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"so as you increase the value of the FB resistor the effective plate load approaches 25K, thus an increase in gain. If you decrease the FB resistor, you lower the effective plate load which = less gain." Apologies if I'm over thinking this....
You are over-thinking.
The in-circuit plate resistance is never the value on the data-sheet. (Find a triode sheet which plots Rp against current.)
In triodes, wide change of load has little effect on gain.
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just trying get a good grasp on how to break down single stage FB.
Don't get specific (yet).
The idea is: you have an amp with "infinite" (or very-high) gain.
You put a 10:1 divider from output to input, it has gain of 10 (or very nearly 10).
So by diddling that 220K-330K-470K resistor, you are changing the NFB division, and (as long as no-NFB gain is greater than NFB ratio) the with-NFB gain.
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...how to break down single stage FB.
It is not "single stage" (if we are looking at the plan Doug posted in #13). Gates' drawing-style is more pictorial than analytical, and I'm not going to re-draw it, but I've shorn some irrelevancies:
V3 is a Pentode with signal input at grid and NFB input at cathode. Its plate drives V4, a fat triode. Signal path is blue line. NFB runs from V4 plate to V3 cathode, red line.
What is gain of V3? Probably 100. What is gain of V4? Probably 10 or 13. Total (no-NFB) gain, over 1,000.
What is NFB ratio? 220K/750 293. (Well, +1 or 294.)
The forward gain is 3 or 4 times higher than the NFB ratio.
If the forward gain were more like 100,000 (chip), 300:1 excess gain, then the closed-loop gain would be very-very close to 294. No matter if the actual gain were 50,000 or 500,000, we'd still get 292-294.
We only have excess gain of 3 or 4. Closed-loop gain will be closer to 230 or so.
Change 220K to 470K. NFB ratio is 470K/750= 627. Open-loop gain hardly changes (V4 gain is all about its Rp and load, the 220K-470K change is nothing). Still about 1,000. That is greater than 627, but not by a lot, less than 2:1. Closed-loop gain will be around 400.
Changing 220K to 470K has made closed-loop gain go up almost 5dB. The original value reduced THD 3 or 4 times, the higher value reduced THD by half. The original reduced output impedance by 3 or 4, the revised by only half. The higher THD may fail some arbitrary broadcast spec. The higher output impedance may allow a level-jump if you drive two loads and one of them falls off. None of that is important when using vintage gear today.
The original network was 220K+0.05uFd. So it isn't 220K to infinitely low frequency, the ratio rises below 14Hz. Alone, that might be bad. But we have other roll-offs. It may-or-may-not be that the 220K+0.05u was selected to cancel some of the coupling and OT roll-off. In this case, that probably does not matter. even if they were trying to fudge the 32Hz and 20Hz response for broadcast tests or cascade use, such errors are unimportant for a single "flavor" amp in modern studio use.