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-   -   Local feedback between grid-cathode (https://www.diyaudio.com/forums/tubes-valves/340415-local-feedback-grid-cathode.html)

indaco 19th July 2019 02:48 PM

Local feedback between grid-cathode
 
1 Attachment(s)
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.

DF96 19th July 2019 03:12 PM

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.

SpreadSpectrum 19th July 2019 03:33 PM

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!

rayma 19th July 2019 03:45 PM

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.

Mark Tillotson 19th July 2019 04:40 PM

Well it loads the output whatever...

6A3sUMMER 19th July 2019 06:00 PM

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.

kodabmx 19th July 2019 06:15 PM

Indeed. This is a real grounded grid :)
https://www.tubecad.com/2014/11/08/R...0Amplifier.png

SpreadSpectrum 19th July 2019 07:01 PM

Quote:

Originally Posted by 6A3sUMMER (https://www.diyaudio.com/forums/tubes-valves/340415-local-feedback-grid-cathode-post5857041.html#post5857041)
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? :eek:

MagicBus 19th July 2019 07:35 PM

Quote:

Originally Posted by indaco (https://www.diyaudio.com/forums/tubes-valves/340415-local-feedback-grid-cathode-post5856822.html#post5856822)
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

Quote:

Originally Posted by rayma (https://www.diyaudio.com/forums/tubes-valves/340415-local-feedback-grid-cathode-post5856888.html#post5856888)
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?

rayma 19th July 2019 08:02 PM

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.


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