Down the Amp Repair Rat Hole

My buddy's amp (Hot Rod Deluxe) was sounding pretty weak and would intermittently squeal. In addition anytime his One Spot was plugged into a pedal that was plugged into his amp, anything else plugged into the One Spot or the power strip would hum like crazy.

So I figured, best place to start would be with replacing the tubes (he bought it used and it definitely needed new power tubes, and it sounded like the tube in V1 was microphonic.

So I drop some new tubes in and the amp starts to unravel. First thing I noticed was that when you turn the power on, you can feel it buzzing. Then after biasing it out, it played for all of about 2 minutes then went out. Something in the preamp section was not right, because if you plugged into the Power amp in, you got sound out of it, albeit at about half volume.

Then I plugged back into the input and it was working again. only to start getting really farty and crap out again. Now, when turning the power on, it crackles a bit when on standby. When you flip it off standby, after a few seconds you get a loud buzz and the voltage at the bias point spikes.

Any thoughts on where to start with this mess? My skill set tends more to the low voltage than the high, although I do want to dabble into amp building as soon as the kids get a bit older. So, I am looking at this as a good learning experience.

Buy a 'scope and signal generator, build a dummy load.

I recently fixed a Blues Deluxe that had a sandcast power resistor in the solid state power supply open up. Fender uses the absolutely worst and cheapest method of voltage regulation with barely acceptable ratings on some of the parts. I'd start by measuring pin 4 (V-) and pin 8 (V+) on one of the op amps to make sure you are getting +/- voltage there. Then check that the screen dropping resistor hasn't failed (a very common problem with Hot Rods).

Sorry for abandoning the thread, my home was burgled (don't worry, laptops, iPhones, etc. no guitars) then got hit with snowmageddon here in Chicago.

I will try to take some readings, but I might just have to pass this one onto someone more qualified, with mire time. Thanks for the responses.