Variac?

[Witek]

Why do some use these on their amps? and what exactly does it do? I know it's something to do with playing around with their gain, but doesn't it damage the tubes because they operate too cold and shorten their life?
So what's the deal?

[Blue Strat]

Variacs plug in between the amp and wall socket. It allows you to adjust the line voltage to your amp. They can be useful for vintage amps which were designed to operate at 110 Volts (today US line voltages are closer to 120 volts and sometimes higher).

Lowering the voltage to your amp lowers the internal voltages and allows you to achieve the "brown sound" (as in a "brown out", when your power company is unable to meet demand and line voltage drops) which means earlier breakup and less high frequencies (treble).

Eddie Van Halen was famous for using this technique.

[nickreynolds]

can anyone recommend me a good variac to buy and where?

[MRX2099]

Does it hurt the amp/tubes? I have a variac but not used it yet on my 50W Plexi clone I was waiting to see the pros/cons before I end up maybe screwing up my head

Jeff

[vibroverbus]

Does it hurt the amp/tubes? I have a variac but not used it yet on my 50W Plexi clone I was waiting to see the pros/cons before I end up maybe screwing up my head

Jeff

yeah, it's terrible for your tubes to run it at extreme levels that really tweak the sound. it will change your bias setting unpredictably, and runs the heaters at levels outside the recommended range. using one +/- 10-15% of original rated voltage is probably fine. I have one for tech service purposes, and frankly don't see much difference in tone within that range - there are plenty of other things I can/will do to an amp to tweak it before using a variac...

the Eddie V story is pretty much a debunked urban legend and at some point he went on record admitting no he never really used a Variac like that, but who knows - I suppose he could have blotted out all memory of what he did or didn't use, personally I wouldn't place much stock in what he claims one way or another - but personally I think it's one of those fiction-has-become-truth things...

on the other hand, mods like KOC's (London Power) "Power Scaling" setup are oriented around safely adjusting plate voltages and simultaneously trimming the bias to match while not affecting the heater voltages. basically it's like a 'smart & safe variac mod'.