Variac Variable Transformer

It's mainly for older amps with the paper/oil caps. A sudden voltage surge can cause older caps to short out-which can be very bad. I would worry mainly about the tube gear but if you buy a variac, use it only any solid state gear too.

By very slowly bringing the voltage up, it will allow the caps to "reform" internally and avoid costly shorts or having to replace the caps.

I always use mine, however the reason is to reform the dielectric on the power supply caps to avoid hum.

It's a problem with electrolytic caps. Layer of aluminum oxide that serves as isolator diminishes over time (years) without voltage and capacitor looses its voltage rating. Applying voltage and keeping voltage at lower levels (for days) will replenish layer of aluminum oxide and capacitors will be OK. Applying full voltage to unit that was in storage for many years can cause capacitor short, thermal build-up and even explosion. Capacitors have fuse (vent) to prevent it (explosion) but it's wise to to test it.

How long should I leave the Variac on for? Bringing up the voltage slowly to 120 volts...Mike at Quicksilver recommended ten minutes. They have been in storage over a year now, originaly built in the last several years. I have an older ARC SP-6B tube preamp stored for over 7 years as a back up preamp. Any recommendations? Thanks..

My personal preference is to bring the voltage up very slowly, over several days, or a week. Have seen guys in a hurry to hear their new (old) tube amps bring the voltage up to 120 and smoke the capacitors. IMO all of the internal components require small voltage increases to prevent malfunction.