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Old 23rd April 2009, 03:59 PM   #11
Minion is offline Minion  Canada
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Great , thanx a lot guys ....I"ll try to keep you posted on this project...
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Old 23rd April 2009, 04:18 PM   #12
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Zobsky used a 12AX7 in that manner and had some problems
with his wire layout and oscillation. (-100) * (-100) = (+10000).
Too much non-inverting gain for any part of it to leak back into
the input, even by accident. And accidents happen...

We ended up paralleling both halves of his 12AX7, such that
the wire mess was coupling back only negative polarization.

You could still get your full 10000 voltage gain with a cascode
of triodes and have all the advantage of an inverting output.
But would need a taller working voltage for the plate.

Or maybe unity invert yet again, with an opamp inbetween
the triodes.... That might get you there.

Or pay very close attention how you shield and route those
wires wherever gain is so high.
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Old 23rd April 2009, 05:06 PM   #13
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Hi kenpeter,
do you mean to say that if the input wires (actually, I think shielded cable would be appropriate) of the first stage are too close to the output wires (again cables) of the second stage, you'll induce oscillation?

I'm asking because I have essentially the same design in one of my amplifiers and I used to have this very high-pitch wistle-like sound when the gain was maxed out. I managed to tame it with a 51 pF cap from the plate of my second stage to the B+ line. Inother words I paralleled the Plate resistor with a 51pF cap. I noticed that it did suck away a little bit of high end, but not enough for it to be "dull".

In any case, thank you. I can't believe how much I learn every time I read stuff on diyaudio!
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