Sorted.. Finally!
Well I installed the larger input capacitor, the choke, and determined that this killed all of the buzz on the amplifier output. It is dead silent now.
Did some further investigating with the scope and discovered to my considerable chagrin that I had somehow had swapped pins 1 & 8 on the 6J7 and nothing else. This had the unfortunate effect of running the 6J7 in zero bias mode which isn't linear above a certain signal amplitude and also caused the LF response errors I was complaining about in earlier posts. The amp worked despite this error - rather wish it hadn't, my error would have been so much more obvious.. All problems reported stemmed from this mistake.
Anyway the amplifier now cranks out 4.5Wrms over the entire audio band and all the way down to 20Hz. I have not redone the response measurements, but I assume they will be good when I do get around to doing it.
Noise is inaudible, (no discernible ripple on the output) and the amp sounds very clean and detailed. All reservations now removed. The amp doesn't sound sound like a typical triode SE, but it doesn't sound like a typical pentode amp either. It's quite good.
The schematic as shown in the final version is correct, and I will now disclose that the OPTs were made by Twins Audio and are quite excellent, and are exactly as they described. I can heartily recommend them based on what I am hearing!
Well I installed the larger input capacitor, the choke, and determined that this killed all of the buzz on the amplifier output. It is dead silent now.
Did some further investigating with the scope and discovered to my considerable chagrin that I had somehow had swapped pins 1 & 8 on the 6J7 and nothing else. This had the unfortunate effect of running the 6J7 in zero bias mode which isn't linear above a certain signal amplitude and also caused the LF response errors I was complaining about in earlier posts. The amp worked despite this error - rather wish it hadn't, my error would have been so much more obvious.. All problems reported stemmed from this mistake.
Anyway the amplifier now cranks out 4.5Wrms over the entire audio band and all the way down to 20Hz. I have not redone the response measurements, but I assume they will be good when I do get around to doing it.
Noise is inaudible, (no discernible ripple on the output) and the amp sounds very clean and detailed. All reservations now removed. The amp doesn't sound sound like a typical triode SE, but it doesn't sound like a typical pentode amp either. It's quite good.
The schematic as shown in the final version is correct, and I will now disclose that the OPTs were made by Twins Audio and are quite excellent, and are exactly as they described. I can heartily recommend them based on what I am hearing!
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Nice Kevin,
What voltages are you getting at the 6J7 plate and G2?
Have you considered regulating the 6J7 G2?
After playing with a Sun Audio(?) clone circuit which uses a // 7N7 driving a 6V6 I did something similar to you. It has since been torn apart to be rebuilt...its in the queue (famous DIY saying).
I am not a DHT fan, or perhaps I should say have not been particularly enamored by any of their ilk I have heard, including those which I built,
however, these small tetrode / pentode amps have a sound I really like.
Andrew
What voltages are you getting at the 6J7 plate and G2?
Have you considered regulating the 6J7 G2?
After playing with a Sun Audio(?) clone circuit which uses a // 7N7 driving a 6V6 I did something similar to you. It has since been torn apart to be rebuilt...its in the queue (famous DIY saying).
I am not a DHT fan, or perhaps I should say have not been particularly enamored by any of their ilk I have heard, including those which I built,
however, these small tetrode / pentode amps have a sound I really like.
Andrew
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Well I installed the larger input capacitor, the choke, and determined that this killed all of the buzz on the amplifier output. It is dead silent now.
Did some further investigating with the scope and discovered to my considerable chagrin that I had somehow had swapped pins 1 & 8 on the 6J7 and nothing else. This had the unfortunate effect of running the 6J7 in zero bias mode which isn't linear above a certain signal amplitude and also caused the LF response errors I was complaining about in earlier posts. The amp worked despite this error - rather wish it hadn't, my error would have been so much more obvious.. All problems reported stemmed from this mistake.
