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Schade Feedback In A Push Pull Differential Amplifier?
Schade Feedback In A Push Pull Differential Amplifier?
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Old 17th May 2016, 10:34 PM   #1
mr2racer is offline mr2racer  United States
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Default Schade Feedback In A Push Pull Differential Amplifier?

As you add stages in an amplifier there is enough phase change between the input and the output so that negative feedback becomes impossible. The feedback becomes in phase and positive so the amp becomes unstable. I've built a few SE amps using Schade, or plate to plate feedback. Is there any reason you couldn't use this feedback in a differential push pull design?

Since the feedback loop is confined between the plates of the voltage amplifier and the output tube the phase difference is, I think, 180 degrees out of phase. But my real question is would this also work in a differential PP design?
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Old 17th May 2016, 10:35 PM   #2
SY is offline SY  United States
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Schade Feedback In A Push Pull Differential Amplifier?
See the 50W amp in the back of the RCA Tube Manual.
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Old 17th May 2016, 10:37 PM   #3
mr2racer is offline mr2racer  United States
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Thanks SY.
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Old 17th May 2016, 10:50 PM   #4
mr2racer is offline mr2racer  United States
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Sy, is there an electronic version of this manual available?
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Old 17th May 2016, 10:54 PM   #5
SY is offline SY  United States
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Schade Feedback In A Push Pull Differential Amplifier?
First hit on Google.

http://www.tubebooks.org/tubedata/RC30.pdf
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Old 17th May 2016, 11:29 PM   #6
mr2racer is offline mr2racer  United States
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Time to give up on Yahoo. Thanks again!
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Old 18th May 2016, 12:40 AM   #7
mr2racer is offline mr2racer  United States
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I've looked at the schematic and there seems to be a mistake in the description of the circuit? It appears to me that there is output tube plate feedback to both the plate and the cathode of 6CB6A. Via r21 to the plate of the 6CB6A and through r22 to the its cathode. I've included the excerpt from the RCA description. Am I missing something here?

What would be the purpose of feedback to both the cathode and plate of 6CB6A?
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Last edited by mr2racer; 18th May 2016 at 12:48 AM.
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Old 18th May 2016, 01:07 AM   #8
Wavebourn is offline Wavebourn  United States
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Schade Feedback In A Push Pull Differential Amplifier?
I use similar feedback in my Pyramid power amps, from anodes of output Gu-50 tubes to anodes of 6P15P driver tubes that work in a pentode LTP. I would not recommend to surround more than one stage by such a feedback, to avoid harder clipping on overload.
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Old 18th May 2016, 01:12 AM   #9
mr2racer is offline mr2racer  United States
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I wasn't thinking of using the global feedback at all. Wavebourn is there any reason to use feedback at both the plate and the cathode?
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Old 18th May 2016, 01:22 AM   #10
richardburak is offline richardburak  Australia
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Hi mr2racer,

It works, listening to one now - a pair of 8CS7 (dual dissimilar triode) per channel, line matching transformer (Altronics M1115) driving near field speakers either side of the monitor. The voltage stage is CCS/LTP, the power stage is CCS/LTP and the VA and PA plates tied.

Also see Baby Huey for an example of feedback.

Scary:
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