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Local feedback between grid-cathode
Local feedback between grid-cathode
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Old 19th July 2019, 08:12 PM   #11
SpreadSpectrum is offline SpreadSpectrum  United States
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Have a look at the gain formula for an inverting op amp. Make Rin = 0.
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Old 19th July 2019, 09:10 PM   #12
MagicBus is offline MagicBus  Greece
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I see, thanks! Well, one question brinks another so, is the attached schematic a possible way to apply local feedback to a power stage with fixed bias when the driver's output impedance is low?
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File Type: jpg Local Feedback.JPG (47.0 KB, 261 views)
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Old 19th July 2019, 09:15 PM   #13
rayma is online now rayma  United States
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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 by rayma; 19th July 2019 at 09:19 PM.
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Old 19th July 2019, 09:18 PM   #14
MagicBus is offline MagicBus  Greece
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Great! Many thanks for your time!
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Old 20th July 2019, 09:35 AM   #15
DF96 is offline DF96  England
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Quote:
Originally Posted by DF96
it is actually just ordinary voltage series feedback
Sorry, I got that wrong; it is actually shunt feedback.

Quote:
Originally Posted by SpreadSpectrum
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.
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Old 20th July 2019, 10:15 AM   #16
indaco is offline indaco
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Thanks for enlighting me. I took a look on the link of Magicbus, something is clearer now...it seems to me to be "exotic" and poorly used though in hi-fi, as to reduce distortion compared to other ways (since you need to have strict conditions to respect).
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Old 20th July 2019, 11:07 AM   #17
JoeAlders is offline JoeAlders  Netherlands
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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.
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
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Old 20th July 2019, 12:18 PM   #18
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by indaco View Post
it seems to me to be "exotic" and poorly used though in hi-fi, as to reduce distortion compared to other ways (since you need to have strict conditions to respect).
Well there is the RH84 and derivates, build many times.
Mona
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Old 20th July 2019, 02:34 PM   #19
LJT is offline LJT  Norway
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Default 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?
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Old 20th July 2019, 03:12 PM   #20
MagicBus is offline MagicBus  Greece
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Quote:
Originally Posted by Ketje View Post
Well there is the RH84 and derivates, build many times.
Mona
Back to starting point RH84 seems to have plate to grid feedback without a resistor in series with the low Zout driver. How does this work?
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