For the sake of anyone who might come across this thread in the future with similar problems, thought I would update to say I finally sent it off and got it fixed by
Lynn Rochester (Ampworks) near Greenville SC for $120. Working like a charm now, and I'll just post our email exchanges below for info about what he found and did:
"I think the problem started with the two 6.3 volt supply lines from the transformer to the filaments of the tubes (filaments are what lights up). In my 41 years in business, I never gave 6.3 volts to light the tubes and pilot amp much thought. But a few years ago, everyone started putting all of the transformer wires into multi-pin connectors and trouble started. Yours is wired like that. The 6.3volt wires would cause things to burn if they were not simply soldered directly to the tube sockets. The first warning: Fender started putting a 10-amp fuse in line with the wires on their 65 Twin Reverbs.. The fuse holder would melt off the circuit board every time. In Bugeras, Egnators, and many other brands, the connector would melt every time. The fix has always been to simply bypass the connector somehow, at least as far as those two wires go.. I often shutter to think that for many years, I was building small low voltage relay circuits for channel switching. I was often holding the board I'd made in my hand, plugged in, and turned on. It might have been only 6.3 volts but there was 10 amps pulled through it.. The amplifier fuse isn't even more than 3 amps. I went back and checked your connector which is not visibly burned. Sure enough, all the damage was directly connected to those two wires. The pins inside the connector were lightly scorched. i could simply clean them. But it won't last. The correct fix is to solder the wires to someplace solid, straight to the tube sockets.. That should prevent this problem from recurring again. I'm trying to figure out the best place in the amp to solder the two wires."
"There is one fuse inside connected to the filament circuit. It is a small 5-amp fuse. It was not blown.Traces on the underside of the PCB were beyond that were damaged. If anything serious went wrong, it might blow as it should but that is rare. The amp would still turn on but the tubes would not glow-the problem you had this time. Caution: with the tubes not turning on, the high voltages will not bleed off, at least not quickly. The fuse holder is safe to touch under those conditions but not the tube socket pins.
I would like to strongly advise that you get rid of the JJ6V6 tubes and get Genalax Gold Lion 6V6s. You'll notice an improvement in the sound immediately. They'll sound better and last a whole lot longer. They cost a little over $60 for a matched pair. My experience with JJ 6V6s is an 80% failure rate in the first three months, 50% bad right out of the box. They are horrible. They may have even been a contributing factor in the failure this time. I did play it for a few hours with no problems. But nobody plays it the way you pay it. And I just don't trust those tubes. Fender uses them and labels them as Groove Tubes. They have a lot of failures while still under the 90-day tube warranty."
