Variac Variable Transformer

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.

If the amp uses a GZ34/5ar4 rectifier it will not do a slow start with a variac.

Think this may be true for other indirectly heated cathode rectifiers as well.

I substituted a directly heated 5Y3GT, for a GZ34, when slowly bringing up a Pilot 240 and it worked (someone @ AA made the suggestion).

A local tech used a DIY solid state plug-in GZ34 for the same reason when using a variac on my Pilot 232.