. If you try to cover all fronts, you may end up with a _camel_. A cammel, you ask? Yep, a desert horse, designed by a committee...


----------



## aerius

Quote:


  Originally Posted by *philodox* 
_How is the push-pull diagram not class A?_

 

Funny thing happens when you run push-pull in class A. The waveform will look like the one for a balanced amp until you exceed a certain power and one side starts clipping or going into cut-off. Found out about that one when I had an interesting failure on my test amp and did the research to find out why.

  Quote:


 Are we going to have a shortage of current in this design? Or is that largely dependant on the tubes? 
 

I'd say it's quite doubtful, you'll probably rupture your eardrums before that happens.

  Quote:


 Well, I suppose it sounds interesting from that point of view, but there are two major flaws with this design that sort of rule it out for me. One, this will effectively make the amp only good for the K340 [or other very high impedance headphones] which limits its versatility quite a bit. And two, what happens if that cap fails? Bye bye headphones. 
	

	
	
		
		

		
		
	


	



 

So what? Use another amp for your other headphones. Specialize and tune them. You don't need an amp that's good with everything unless you plan to collect or rotate a harem of headphones. And the chances of a film cap failing are vanishingly small, you have a much better chance of dying in a horrible bus crash.

  Quote:


 So it might be possible to get by without the servo or a cap? Interesting. 
	

	
	
		
		

		
		
	


	



 

Yup, but better make sure everything's really well matched and you gotta keep an eye on things since the tube & other parts will drift a bit as they break in & age, especially the tubes. The KGCA for instance, can be built without a servo, it's been done before.


----------



## philodox

Elmer the safety elephant!? 
	

	
	
		
		

		
		
	


	




 Quote:


  Originally Posted by *Ori* 
_You noticed there are so many options and not all of them are appropriate for all types of headphones. As a DIYer, you can build a *point solution*, a design that is optimized for a specific narrow set of requirements - and do it well. If you try to cover all fronts, you may end up with a camel. A cammel, you ask? Yep, a desert horse, designed by a committee..._

 

 Quote:


  Originally Posted by *aerius* 
_So what? Use another amp for your other headphones. Specialize and tune them. You don't need an amp that's good with everything unless you plan to collect or rotate a harem of headphones. And the chances of a film cap failing are vanishingly small, you have a much better chance of dying in a horrible bus crash._

 

Hmmm... maybe you guys are right. I could always just use this amp for the K340's and make another amp or use my dynalo for other headphones. I'll think about it.


----------



## philodox

I'm warming up to the idea of building the amp specifically to drive the K340s. One question: I understand that there will be some change as components age or burn in and the bias may need to be adjusted over time. Could we mount the pots externally and have some sort of readout so that I wouldn't need to bust out the multimeter?

 When talking this over with aerius [my other 'advisor' on this] a while back he mentioned a variable startup/warmup process that we could implement. I am envisioning something like the following based on what aerius suggested before:

*4 position rotary switch*
 Position 1: Off. No power goes anywhere.
 Positinn 2: Low power standby. Resistors in series with tube heaters. High voltage section off.
 [Check readout and dial in bias at this point.]
 Position 3: Standby. Full power to tube heaters. High voltage section off.
 Position 4: On. Full power to everything.

 What do you think?


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_I'm warming up to the idea of building the amp specifically to drive the K340s._

 

A smart move! Think of it as putting together a _super-Orpheus_ for a fraction of the cost!
  Quote:


 Could we mount the pots externally and have some sort of readout so that I wouldn't need to bust out the multimeter? 
 

If you want a classic look then build an analog readout into the chassis. If you just want function, then the same can be done with LED indicators.
  Quote:


 variable startup/warmup process 
 

The options you outlined make a lot of sense. Incorporate them into a multi-wafer rotary selector with one wafer for the power, so all the "controls" are on one knob.
 The only issue with a manual selector is that you may make a mistake and switch it from off all the way to the full-on position with no warm-up delay. The same can be done with relays and some circuitry that will force delays between steps, but that would be more complicated. You might want to 'recruit' someone familiar with microcontrollers, instead of doing this by analog means.


----------



## philodox

Quote:


  Originally Posted by *Ori* 
_If you want a classic look then build an analog readout into the chassis. If you just want function, then the same can be done with LED indicators._

 

I think the analog readout would look great, sort of like a VU meter then? Quote:


  Originally Posted by *Ori* 
_The options you outlined make a lot of sense. Incorporate them into a multi-wafer rotary selector with one wafer for the power, so all the "controls" are on one knob.
 The only issue with a manual selector is that you may make a mistake and switch it from off all the way to the full-on position with no warm-up delay. The same can be done with relays and some circuitry that will force delays between steps, but that would be more complicated. You might want to 'recruit' someone familiar with microcontrollers, instead of doing this by analog means._

 

I think I'd rather just do it manually with the knob... simplifies things and it's not like anyone will be using the thing without my assistance anyways. Technology frightens my Wife... and the 'lightbulbs' will make it even worse.


----------



## Ori

Quote:


  Originally Posted by *philodox* 
_I think the analog readout would look great, sort of like a VU meter then?_

 

If you find a decent ham swap meet, you might be lucky and find real cool meters. My stash includes brand new US Navy meters from 1945 and a large 4x4 inches unknown unit in _real trendy_ Heathkit green. Now that's what I call *cool*!!! 
	

	
	
		
		

		
		
	


	



_I tried to attach a pic but it's beyond me..._




  Quote:


 I think I'd rather just do it manually with the knob... 
 

Sure, just _don't drink and drive_...


----------



## philodox

Quote:


  Originally Posted by *Ori* 
_If you find a decent ham swap meet, you might be lucky and find real cool meters. My stash includes brand new US Navy meters from 1945 and a large 4x4 inches unknown unit in real trendy Heathkit green. Now that's what I call *cool*!!! I tried to attach a pic but it's beyond me..._

 

That sounds very cool... If you wan't to send me the picture, my email is:

 jason DOT smyth AT gmail DOT com Quote:


  Originally Posted by *Ori* 
_Sure, just don't drink and drive..._

 

Luckily I don't drive... but if you checked out the recent Hamilton meet impressions thread, the drinking thing might be tough. 
	

	
	
		
		

		
		
	


	




 I might just have to put a warning label on it.


----------

