1) Using a variac to drop the mains voltage on your amplifier is not good for it. It's not good for the power tubes, and since the power tubes control how much current the power supply can pull through the power circuit, more than likely what's not good for power tubes probably isn't good for output transformers either.
2) Using a variac to drop the mains voltage not only drops the voltage of the main high voltage supply, but it drops EVERYTHING...bias and heater voltage just as well. Too low of a heater voltage and you run into what is called "cathode stripping", where the heater doesn't have enough voltage to heat the cathode to the temperature required for the cathode to thermally emit electrons. When this happens, the cathode cannot get hot enough to efficiently emit electrons and the high positive voltage on the plate will literally try to "strip" electrons from the "not so hot" cathode. It also makes the bias voltage less negative...too much of that and your power tubes will go into meltdown as they won't have enough negative voltage to keep them from pulling too much current.
3) Using a variac to raise the mains voltage...don't even go there.
Long and short of it...using a variac on your amplifier is not a good idea and definitely not recommended. It will tax your tubes as well as the rest of the amp. EVH himself stated that they were constantly having to replace tubes among other things in the amplifier because of operating it on a variac. If you should decide to do it anyway...you've been warned.
4) EVH was inconsistant in his interviews...some people claim that he's even lied about what he truly did to get that first album tone. In one interview he was quoted as saying "We used to crank the variac all the way up to 140 volts and watch the tubes melt" while others claim that he would drop the mains voltage down to 80-90 volts. I've even read an article that suggested that the variac wasn't even on the mains...but rather used as a power soak between the amp and the speaker cabinet. So who the hell knows what he really did...something tells me he doesn't even know anymore.
5) There are much better and safer alternatives to lowering your plate voltage (which was the main idea behind what EVH was trying to do in the first place) without dropping your heater and bias voltages. Metro makes the dual voltage secondary power transformer (Metro 1203-80-ML or MS...switchable between 495 and 420VDC) for this reason. There are other tricks as well, such as referencing the negative side of the bridge rectifier above ground by so many volts via a zener diode (or the center tap for amps with a full wave center tap rectifier), or if you really wanna drop the plate voltage, try converting to a full wave center tap rectifier setup with one of George's transformers (should drop it into the neighborhood of around 250 volts or so).
6) The whole reason behind dropping the voltage on the mains was to allow EVH to run with everything cranked at a lower volume. There are other mods such as the post phase inverter master volume that will achieve this, as well as power soaks available for this.
7) EVH used Celestion G12M-25 Greenbacks...these speakers have a very signature sound to them that I feel is a key ingredient to the EVH Van Halen 1 sound.
Last but not least...much of EVH's sound is in his technique rather than the gear he plays (EVH himself will tell you that). Ted Nugent played through EVH's gear expecting to sound like EVH and he ended up sounding like Ted Nugent. It's all in HOW you play. 85% technique, 15% gear is the way it's done.Just my $0.02.
