Tremelo not working on my Gibson GA-20T

[Curbdog]

I have this amp and the tremelo is not working. When I step on the footswitch, I get a second or two of tremelo, and then it fades away very quickly. I have replaced both the 5879 and the 6SQ7 pre-amp tubes to no avail. Any ideas about what I should do next to try and find the problem? I have attached the schematic. Thanks.

[Firestorm]

Replace the shared cathode bypass cap to make sure the oscillator tube has enough gain to function. Then replace the .047 caps in the phase loop to rule out the chance that one or more is leaking current. One of those steps usually does the trick.

[Curbdog]

Ok, thanks, I will sort out the tremelo problem.

This old amp has been hacked a bit over the years, but I am getting it back into shape.

The microphone channel has some weird contact bias circuit going on, and doesn't sound good for guitar.

Any suggestions on how I might modify this channel to be better for guitar?

Any other suggestions on how to improve this simple circuit for guitar? I understand that some of the Gibson circuits leave something to be desired... Thanks !

[Firestorm]

On the Mic. channel: loose the .01 input caps and either leave the grids naked or stick 33k resistors on 'em (or wire them Fender style with paralell 68K grid resistors). Get rid of the 10M grid leaks and replace with 1M. Rebias V1 with an 820R, bypassed by 25uf (or more) cap. That'll get you somewhere. Then try replacing the 12AY7 with a 12AX7. Somewhere in this mix, you should find a guitar sound you like.