Anyway the amplifier now cranks out 4.5Wrms over the entire audio band and all the way down to 20Hz. I have not redone the response measurements, but I assume they will be good when I do get around to doing it.
Noise is inaudible, (no discernible ripple on the output) and the amp sounds very clean and detailed. All reservations now removed. The amp doesn't sound sound like a typical triode SE, but it doesn't sound like a typical pentode amp either. It's quite good.
The schematic as shown in the final version is correct, and I will now disclose that the OPTs were made by Twins Audio and are quite excellent, and are exactly as they described. I can heartily recommend them based on what I am hearing!
Reminds me of the time last week when I switched the signal and chassis ground pins in a two chassis project, took me 3 or 4 days to figure that one out.
Congrats, I'll post my progress once I start on this amp.
Athos
Completely, in fact the bass is very good. Almost all of the issues with the exception of the initial stability issue and output buzz can be traced back to my miswiring.
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Hi Andrew,
I had to remeasure the voltage, but the 6J7 plate is ~ 105V, and the screen is ~63V. The supply to the output screens and the driver stage is approximately 270V. Plate current is around 1.7mA.. As configured this driver will swing at least 80Vpp into a 270K load which is of course far more than required. I can't remember where I saw the graphs, but I remember seeing some that indicated that this tube at the particular operating point was pretty linear, and it sounds that way. I recommend it.
This thing performs well, and is only one of two SE amplifiers I own that work well with the Onkens which tend to be fairly merciless at revealing amplifier weaknesses. (The other amps found new homes)
I am listening to Bedrocks "Beautiful Strange" which amongst other things is a good demonstration of phantom imaging, (cut 3 in particular) and this amplifier seems to preserve those imaging cues.
I am pretty firmly in the DHT camp, but almost all my designs feature fixed bias as indeed this one does. I feel pretty strongly that almost any bypass cap in a traditional cathode bias setup is a serious sonic compromise, so I tend to use fixed bias a lot. (including low level phono stages and line stages)
I had initially planned on using regulated screens for the output tubes, but not for the 6J7 which apparently performs better with the screen AC coupled to the cathode which in this case is not at ground potential due to the unbypassed cathode resistor and the feedback connections.
Nice to hear I am not alone.. This was an embarrassing mistake, and in my eagerness to get the thing done I did not initially investigate closely enough. OTOH I did find the problem and rather quickly once I decided that something was decidedly not quite right.
I hope you do build one of these, I think you'll be pleased by the result.
Kevin,
I'm repairing a vintage Aussie MATON suitcase style guitar amp which is a 6J7 driving a KT61.
It is oscillating like a "bi.ch". Will no doubt solve that later.
For now - I've reviewed this thread but was uncertain what you did with the 6J7 Internal Screen pin 1. I initially had it grounded and then tried it connected to pin 8 (Cathode). It was much quieter with it wired to ground. What did you do with pin1?.
This did not change the oscillation problem at all. That I think is a lead dress issue.
Cheers,
Ian
I'm repairing a vintage Aussie MATON suitcase style guitar amp which is a 6J7 driving a KT61.
It is oscillating like a "bi.ch". Will no doubt solve that later.
For now - I've reviewed this thread but was uncertain what you did with the 6J7 Internal Screen pin 1. I initially had it grounded and then tried it connected to pin 8 (Cathode). It was much quieter with it wired to ground. What did you do with pin1?.
This did not change the oscillation problem at all. That I think is a lead dress issue.
Cheers,
Ian
Hi Ian,
I tied pin 1 of the 6J7 directly back to my ground buss through a short piece of wire.. I had a lot of oscillation problems and found that shielded wire, and careful lead dress particularly around the 6J7 were the key.. The suppressor is connected directly to the cathode, and the screen directly bypassed to the cathode. I have a 10K grid stopper mounted right at the grid cap. It is rock stable now.
