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.
Vintage tube amp resurrection with Variac?
I have acquired some vintage tube equipment that has not been turned on for several decades. I bought a Variac on eBay having read many times to be sure to "bring up" old tube gear "slowly on a Variac". What exactly constitutes "bring up slowly"? Is it some sort of staging perhaps? If so, what voltage at each stage, number of stages, how long per stage. Or does it mean to continuously, but very slowly, crank the Variac from 0 to 117 volts?