Running ST-70 in Class A

knowledge base for the classic Dynaco ST70

Postby burnedfingers » Tue Feb 16, 2010 5:45 am

quote:
It is close. The primary impedance is 4300 ohms. There is a short thread on diyaudio, talking about using 6L6s and the A470.

The 6L6GC's and 6BG6GA's will work fine in the 70 as will a few other tubes.

I personally think you are putting too much stock in the class A1 claims of that particular manufacturer.

If you are talking class A1 then address it as A1 and not A. There is a difference.

Without mentioning names here there are some manufacturers that seem to deal in hype and not facts. I've heard the board in question and wasn't impressed. If distance wasn't a factor I could set up a listening A/B test against it and a modified 70 that has a good driver board and power supply. The front end in my opinion has a lot to do with the sound of an amplifier and there are clearly better front end designs than the one you are thinking about.
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Postby Slartibartfast » Tue Feb 16, 2010 6:38 am

You sound like the one, who is hung up on "nomenclature". "Class A" assumes Class A1, as most amp designs find grid current objectionable.


I have no doubt that the driver board has a huge effect on the sound of the amp. In time I will install a different board. What I was looking at, was just a starting point for experimentation.
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Postby Slartibartfast » Tue Feb 16, 2010 6:39 am

TomMcNally wrote:
Slartibartfast wrote:

Calling it Class A at the beginning of the thread sent everyone
on a wild goose chase.


nevermind. when I say Class A, I am referring to Class A1. Who wants positvie grid current flowing in an audio amp? I did not think anyone would have a problem figuring that out. Let's not all get pedantic, okay?
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Postby Slartibartfast » Tue Feb 16, 2010 8:37 am

Even Langford Smith loosely uses both terms, when writing about Class A (A1) Push Pull Triode amps.
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Postby mesherm » Tue Feb 16, 2010 11:11 am

I have used El34s in single ended class A1 by using a high voltage 3 terminal regulator (TL 783) as an adjustable current sink in series with a 10 ohm resistor. I have also used them in the same manner for a small PP amp and in both cases the sound was quite good.
Each TL783 has to be bypassed with a large value cap just as if it was a cathode resistor but once adjusted, the idle current stays rock steady even if you change tubes. The TL783s need to be heat sinked however so I'm not sure how all that extra stuff could be fitted into a stock ST70 chassis. The bias circuit could be removed as it is not needed.
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Postby EWBrown » Sun Feb 21, 2010 3:25 pm

I've generally used "Class A" for what is really Class A1, just being lazy, I guess... Grid current is essentially "evil" (666) in out intended purposes

The Ixys IXCP 10M45S is also a good cathode current regulator, for Class A1 (OK, I'll say it) operation, these take a lickin' and keep on tickin', easy to use and relatively cheap. Make good plate ccs regulators too, for VA and driver circuits. If you're into HV there is also a 900V version, IXCP 10M90S.

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