I agree with tomelex. Just raise the voltage slowly. A minute sounds about right. You just don't wang to "shock" the unit after being off for a long time. You may have some bad caps and some marginal caps that will result in less than the unit's potential performance that may go unrecognized.
On the other hand . . . if you just fire it up at 120 volts and parts fail, you'll need and get a definitive repair if parts are marginally operational.
A minute is about right. Most variacs won't start from zero - and some will go above line voltage, so be careful cranking it up all the way.
A friend just borrowed my variac to check out an entire life time's collection of vintage hifi (selling 'cos he's migrating/retiring to Thailand) and everything still worked except for an FM tuner from the 60's. It's usually the power supply caps that have deteriorated, but even old deteriorated caps should still be able to power up gently.
Now that the main question has been answered fully, may I expand the topic a bit?
I recently had an old solid state amp repaired locally. When I picked it up, I asked the repair tech if he recommended plugging the amp directly into the wall, and all other equipment though some sort of power treatment box or protection strip (as I currently do).
The tech said that he recommends to customers which ask about such things, and which own vintage SS receivers/ amps that they purchase a Variac, and run it at a slightly lower voltage than what comes out of the wall (going from 120V down to 110 or 100V).
He claims it's puts less stain on the equipment and extends the life of the equipment, and you will not hear a difference.
The tech guy which also sells on ebay, deals with a lot of mid and late 1970's solid state receivers. His collection absolutely delights me when I see it laying around the place waiting for it's new buyers.
Thoughts on running solid state amps though a Variac all the time?
I've always thought that running stereo equipment, and any equipment (computers, refrigerators....etc.) on low voltage or brown out like conditions was taboo.
Interesting. There was a guy on the radio who said that the utilities used to push the voltage up slightly in order to make the power cost more so they could charge more (a very simple description here).
IIRC, running something slightly lower (105v-110v) won't really hurt anything, just don't go too low.
I'm venturing unto unknown territory here so I'll just shut up about it. My main concern was powering up vintage equipment and abusing old capacitors and such.
A long, long time ago, the voltage standard in the US was 110V then 115 or 117V more recently it's 120V. But now we see electrical items listed at 125V and more and more power companies providing about 125V.
So if you are in a higher voltage neighborhood and you have legacy equipment designed for maybe 115V I could see a line voltage adjustment.
Power amps need bigger Variac's than their ratings.
For those that can work with power line voltage equipment, a boost-buck transformer is the way to go. But safe wiring can be tricky.
