Hi Z: hold the beer for now (burp ...). Of course it could be this ... or ... it could be that ... or ... it could be this and that ... or ... it could be something else. :wink: If the come-and-go is accompanied by staticy snap, crackles, and pops, it's likely dirty tube pins and sockets. If not and it's just a guess but it sounds like one or more sections of the primary multi-section can capacitor [the reverb section has its own can cap] are weak or one of the cathode resistor bypass caps is giving it up.
The sections of the power supply can cap filter or smooth the DC by charging and discharging. The cathode resistor bypass capacitors are also electrolytic ... filled with an acidic paste ... and, like filter capacitors, wear out or dry out from years of use, abuse, or extended lack of use. If a cathode bypass cap fails, it can provide the signal a path to ground around the cathode resistor effectively grounding out the signal. The rest of the amp ... including the power supply caps can be fine but the signal has been grounded. The power supply and cathode bypass caps share some of the properties of failing batteries; they work but then have to recharge and then function for a while longer.
The exact replacement power supply can cap is out of production but this one is pretty close:
2 40uf sections and 2 20uf sections / 525vdc; the original is 4 sections at 40uf. The difference between 40uf and 20uf is immaterial; those sections filter power for the two preamp supplies - any tone distinction would be immaterial and at no reduction in safety. That model also uses a single 40uf/450V cap that sits in the space between the rectifier socket and the transformer ... like
this one ... Some creativity needed to figure out how mount/stabilize it. If you post pics of the underside of the chassis, I can spot the two (or three?) cathode bypass caps.
Route 66: If you go this route, the original can cap has 3 or 4 resistors tied to its hot posts; they have to come off and be remounted or you have to buy fresh ones. Since any one of these could either be intermittent or drifted out of tolerance and since they are cheap and since you are re-building the power supply anyway, re-using the old ones is a waste of time. The cathode bypass caps are pretty easy. The mechanics aren't that tough; you are just putting back what was already there; you can help yourself by taking lots of pics and making sketches.
Hunt and Peck: if re-building the power supply strikes you as tedious, they you can always turn the amp on and do hot resistance checks on every resistor or turn it off, disconnect one end of every signal cap, and check it. As you can imagine, this is time consuming and carries some risk of error ... not to mention the fact that there's at least an even chance that it isn't a resistor or capacitor and you are stuck rebuilding the supply anyway. You're right; you might get lucky and find that it's an inexpensive defective part ...
well .... do you feel lucky?
Dodge Ball: It might cost you $150-$175 or so for a tech to provide new power supply and reverb caps; whether you did it or somebody else, doing so would put 20 years of life on the amp. If you have a tech do the work and the problem isn't in the caps, then it'll take them no time to find the defective resistor / capacitor and the amp will sound as good as new.
Power supply caps are a cost of ownership of any tube amp. The biggest reason people put 2nd-tier '60s tubers on eBay is wanting to avoid paying as much in maintenance as the amp is worth. No matter who does it, with a refreshed power supply, the amp will sound dramatically better; cleaner, tighter bass and tinklier highs ... it really makes a big difference ... good luck with it. John