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Walt

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Walt

Stuart you are a gentleman
thanks

One interesting note is, let's say, you were to start with an amplifier that had loop feedback from the sec of the output transformer brought back to the input stage cathode. Now let's say the second stage had a cap-bypassed resistor to run that second stage at the highest gain. You would then have an amplifier with a certain amount of overall (global) feedback. Now let's remove that bypass cap in the second stage. This would reduce the open-loop gain and would reduce the global feedback but shift some feedback to local where there was none before (at least in the second stage). One thing that would happen is the output impedance would increase. Another would be likely increased stability, although not guaranteed if a bad design fundamentally. Both of these and possibly other effects such as overall bandwidth would likely change the sound, better to some listeners, worse to others.

Paul, to answer your question, the term degeneration is used to refer to degrading the gain of the tube by effectively reducing transconductance. Interestingly, when a tube's transfer characteristics are observed on a curve tracer, you get one set of curves. Now add a resistor in series with the cathode and those characteristics will look similar to those of a bad tube. On close inspection the spacing between the various curves will improve in linearity. But a certain amount of this will happen with a bad tube in a rather uncontrolled way, but superficially they appear very similar.

Thanks, David for putting this bed.
It is certainly more understandable that amp design is not a walk in the park. These very subtle things is probably why some amps sound great and others not.

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Walt

I have actually seen this topic come up frequently, when people are discussing cathode followers and where degenerative feedback reduces gain to (effectively) unity.