OK, lets clear this up a bit?
The 100K is the stock Express value and billyz runs the stock value. Since the NFB resistor, circuit-wise, is looking at the 5K Presence pot as the ground of the PI tail, its the ratio of these two components that determine how much energy from the OT is fed back to the PI. (BTW, this energy is now retuning to the circuit with a reversed polarity of what it was when it originated - important!) Which in turn either helps tighten things up (more signal fed back>smaller ratio), or loosens things up (less signal fed back>larger ratio).
(Also, when billyz says he prefers amps 'without any feedback', he is saying he likes an amp that doesn't have this circuit added. An AC30 is a good example of such an amp, as are a lot of older Class A amps, P/P or SE.)
So 100K/5K = 20:1 ratio, and 68K/5K = 13.6:1 ratio.
This means that increasing the NFB resistor to 200K would make the amp more 'ragged/loose' than the stock 100K value. Which means that the same amp with a 68K NFB resistor vs. a 100K value would be 'tighter' overall.
So when billyz says: "Regarding what I said about the NFB resistor. 100k will not get you a cleaner sound. You would go to a lower value like Dana mentioned." - what he is saying is that if you want a 'tighter' sounding amp, make the ratio of these two components smaller....
The component that billyz does talk about changing is the 'tail' of PI. He changes the stock value, 10K, to 22K. This change will make the PI have less 'gain' than stock. If you went to a 47K value, you would have less gain still, given the same PI.
OK, since we are here... Going back to the AC30, it has no negative feedback circuit. So here's an option for a kind of 'loose' sounding amp. But then its PI has the 47K resistor tail creating less gain than 10K resistor would in the same circuit. ?? Yeah, eventually came the TB channel, bumping up the preamp and making it something many people think is very cool. But before this, in its middle period, the non-TB version AC30/6 was a really quiet and also somewhat loose amp. Interesting combination and, of course, definitely the kind of girl Brian May wanted to take to the Prom...........
OK, you are bored, but I feel better.
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Most people stall out when fixing a mistake that they've made. Why?