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.
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.