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Variacs and other things
Hi Guys

Everyone has heard the story that Eddie van Halen used a variac to dial the mains voltage down a bit so his amp would overdrive at a lower loudness. Does this work?

Short answer: yes.

Long answer: Only to a point.

Drops in the mains voltage effect ALL the internal voltages in a typical amp that does not have active voltage regulation. This means the heater voltage will drop, leading to slightly cooler cathodes and reduced emission of electrons. There is a 10% tolerance listed for heater voltage, beyond which the heater itself may not work but certainly beyond which the cathode stops functioning as it should.

When the plate supply voltage drops, the tube stages generally keep working as they should provided that bias supply tracks the plate change for fixed-bias amps. The tone will change and become a bit softer, rounder, and easier to overdrive. The power output may not change radically enough to attain "quiet" SPLs, but it might be enough to make the excess loudness tolerable. In any case, the amp tone changes with this type of power control.

In the refined version of the variac approach, a separate filament transformer is added to maintain heater and cathode temperatures. The variac now changes plate and bias voltages for the entire amp. The loudness control range may be extended but again tone changes as you dial down. Another issue arises that pots become "scratchy" even on your guitar !! This is a problem users of vvr encounter, and they have to add DC blocking caps to the input of the amp and where pots interface directly with tube grids. The latter requires also adding grid-leak resistors to maintain the tube bias and normal operation.

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