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Old 14th March 2012, 11:32 PM   #11
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Quote:
Originally Posted by DF96 View Post
Temporarily remove the 39pF across the 10k feedback resistor, then try increasing the 10k until the amp becomes stable. This will give you an idea of how far off stability you are.

One option to try is splitting the 10k into two resistors, with a small cap across one of them.
I never do such things, with caps in feedback. They are good for oscilloscope pictures, no more. Otherwise it is harmful, requiring more of compensation for unity gain stability that is not needed. Also this cap feeds signals from cellphones to the amp's input.
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Old 14th March 2012, 11:52 PM   #12
BRSHiFi is offline BRSHiFi  United States
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So you are saying leave out the 39pF cap all together?
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Old 15th March 2012, 12:05 AM   #13
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Suggestions,
Make sure the 10R Anode stoppers are right on the tube socket pins.
Up the screen stoppers to 1K. Ultralinear Mode REQUIRES larger values of screen stoppers.
ASIDE: I had a 4 x EL34 HiFi Amp with 150R screen stoppers. It was fine in triode ande Pentode Mode but oscillated like crazy in Ultralinear Mode - increasing the screen stops from 150R to 1K fixed the oscillation in that amp.
Up the grid stoppers to at least 4K7 or even 10K

Cheers,
Ian
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Old 15th March 2012, 12:10 AM   #14
BRSHiFi is offline BRSHiFi  United States
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Working on it right now.

Installing 1k screen stoppers and 10k grid stoppers.
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Old 15th March 2012, 12:23 AM   #15
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Quote:
Originally Posted by kingneb View Post
So you are saying leave out the 39pF cap all together?
Yes. Forget about this nonsense.

Aand go this way:

Quote:
Originally Posted by Wavebourn View Post
Play with R12 and C31.

One more option is to add a loop across output tubes, like 240K 2W resistors from anodes of output tubes to anodes of driver tubes.
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Old 15th March 2012, 01:42 AM   #16
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The LTP second grid is tied to ground. My circuits has it biased to the other grid with a 1M and a .1u to ground. That ensures proper bias and LTP function.
Is it stable in triode mode?
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Old 15th March 2012, 02:08 AM   #17
BRSHiFi is offline BRSHiFi  United States
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Upped grid stoppers do not help.

As far as R12 and C31 go I have a 470pF cap in there right now and a 8.2k ohm resistor.

I tried a 10k ohm pot in there and swept it. The oscillation persisted. I will try a few more caps in there.
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Old 15th March 2012, 02:16 AM   #18
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What about RC grid to anode on the 12BH7s? Say 27p + 100k.
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Old 15th March 2012, 02:18 AM   #19
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I will try and see.
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Old 15th March 2012, 02:19 AM   #20
Salas is offline Salas  Greece
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Also U56 can be the culprit? Stick the probe at the cathodes node it sinks and see if there is some oscillation there?
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