Hi Mr. Bo; okay ...
I'll take Gibson amps for $300 please :wink:
Easy points first; the tone control is common to both channels; changing to the instrument jacks won't make a difference. I note on your schematic the 120K and 22K resistors on the instrument jacks in lieu of the 47K grid-stoppers on the schematic; somebody's effort to get a little tone distinction among the jacks?
The input of the microphone and instrument channels are wired like several other Gibson models including the GA20T ... capacitor input with 'grid-leak' bias ... connecting a resistor from the tube section's input across to the cathode/ground instead of 56K-68K grid stoppers, +/- 470K grid-to-ground, and a 1.2K-2.2K cathode bias resistor. The difference between the values of the input capacitors (.01 v. .05) is all you will hear by changing from the mic to the instrument channel.
Some charming chit-chat from the GA6 Owners Manual about the tone control and mic channel
(from: HarpAmps)
Gibson's suggestion is rolling off all the treble when playing on the neck pickup and rolling off all the bass when on the bridge ... although nobody knows what in the heck Gibson was thinking when it references you and three buddies all jacked in and 'blended' ...
The tone cap: the principle is that small-value caps block low frequencies in the signal and large-value caps block high frequencies. The cap and pot are supposed to bleed off whatever it is you don't want so ... to diminish the bass by bleeding it off, your suggestion to go to .02 is headed in the right direction ... relatively speaking, the bass will more easily pass through the .02 and the treble, finding resistance at the cap, will turn around and head back towards the line.
In fairness to Gibson, they think you're Wally Cleaver and you and your friend Dobie Gillis are going to be entertaining at pool-side ice cream parties where clean and woody - but dark by modern standards - was the desired tone ... without any of those nasty, distorted 'wolf tones'
(Ted McArty). Finally and maybe before you mess with the tone cap, the 12AY7 preamp tube has an amplification factor of 40; a 12AT7 an AF of 60 ... both have the same pinouts and are drop-in substitutes. You might try a 12AT7 there to produce a gainier tone ... won't change the amp's frequency response but might perk up those instrument jacks.