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NFB net gain or loss

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Here's a formula for output resistance (with feedback) given output resistance (without feedback), the feedback loop gain, and the voltage gain of the enclosed stages, but I'm not able to apply it.
 

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PRR

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I'm not going to try math.

That formula does not separate naked source impedance from load impedance. The load varies a lot (why we may want damping).

In practice, the change-of-gain is more significant than the change of damping. Be prepared to make-up gain another way.

Here's a 32 Ohm source feeding a "8 Ohm loudspeaker" load. I scaled the open-loop gain and impedance so it would approximate a gain of 10 into an 8 Ohm resistor (I may have approximated too much). As we change NFB gain drop very significantly. And the bass-bump shrinks from 9dB to 3dB to 1dB.
 

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LetItGrowTone,

6L6 plate resistance, rp, is about 22k Ohms in Beam Power mode, just like your schematic.
For example, lets use a 6k plate to plate output transformer.

We have 22k Ohms + 22k Ohms driving 6k Ohms (44k driving 6k).

6k / 44k = 0.136
The damping factor will be 0.136 (far less than 1).

Well, that sounds like many speakers are going to need the 6L6 Beam Power amp to have quite a bit of (global or output stage local) negative feedback.

The same push pull amp with 6L6s in Triode Wired mode has plate resistances, rp, of perhaps 1.3k.
6k / (1.3k + 1.3k) = 2.3
The damping factor is 2.3, even without any negative feedback.
It will be a little less than 2.3 because of the DCR losses in the primary and DCR losses in the secondary of the output transformer.
And of course, you will get less power out with the 6L6s in Triode wired mode.
 
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