Bad Transformer?

Hi, Last night as I was listening to my old dynaco St-70 (circa 1963), the amp went dark (all tubes went dark) for about a minute, it then revived and played for another hour perfectly before I turned it off. It wasn't the power supply to the unit as my preamp and cd player are also plugged in at the same place. I was wondering if this unit has a "cut out" switch when it begins to overheat. I have it hooked to my Vandy 2ce' which are pretty demanding. But then again, it just lit right back up after a minute so I don't know what was up. Any ideas?
I kind of doubt is the transformer. Since the amp is pretty old, my guess is the power supply is failing. The capacitors may be drying out and retain much charge anymore. If you replace the caps, I would also consider replacing the rectifiers. The other problem could be the you have bad soldering joints which may be oxidizing (sp?) due to the age of the amp. Good Luck, Richard
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.