Vintage tube amp resurrection with Variac?

I have used a variac to bring back equipment that hasn't been turned on for a year or two, but decades is another story! My method is to start out at zero and bring up in either six or twelve volt increments (I use these values because they divide into 120 VAC by factors of ten and five respectively) depending on how long since the unit was last energized. Twelve volts if it has been a year, six volts if it has been more than a year. I also vary the length of time at each increment depending on how long since last energized, 10-15 minutes for a year or less and 15-30 minutes for more than a year. I would suggest you move up in 6 volt increments and leave it at each increment for at least 30 minutes. The whole point of this is to reform the capacitors, after decades they may very well be bad, but it's worth a try. One other comment, my variac has a tendancy to blow the circuit breaker when first plugged in if the load is attached, not sure why, but I start out with no load, make sure it is at zero volts, then plug in the load.

That paranthetical bit should be factors of twenty and ten, not ten and five. Hit the Submit button when I should have hit the Preview button!

Don't forget earplugs and safety goggles.

I have done this a few times using this technique. There are different opinions out there, but an excellent audio technician told me this is what he did, and never had a problem, so that was good enough enough for me. Make sure you have speakers or load resistors on the outputs. Not having a load on the outputs could cause a serious problem. Also, watch the tubes as you increase the variac. If there is a major bias issue, the plates of an output tube may start to glow red. If this happens, definitely stop the process, as it will be best for a technician to take over.

@OP, I'm not a tech and that's what makes my post worthwhile.

Knowing what I DO know is enough to say that rather than screw around with your old amp, I'd have it looked over by a reputable tech. If the manufacturer is still around... even better. I bet the old PS and coupling caps are dried out and need replacing. Other parts may need to be replaced too. Here's a quote from TLS49's article:

"You should probably consider replacing the capacitors on an old piece of equipment even if it doesn’t hum. Old filter caps will fail sooner or later, so you may save yourself some time and trouble later on."

Seems like good advice to me.