I tied pin 1 of the 6J7 directly back to my ground buss through a short piece of wire.. I had a lot of oscillation problems and found that shielded wire, and careful lead dress particularly around the 6J7 were the key.. The suppressor is connected directly to the cathode, and the screen directly bypassed to the cathode. I have a 10K grid stopper mounted right at the grid cap. It is rock stable now.
The amp is working well, and after a couple of weeks of steady use I can state that it is reliable.
The bass is extended and well controlled with an authority when called for that belies the simplicity and small size of the amplifier. Imaging is relatively solid, sound stage has some discernible depth, and tonal balance is just a little forward, but warmer than I expected. The highs are clean and extended.
The overall results definitely seem to justify the effort I put into this project.
The bass is extended and well controlled with an authority when called for that belies the simplicity and small size of the amplifier. Imaging is relatively solid, sound stage has some discernible depth, and tonal balance is just a little forward, but warmer than I expected. The highs are clean and extended.
The overall results definitely seem to justify the effort I put into this project.
Hi Andrew,
I definitely consider this a keeper, and in a worst case scenario I could probably live with it, but I do prefer a really good DHT to this amp which honestly would cost about an order of magnitude more. This one offers quite a lot of bang for the buck, and with care can be built with all new parts for around $450.. It sounds very good, but ultimately has neither the depth/width of soundstage nor the overall resolution of my 300B, but it comes a lot closer than I expected. It does very good bass, recovers from clipping very gracefully, and manages to stay uncongested sounding even on very complex material at high levels. It probably is only the first experiment along this line - I would like to try a higher powered version with a KT-66, bigger supply, and larger OPT, bet that would be good for 10Wrms or more..
I actually started with the plan to regulate the screens, and recommend it, however a low drop out 250V regulator (the supply is 280V) would be required, and I found in the end that my supply is stiff enough that it wasn't needed.
I went through the try a zillion different cap thing not too many years ago, and came to the conclusion that I am not a fan of the PIO sound, so in answer to your question.. No.. I am a strong proponent of teflon dielectric caps for all coupling chores - that's just my preference based on a lot of listening experience and for the sonic attributes I prefer. This amp currently has Russian FT-3 teflon caps, and I am extremely pleased with the audible improvement they made over the polystyrene RTX I originally used. You'd have thought with all of that nested feedback (not to mention the high open loop gain needed to facilitate that) that the caps would have largely been inaudible, but they weren't..
I recommend it without hesitation, it sounds clean enough to work well with my all JBL horn mids and highs which says something.
I definitely consider this a keeper, and in a worst case scenario I could probably live with it, but I do prefer a really good DHT to this amp which honestly would cost about an order of magnitude more. This one offers quite a lot of bang for the buck, and with care can be built with all new parts for around $450.. It sounds very good, but ultimately has neither the depth/width of soundstage nor the overall resolution of my 300B, but it comes a lot closer than I expected. It does very good bass, recovers from clipping very gracefully, and manages to stay uncongested sounding even on very complex material at high levels. It probably is only the first experiment along this line - I would like to try a higher powered version with a KT-66, bigger supply, and larger OPT, bet that would be good for 10Wrms or more..
I actually started with the plan to regulate the screens, and recommend it, however a low drop out 250V regulator (the supply is 280V) would be required, and I found in the end that my supply is stiff enough that it wasn't needed.
I went through the try a zillion different cap thing not too many years ago, and came to the conclusion that I am not a fan of the PIO sound, so in answer to your question.. No.. I am a strong proponent of teflon dielectric caps for all coupling chores - that's just my preference based on a lot of listening experience and for the sonic attributes I prefer. This amp currently has Russian FT-3 teflon caps, and I am extremely pleased with the audible improvement they made over the polystyrene RTX I originally used. You'd have thought with all of that nested feedback (not to mention the high open loop gain needed to facilitate that) that the caps would have largely been inaudible, but they weren't..
I recommend it without hesitation, it sounds clean enough to work well with my all JBL horn mids and highs which says something.
