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Roscoe Primrose
PostPosted: May 1st, 2020, 6:57 pm 
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Pelliott321 wrote:
So every tube in the amp has this local feedback. Seems like a bad choice of words


No, that's why we (usually) use cathode bypass caps. They short the cathode to ground at AC, so no degeneration...

Roscoe

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tomp
PostPosted: May 1st, 2020, 7:32 pm 
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Stuart Polansky wrote:
At first, it appeared to be Tom P., because he uses real purty words. But by the end it was obvious. It's Steve, isn't it?


If it is hollow state it would never be me. I gave that stuff up eons ago.
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HAL
PostPosted: May 2nd, 2020, 4:25 pm 
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If the cathode resistor has been bypassed with a capacitor, then the feedback maybe 0 in the audio range and just there for DC biasing the grid negative relative.
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SoundMods
PostPosted: May 2nd, 2020, 4:59 pm 
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Roscoe Primrose wrote:
Pelliott321 wrote:
So every tube in the amp has this local feedback. Seems like a bad choice of words


No, that's why we (usually) use cathode bypass caps. They short the cathode to ground at AC, so no degeneration...

Roscoe


Plus the cathode bypass enables the tube to run at full gain.

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Pelliott321
PostPosted: May 2nd, 2020, 7:21 pm 
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maybe I am asking the wrong question
what is degeneration
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SoundMods
PostPosted: May 2nd, 2020, 8:19 pm 
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Pelliott321 wrote:
maybe I am asking the wrong question
what is degeneration


Maybe this will help:

https://www.google.com/search?client=fi ... n+feedback

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Pelliott321
PostPosted: May 2nd, 2020, 9:16 pm 
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I just never saw this type of feedback before, just the global type from output tranny to grid of first tube.
I wonder what the advantages or disadvantages are for each type
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SoundMods
PostPosted: May 2nd, 2020, 9:41 pm 
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Pelliott321 wrote:
I just never saw this type of feedback before, just the global type from output tranny to grid of first tube.
I wonder what the advantages or disadvantages are for each type


Unless the tubes have a grounded cathode and fixed bias at the control grid there is no local feedback. When the bias is set by a cathode resistor, you've got local degenerative feedback. Like Roscoe pointed out -- if the cathode resistor is bypassed with a capacitor the cathode is essentially tied to ground but only for the AC signal. You still have the cathode resistor doing its job to provide a bias differential, but no local feedback. Most tube amplifiers will have cathode bias on the line-level gain blocks and the attendant "local feedback." Except maybe the first stage that gets the global feedback feed tied to the cathode.

The subject should have never come up in the first place. This is one of those unsung-hero design features that essentially go un-noticed and taken for granted. It is what it is unless you are designing from scratch like our friend David Berning.

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Pelliott321
PostPosted: May 3rd, 2020, 7:27 am 
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Hey I am just trying to learn here
I did not understand the quote so I asked the question
I think I have a better understanding now and I thank you guys for your answers,but I do not like being belittled for asking a question
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Stuart Polansky
PostPosted: May 3rd, 2020, 8:57 am 
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Local feedback does not have the negative effects that are ascribed to global feedback , as you have described, Paul. At least in my admittedly limited experience. Actually a great question you asked, that many here probably didn't know the answer to.

I learn a lot here by reading threads like this. So, thanks for asking.
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