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Old 15th March 2012, 01:23 AM   #21
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Originally Posted by kingneb View Post
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.
Did you try removing the 39p cap that Wavebourne suggested?
I never understood why people add those caps either TBH.

But there again my general comment about your amp is 'Good God how many tubes, CCS's, capacitors and transformers does that feedback loop travel through?!

I'd scrap the global feedback entirely and just use localised ones instead.
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Old 15th March 2012, 01:26 AM   #22
BRSHiFi is offline BRSHiFi  United States
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Yes, all my tests were without the 39pF cap.
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Old 15th March 2012, 01:34 AM   #23
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The sine wave shows up at the cathode as well on the 12BH7, amplitude smaller of course.
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Old 15th March 2012, 01:37 AM   #24
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Quote:
Originally Posted by kingneb View Post
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.
Increase C31 until it stops. If dies not, you have some ground loop.
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Old 15th March 2012, 01:38 AM   #25
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Quote:
Originally Posted by Globulator View Post
Did you try removing the 39p cap that Wavebourne suggested?
I never understood why people add those caps either TBH.

But there again my general comment about your amp is 'Good God how many tubes, CCS's, capacitors and transformers does that feedback loop travel through?!

I'd scrap the global feedback entirely and just use localised ones instead.
As I suggested before, additional local loop may help.
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Old 15th March 2012, 01:40 AM   #26
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Will try some larger caps. I have always been under the impression that ground loops cause low frequency instability.

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As I suggested before, additional local loop may help.

So you mean change the design so a local FB loop runs to the phase splitter instead of the global?

Last edited by BRSHiFi; 15th March 2012 at 01:42 AM.
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Old 15th March 2012, 01:53 AM   #27
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Quote:
Originally Posted by kingneb View Post
Will try some larger caps. I have always been under the impression that ground loops cause low frequency instability.

Quote:
As I suggested before, additional local loop may help.

So you mean change the design so a local FB loop runs to the phase splitter instead of the global?
No, I am suggesting to add one more, internal loop, through 240K 2W resistors from anodes of output tubes to anodes of driver tubes. Short parallel feedback by voltage across output tubes only. It shifts up pole caused by output resistance of driver and Miller capacitance of output triode. And it does not involve parasitic loss inductance of transformer.
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Old 15th March 2012, 02:01 AM   #28
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There's a general procedure that allows you to actually see what you're doing, rather than just shooting in the dark, and it requires a dual-trace scope. Don't say you don't have one - you need one to do the level of work you're wanting to do.

The feedback network is disconnected at the input cathode, then loaded with a resistor equal to the load it would normally see (cathode resistor in parallel with 1/gm). One scope probe on amp input, the other on the new feedback output. Observe phase shift at various frequencies and various loads while varying lead and lag options inside the amp. Some people can work with a square wave of about 10 to 20 KHz (easily made by any computer sound card) but that takes practice and luck. Best learning and realest results come from actually plotting frequency vs. phase shift. It's easy, fun and answers your questions FOR REAL.

All good fortune,
Chris
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Old 15th March 2012, 02:07 AM   #29
BRSHiFi is offline BRSHiFi  United States
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I have all the equipment.
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Old 15th March 2012, 02:11 AM   #30
Salas is offline Salas  Greece
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Quote:
Originally Posted by kingneb View Post
The sine wave shows up at the cathode as well on the 12BH7, amplitude smaller of course.
Is it easy to damp between U56 and the cathodes with a small resistor just in case the 334 chip is acting up?
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