Kevin,
Thanks for the detailed response. I took a look at your web page and see where you are coming from with the DHT amps, basically an all out assault with the hi end Japanese transformers and uncompromising power supplies et all.
I can see the difference in building costs now
Perhaps it is time I revisit the DHT thing. I gave up on them a few years ago. I tried IT and RC coupling, SE and PP, CLC and LC power supplies but nothing regulated, 300b, 2a3 and 6b4g's (the usual victims) were the tubes I tried.
I still have a pair of Softone RW-20 OPT's and some Electraprint iron as well.
I never tried a Tetrode / Pentode driver stage though, which I am seeing a bit of now, not sure if thats just fashion or there is some synergy. I might try an EL84 or something as driver, or perhaps a 6V6
Andrew
Thanks for the detailed response. I took a look at your web page and see where you are coming from with the DHT amps, basically an all out assault with the hi end Japanese transformers and uncompromising power supplies et all.
I can see the difference in building costs now
Perhaps it is time I revisit the DHT thing. I gave up on them a few years ago. I tried IT and RC coupling, SE and PP, CLC and LC power supplies but nothing regulated, 300b, 2a3 and 6b4g's (the usual victims) were the tubes I tried.
I still have a pair of Softone RW-20 OPT's and some Electraprint iron as well.
I never tried a Tetrode / Pentode driver stage though, which I am seeing a bit of now, not sure if thats just fashion or there is some synergy. I might try an EL84 or something as driver, or perhaps a 6V6
Andrew
This week-end the amp gets its moment of glory at the local audio club gathering at my place. It is now equipped with a pair of late 1930s Kenrad 6V6G.. Sounds good, but after having set it aside for a month or so when listening through the Onkens there was no mistaking it's pentode signature.
I also concluded that adding the triode/pentode switch was probably a step in the wrong direction as it is neither quite triode nor pentode, and lacks the individual strengths of either.. Perhaps it is just not the ticket for driving the Onkens, will report back next week. Source will be an ancient massaged PAS-3 and my diy TT..
I also concluded that adding the triode/pentode switch was probably a step in the wrong direction as it is neither quite triode nor pentode, and lacks the individual strengths of either.. Perhaps it is just not the ticket for driving the Onkens, will report back next week. Source will be an ancient massaged PAS-3 and my diy TT..
Well the amp was trotted out for the annual audio-fest I host, and to my considerable surprise triode mode and pentode mode were both used extensively depending on the type of music being played. The amp was quite popular and I received a fair number of complements. It was played mainly with a pair of 1970s vintage ESS AMT-1B which are surprisingly efficient, and the amplifier was a great match with these speakers in both modes of operation.
If it wasn't clear from the previous post the triode/pentode mode switch proved most useful, and triode mode sounded very good with all speakers tried.
I was of course busy hosting but observed that the general trend was towards pentode mode with rock and anything with a strong bass line, and towards triode for acoustic music, female vocals, jazz, and classical pieces except those with very powerful bass.
I did notice that the image depth seemed somewhat enhanced in triode mode on some material.
The amplifier sounded extremely dynamic driving both the Half Chili Changs and the ESS AMT-1Bs. The AMT-1B however sounds far better (light years) than the HCC even over the limited range where the HCC should have been good. The relatively poor performance of the HCC was one of the major disappointments of the event.
I was of course busy hosting but observed that the general trend was towards pentode mode with rock and anything with a strong bass line, and towards triode for acoustic music, female vocals, jazz, and classical pieces except those with very powerful bass.
I did notice that the image depth seemed somewhat enhanced in triode mode on some material.
The amplifier sounded extremely dynamic driving both the Half Chili Changs and the ESS AMT-1Bs. The AMT-1B however sounds far better (light years) than the HCC even over the limited range where the HCC should have been good. The relatively poor performance of the HCC was one of the major disappointments of the event.
ghpicard inquired:
The Russian 6П1П (6P1P) has similar operating characteristics to 6AQ5 and 6V6, but it is a 9 pin miniature, with a unique pinout, and as far as I can determine, it has no direct USA made "plug-in-and-play" equivalent. The Chinese 6P1 is the same thing. Their tube numbering system generally (but not always) folows the Russian system. One notable exception is the Chinese 6N4, which is a close equivalent to 12AX7.
