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Too late now that you've done it, your were lucky.
Good idea to power up slowly (over a few hours) with a variac after long period of non use.
But!! being a big tube amps or any tube amps for that mater, one should never power them up without a speaker load or 8ohm power resistors attached to the speaker terminals, and also for good measure input shorting plugs.
As some tube amps can go into oscillation without a speaker/resistor load, and can take out the output transformer.