AB165>Express

[Smokebreak]

I've built an amp for a friend that loves the AB165 circuit. .I couldn't resist just going ahead and doing the whole AA864/AB165 best of both worlds thing that seems to be common. The pic is what I built. It sounds good, and way too loud, but I can't get it to him for a few days, which means it will probably become a few other amps in the meantime.

I couldn't help but notice the staging of the AB165 is very similar to the Express. I'm not going to build him an Express, but at this point I would like to borrow ideas from both of these amps.
So, far, Ive only changed the interstage between 2nd and 3rd stage to be like an Express, and it immediately acted like one....on the brink of insanity, V1 tube shows it's true colors, and totally rockin. All the funky local FB stuff from AB165 is removed, too, at this point

Now I'm at the global NFB and PI, and I'm not sure what I want to do, and not exactly sure what my question is, but hopefully some of you will see my confusion and chime in. I do know that I want to keep a lot of NFB, like the Fender, and maybe add presence pot, instead of fixed shunt

So Express has 100K FB resistor, 10K tail, and 5K presence. I f you were to ignore the frequency shaping of the presence(remove the cap), and replace the pot with a fixed value, 2.5K would be appropriate, right?...for the purpose of inserting the NFB?
I like the huge amount of NFB in the Fenders, though(for this amp), which has 820r FB, 22K tail, and 100r shunt tail.

So I guess I'm confused about why the ratio of tail>shunt is so massively different in these(and most BF/SF) Fenders as compared to most other amps with a NFB/presence setup. I mean, if I were to put a presence(scratchy style) on a typical BF, would I really use a 200r pot??

[billyz]

See the longhorn bassman mod on my website.

[Smokebreak]

Yes! This is what I'm talkin about. How did you come to the 2.2k at the bottom of the tail? Is that an approximation of a 5k presence pot halfway up? And does that(5k/47K)yield the same results as the original BF 100r/820r (50x multiplier)?

[billyz]

For me it's a ratio (20:1) approximate and using what ever I got in the parts bin . when doing a nfb mod , I usually start with whatever is already
In place and just change one resistor to get whatever ratio I'm looking for. Often I like 20:1 for spanky rock tones and nasty blues. For country maybe less. Jazz = 10:1 or less. Bass = blackface spec.
These are for medium gain amps. For lower gain and small amps I often use no nfb.
Of course it is the clients taste that ultimately dictates the tone mods I do.