There's a cheap why to start this up without a variac. You use a 40 watt incandescent light bulb.
It has to be wired in series with the AC power cord. By that I mean that if you turn on the amp, the current for the amp should also flow through the lamp. The lamp should not be able to light up if the amp is not on.
Then you remove all the tubes and start up the amp. The light bulb will light up, limiting the current so as to prevent damage to the caps. As the caps 'form up' the bulb will go out. When it is no longer lit, then the amp can be safely operated. Put the tubes in it and see how it does.
If the bulb does not go out after an hour, the caps are shot and should be replaced before the amp is used.
Now its a simple fact that the amp is ancient, and the caps are probably shot as a result anyway. So this method or that of a variac is probably moot as that service should be performed. If not the power transformer can be destroyed and that will be a very expensive and hard to find part, not to mention a bit of a tragedy.
Expect to pay between $200 and $350 to get the amp serviced out. It is worth it.