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.

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.

I agree with Commcat. I found rule of thumb on the internet to apply 5 min. plus 1 min for every month of storage. This would call for 41 min of reforming unit stored for three years. It seems too short and I would do this at least for many hours (if not days). Be patient.

Dekay - similar problem with SMPS power supplies. They will produce full output voltage (regulated) as soon as supply voltage crosses some threshold.

Hi guys. Thanks for this valuable information. I never thought my tube amp and preamp audio addiction would lead to these "other" issues. Oh well, It's still alot of fun even with this added to it. I found and bought a Variable Variac Transformer at Fry's Electronics of all places. It also has an analog meter. Will be using your suggestions. Thanks again...

What I do is start out at 10 volts (or 10 percent, depending on the scale of the variac) for about an hour and then increase by 10 volt increments for about a half hour at each increment. Takes a good six/seven hours.

I would clarify with QS about what they mean by "bring up the voltage slowly". What works for me doesn't mean it's correct for you.

GS5556 does you procedure include solid state power amp that has been sitting unplugged for 2 1/2 years? Thank you for your contribution to this forum. Joe

GS5556 I'm referring to my old Hafler XL280 that has been sitting for all that time. I'm a little worried about throwing all that current, at start up cold, into the PS caps. So I thought if I would just ramp it up in 10 20 vac increments with my 7amp variac for a period of several hours would be prudent. Any thoughts?

This has nothing to do with tubes. It has everything to do with older electrolytic capacitors.

You don't need a variac! All you need is a light bulb (60 watt will do fine) to put in series with the amp or preamp. Pull the tubes out if the unit is not solid state. Turn it on with the lamp in series with the AC line. If the filter caps are having a problem the bulb will light up. As the caps 'form up', the lamp will dim out to nothing (unless you have a solid stage unit, in which case it will remain dimly lit).

This will prevent serious currents from damaging the caps. If the bulb stays lit, you have a capacitor or capacitors to replace.