Have a Marshall 8100 with a dead overdrive channel.  Have plugged into return loop and power amp is ok...so its in preamp.
Have tested the 12ax7. Also tried several known good tubes.

Whats next?   I have whatever gear is needed to test.

Great forum. So glad it is here.  Thank you in advance.
I was playing the other night and my Fender amp started acting up.  Loud humming noise and low output of music as it warmed up.
I can't afford a shop right now.  I have been home injured for 19 months, so any help would be most appreciated.
Where do I start?  Willing to follow directions explicitly.
The amp is  a Fender  Chorus.

Again, thanks in advance.

David