Speaking of 6AQ5s and 6V6s, has anyone here encountered the meral 6V6s which contain a 6AQ5 and a 7 pin to octal converter base? These do exist, they were produced by some of the "lesser" tube companies in the late '40s and '50s.
Once I obtain another pair of 5K SE OPTs I'm going to try the J/J 6V6GTs on my modded Simple SE board, and give a listen to how they play under varying voltage / current conditions.
/ed B in NC
The Russian 6П1П (6P1P) has similar operating characteristics to 6AQ5 and 6V6, but it is a 9 pin miniature, with a unique pinout, and as far as I can determine, it has no direct USA made "plug-in-and-play" equivalent. The Chinese 6P1 is the same thing. Their tube numbering system generally (but not always) folows the Russian system. One notable exception is the Chinese 6N4, which is a close equivalent to 12AX7.
Speaking of 6AQ5s and 6V6s, has anyone here encountered the meral 6V6s which contain a 6AQ5 and a 7 pin to octal converter base? These do exist, they were produced by some of the "lesser" tube companies in the late '40s and '50s.
Once I obtain another pair of 5K SE OPTs I'm going to try the J/J 6V6GTs on my modded Simple SE board, and give a listen to how they play under varying voltage / current conditions.
/ed B in NC
Some favorite tubes were made in smaller envelopes. For example, 6П6С (analog of 6V6) was made as 6П1П. In USA 6AC7 reincarnated to to 6AH6, in Russia it's analog 6Ж4 was later reincarnated to 6Ж5П in 7-pin envelope. It was widely used in military equipment, so later reincarnated to even smaller tube, 1/2 inch one with flexible leads, 6Ж5Б.
Long quiet thread... Yesterday I took this amp to an audio fest where it was well received. It stayed in the system until I had to leave. The speaker system was based on a pair of Altec 604 (later, although I do not know the specific version) in what I recollect are MLTL cabinets. (I will check with the owner and post corrections.)
Strikes me that Electra-Print might be the best source for an output transformer, but Edcor makes something I suspect is comparable to the Twins Audio transformers I used, found here: https://www.edcorusa.com/p/537/gxse15-8-5k
Power transformer: https://www.edcorusa.com/p/598/xpwr009
Reasonable sub for 12H choke: https://www.edcorusa.com/p/776/xc75-15h-75ma
5H choke either Hammond PC-194D from AES or Edcor https://www.edcorusa.com/p/783/cxc100-5h-200ma
Note that if B+ is too high (more than 270V) under load it can be trimmed by adding a small power resistor in series with the HVCT.
Both of the 100uF electrolytics are bypassed with 0.47uF films (not shown) as noted earlier in the thread. Recommend 400 or 450V, 105 degree C rated electrolytic caps if possible.
Strikes me that Electra-Print might be the best source for an output transformer, but Edcor makes something I suspect is comparable to the Twins Audio transformers I used, found here: https://www.edcorusa.com/p/537/gxse15-8-5k
Power transformer: https://www.edcorusa.com/p/598/xpwr009
Reasonable sub for 12H choke: https://www.edcorusa.com/p/776/xc75-15h-75ma
5H choke either Hammond PC-194D from AES or Edcor https://www.edcorusa.com/p/783/cxc100-5h-200ma
Note that if B+ is too high (more than 270V) under load it can be trimmed by adding a small power resistor in series with the HVCT.
Both of the 100uF electrolytics are bypassed with 0.47uF films (not shown) as noted earlier in the thread. Recommend 400 or 450V, 105 degree C rated electrolytic caps if possible.
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