EVH said he stumbled onto the variac thing by accidentally buying a Euro Marshall (220v) and plugging it into the 110v wall socket US voltage.
The clip below gives a demo of a variac used with a Euro voltage amp and he goes from 230v down to 200v and then down to 180v and then down to 150v, and the 150v variac setting is getting down near where EVH's Euro voltage Marshall would have been (110v) and compare the volume drop from the 230v variac setting to the 150 volt variac setting (a voltage drop of 80 volts), the volume drops quite a bit, so that's what EVH used to do.
EVH dropped the voltage down (on a US Marshall) according to the venue and sometimes it would be down to 90v but he could drop it down further, like down to 70v and maybe less.
So EVH had all knobs dimed on the amp and the variac was used as a sort of master volume.
EVH didn't slave anything in the early days and he didn't need to because he had his variac, and it wasn't until around the mid 80s that EVH hooked up with Bob Bradshaw (via Steve Lukather) that he went to slaving for live shows (not the studio), because it had advantages for live shows.
There isn't a huge tone or gain difference at different variac voltages (some tone and gain differences) but the volume can be reduced quite a bit the more the variac voltage is reduced.
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ESQ: You used a light dimmer on what?
EVH: I bought an English version, I had my 100-volt Marshall. I bought one through the recycling or the newspaper that was from England, and it was set on 220 volts. I didn't know. So I plugged the thing in, but I'm going, "Fucking thing doesn't work. I got ripped off." I just let it sit there. After about an hour, there's sound coming out, but it's really quiet, cause it's running on half voltage. So I go, "Hey, wait a minute. It sounds exactly like it's supposed to all the way up, but it's really quiet." So we had a light dimmer in the house, and I hooked up the two leaves from the amp to the light, so I did it backwards, blew out the fuse box. Then I went down to DOW Radio and asked, "Do you guys have any kind of super duper light dimmer?" They go, "Yeah, it's all Variac, variable transformer, you know." And on the dial you could crank it up to 140 volts or down to zero. So I figured, if it's on 220 and it's that quiet, if I take the voltage and lower it, I wonder how low I can go and it still work. Well, it enabled me to turn my amp all the way up, save the tubes, save the wear and tear on the tubes, and play at clubs at half the volume. So, my Variac, my variable transformer was my volume knob. Too loud, [makes knob turning sound] I'd lower it down to 50.
ESQ: That's amazing. But still, that was it? That was the only modification you did?
EVH: Just out of necessity. I need an amp I could play in clubs. We wouldn't get hired, I would play so loud, you know, I'm going, what can I do? What can I do? Okay, I turned the voltage, the wall voltage into my volume knob.
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