Assuming the tubes went dark because the filaments lost power, and not because the amp normally runs with the plates red hot, I have to respectfully disagree about the power supply being the culprit. As I recall, this amp uses AC for the filament supplies, if true, then a failing rectifier or capacitor would only affect the B+. I do agree that a bad transformer is unlikely, since the filament windings are very heavy wire. Opens in the primary (120VAC) or the HV secondary (for the tube plates) are not uncommon, but I don't think this would recover. I would suspect something in the AC line input area, or maybe in the transformer filament to tubes connection. You probably can't tell from the sound if only the filament supply failed, or if both HV and filaments were out. Here's my list of suspects: 1) AC line cord, especially at the plug or where it enters the amp chassis. 2) AC line to transformer, including power switch, line fuse, solder joints or broken wire where the 120V connects to the primary transformer leads. 3) Solder joint or broken wire at transformer filament output leads. I would suggest trying to find the problem, rather than hoping it goes away, since losing filament voltage while the plate supply stays on is not healthy for the amp. Increased hum is one symptom for the bad rectifier or capacitor mentioned above, in an amp this old, you should check them, even if they probably aren't the cause. Hope this helps.