Variac is a variable autotransformer; you can step AC voltage down to near zero, & some can even step up above input line voltage ~10% or so. Unlike a balanced transformer, it does not provide line isloation. You'll need one with reasonable current capacity; several amps seems typical (500 watts or more). Ebay seems like a good place to find one at reasonable cost.
I assume you want to bring up the input voltage slowly in order to re-form the power supply electrolytic capacitors without risking a short. Tubes should be installed. Speakers should not be connected. Signal input (drive) should not be connected, so if this is a receiver, do not set the input to 'tuner'. Starting from zero volts, raise very slowly to around 5V for several (10) minutes at least, then step up ~5 volts at a time. The whole process can take quite some time of course. Do not ever exceed 120VAC max.
I have never actually done this; perhaps someone who has will chime in later. A knowledgable member who has experience with this actually recommended taking several days to raise voltage VERY slowly, only a few volts per step & leaving it there for many hours at a time. Not sure if it's necessary to go that slowly, but you get the idea. That post might yet be in the forum archives; was posted by Sean many years ago. Try a forum search on "Variac" if desired.
