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Old 1st October 2010, 02:20 PM   #71
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Hi Kevin,
In the final analysis do you consider this amp a "keeper" and is it a sound you think you could live with in lieu of a DHT?

Any plans to regulate the 6V6 screen or any other modifications / part substitutions. Would you consider trying pio coupling caps?

Thanks

Andrew
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Old 1st October 2010, 04:20 PM   #72
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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.
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Old 1st October 2010, 05:10 PM   #73
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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
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Old 1st October 2010, 05:23 PM   #74
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And be sure to use fixed bias on those 300B!
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Old 11th November 2010, 05:35 AM   #75
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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..
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Old 14th November 2010, 03:09 PM   #76
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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.
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Old 14th November 2010, 04:29 PM   #77
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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.
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Old 23rd November 2010, 09:00 PM   #78
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ghpicard inquired:

Quote:
Isn't the 6П1П also a close (but smaller) one? Just like the 6AQ5 used to be to the 6V6GT...
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
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Old 23rd November 2010, 09:11 PM   #79
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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.
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Old 17th November 2013, 07:45 PM   #80
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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.
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