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Old 3rd March 2003, 12:35 PM   #1
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Default mmm.... zero-bias class B twin triode amp

Awaaaaaay back in 1974 I built this amp that had a 6N7 (actually metal VT96) output valve. I was fascinated with it because the output stage consisted of a driver / phase splitter tranny, the 6N7, and the output tranny. That's it!! No capacitors, no resistors, no bias supply, no nuthin. Just iron, copper and vacuum. The driver tranny was driven by a single 6A3. I just had to build it to see what it sounded like. Given the signal sources of the day it was really clean and smooth. Way, way better than what I could have expected given it's underwhelming simplicity. Has anyone around here ever built one of those?

I was thinking the other day about 6C33's - now there's a honker of a valve if ever there was one - and IIRC it has two triodes internally, but do they come out separately or are they paralleled inside the bottle? If they are separate then I reckon this would be a good candidate for this kind of setup. Of course the plate voltage would have to be significantly lower than normal. One drawback is the need for a driver transformer because the grids draw current when driven positive wrt the cathode (22mA peak at +41v wrt cathode for 6N7) and so I was thinking you could drive the grids from a normal RC stage with the help of a fet source follower. Not the purist approach but you probably wouldn't get put in jail for doing it. Ahhh.... if only I had a 6N7 and an output tranny nowadays...
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Old 3rd March 2003, 12:55 PM   #2
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Default RE:mmm.... zero-bias class B twin triode amp

Hi,

Quote:
I was thinking the other day about 6C33's - now there's a honker of a valve if ever there was one -
Cheers,
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Old 3rd March 2003, 02:58 PM   #3
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Quote:
Given the signal sources of the day
Huh??!?

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Old 3rd March 2003, 03:20 PM   #4
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I've drawn up a few 6N7 class B2 stages before. It doesn't take much grid current, especially given how far up the grid goes, so it's pretty easy to drive (compare to 12AU7, I dare you!).

- At the time I figured it would only be good for guitar use due to crossover distortion characteristic of class B operation... but we'll see! (Actually, with the load seen there - I think! - it's more like class ABB2. )

Tim
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Old 4th March 2003, 12:52 AM   #5
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Quote:
Originally posted by slowmotion


Huh??!?

No-name ceramic phono cartridge, junk turntable, *very* average recordings. 1974, remember. Good stuff was available but ordinary stuff was absolute rubbish. Many records IME were simply poorly done. Not the vinyl's fault, just that they wouldn't have sounded a whole lot better on most people's record players even if they were perfect. Wouldn't it be cool to take a CD player and a few discs to back then. And a few black frisbees (TM) too.
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Old 4th March 2003, 03:29 PM   #6
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I think I really started to get hooked on this stuff around 1974 -75,
so I had some of the "good" stuff , but you are probably right,
ordinary gear have got much better now. On the other hand,
most of the stuff that I use today could have been from before 1974.

On recordings, well some of my best sounding recordings are from
the '70s , but you are right, the average quality have gone up.
I must say, though, I think the music was much better then than now.
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Old 5th March 2003, 12:49 AM   #7
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Originally posted by slowmotion
I must say, though, I think the music was much better then than now.
That's for sure.

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