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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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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.
You can find information here The Valve Wizard

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?
 
Yes, that's how to do it. You need to consider the DC current through the feedback resistor, and whether it changes the operating point of the first stage (if there is no coupling capacitor in series with the feedback resistor, as is the case here).
 
Last edited:
DF96 said:
it is actually just ordinary voltage series feedback
Sorry, I got that wrong; it is actually shunt feedback.

SpreadSpectrum said:
I'll go along with it I guess, but I don't have to like it!
Yes. I don't like the term "Schade feedback" because there is nothing special about it; it is just normal feedback. It linearises the output stage at the cost of delinearising the previous stage, which on balance may be worth doing.
 
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.
Have a look at this thread perhaps it is of some help:
Trying to solve a Plate Follower Problem

Especially the second pdf file on page 1
 
Transfomer Impedance.

So, given the discussion above, my understanding is that when using local feedback on the output tubes reduce the "effective" plate impedance.

I guess the same effect will be present when using CFB or feedback from output plates to input sage (E.g. Baby Huey)

Will such use of local feedback have any effect on choice of transformer ratio/ plate load?