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Local feedback between grid-cathode
Local feedback between grid-cathode
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Old 19th July 2019, 02:48 PM   #1
indaco is offline indaco
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Default Local feedback between grid-cathode

Hi,
I was investigating about generic feedback and its use and I ran into a particular local feedback loop between grid and cathode of a tube (triode, ecc.). I noticed it's employed especially in certain guitar amps. I wondered if it has any use in tube audio systems too. And how to calculate the the proper cut-off frequency for a generic case. Some informations of the subject are messy.
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Old 19th July 2019, 03:12 PM   #2
DF96 is offline DF96  England
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That appears to be feedback between anode and grid. Sometimes nowadays called "Schade" feedback, although it is actually just ordinary voltage series feedback. See any description of opamp inverting mode.
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Old 19th July 2019, 03:33 PM   #3
SpreadSpectrum is offline SpreadSpectrum  United States
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It really bugs me when people call parallel applied voltage feedback "Schade" feedback since O.H. Schade used series applied voltage feedback in his paper, but I guess at some point if enough people use a word incorrectly, the definition changes and they become correct. I'll go along with it I guess, but I don't have to like it!
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Old 19th July 2019, 03:45 PM   #4
rayma is online now rayma  United States
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To work, there must be an input series resistor, or at least a significant output impedance from the previous stage. If driven by a voltage source, the feedback resistor would have no effect.
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Old 19th July 2019, 04:40 PM   #5
Mark Tillotson is offline Mark Tillotson
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Well it loads the output whatever...
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Old 19th July 2019, 06:00 PM   #6
6A3sUMMER is online now 6A3sUMMER  United States
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SpreadSpectrum,

Thanks for enlightening me about the mis-nomer of applying the term "Schade" feedback.

I have a similar gripe, those who call a long tailed phase splitter a Grounded Grid Amp.

I'm just an old school person.
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Old 19th July 2019, 06:15 PM   #7
kodabmx is offline kodabmx  Canada
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Local feedback between grid-cathode
Indeed. This is a real grounded grid
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Old 19th July 2019, 07:01 PM   #8
SpreadSpectrum is offline SpreadSpectrum  United States
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Quote:
Originally Posted by 6A3sUMMER View Post
SpreadSpectrum,

Thanks for enlightening me about the mis-nomer of applying the term "Schade" feedback.

I have a similar gripe, those who call a long tailed phase splitter a Grounded Grid Amp.

I'm just an old school person.
People call an LTP a grounded-grid amp?
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Old 19th July 2019, 07:35 PM   #9
MagicBus is offline MagicBus  Greece
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Quote:
Originally Posted by indaco View Post
Hi,
I was investigating about generic feedback and its use and I ran into a particular local feedback loop between grid and cathode of a tube (triode, ecc.). I noticed it's employed especially in certain guitar amps. I wondered if it has any use in tube audio systems too. And how to calculate the the proper cut-off frequency for a generic case. Some informations of the subject are messy.
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Quote:
Originally Posted by rayma View Post
To work, there must be an input series resistor, or at least a significant output impedance from the previous stage. If driven by a voltage source, the feedback resistor would have no effect.
Yes, this is what J. Broskie - among others I guess - also points out More Hybrid-Amplifier Design Although I understand the high source impedance mechanism and the related formulae, I wonder why in the low source impedance scenario the feedback resistor won't have any effect. OK, the formulae won't apply, but still a feedback loop. What do I miss?
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Old 19th July 2019, 08:02 PM   #10
rayma is online now rayma  United States
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A very low source impedance (with no series resistor) shorts the feedback signal current to ground,
instead of developing a feedback voltage proportional to that current.

Last edited by rayma; 19th July 2019 at 08:09 PM.
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