If you're talking about old tube gear, here's a tip. I recommend you use a current meter, the same size as the variac is rated (ie: 5 amps). After you've tested the tubes and checked that they're all in the right sockets, and the fuse is the right size, terminate the output with a 10 ohm 10 watt resistor. A simple rule of thumb is to bring it up to 50%, or 60 volts. If you don't see excessive current draw, right away or after 10-20 seconds or so, you're doing well. Let it sit there for a time (say 1/2 hr.) to reform the electrolytic capacitors. Then slowly swing up to operating voltage. Again, watch for excess draw on your meter. It will "bump" a bit as you crank it up, but should settle. Now it's time for the smell test. If it doesn't smell like a toy train that's run too long, or you don't see an orange glow on the output tube plates, you're in the ball park. Next, try a live test, with a durable speaker.