I use one on my 67 Marshall because of the advice from Greg Germino. I do not use the device for any kind of brown sound however. Greg told me emphatically that the old coot wanted to see 110 and nothing else. Who am I to argue!!!!!!!!!!
Here's the issue, let me give you a for instance: You get to the club. Its a hot summer day and the AC and refrigerators are working overtime. All that stuff is hooked to the same supply lines that power the stage. You measure the current at the stage "85 volts! DAMN... that won't do!". Out comes the variac. You dial it in at 110 volts. You start to play the gig, sounds f'n great! Suddenly the AC hits 70 degrees and the thermostat shuts her down. Coincidently, the refrigerators reach the end of their cycle and shut off as well. All of a sudden that voltage that wasn't there when you measured, returns and the variac boosts it accordingly. Now you favorite tube amp is running on 150 volts. Meltdown is near. Fires have been started this way. USE REGULATION WITH A VARIAC!i have one, i bought a rebuilt one on ebay about 3 or 4 years ago. i forget how much i paid, maybe $150.00 with shipping? i used it once to try to reform caps on a vintage 1968 sunn 1000s amp. it didn't seem to help, but i didn't have that much time or patiance that i could let the thing sit there and turn it up 5 or 10 volts every hour or two over the course of a day or two. i decided it is easier to just replace the filter caps.
i don't use it on the new amps i build. when i was first starting to build amps, i asked about buying one at the local electronic supply. the guy was experienced and knowledgable. he told me that for new manufacture amps using new filter caps i could just build and apply power. i have not had any problems, and almost everyone who has tried my amps is impressed by how little hiss and hum they have.
i suspect what you really want to know is about how it sounds running an amp at a lower supply voltage. never thought about it, never tried it before, but i will right now
we have about 125 volt from the wall here. i hooked it to my basic model 5 watt amp and set it for full volume, full treble and 50% bass. when i reduced the varic to 100 volt i noticed the amps volume got just a bit softer and the bass seemed looser. when i went to 90 volt the amp lost a little more volume and got more distortion. when i went to 80 volt the amps volume was a noticeable amout less, maybe 75% or 80% from regular wall voltage. the amp also had a fair amount more distortion. its a useable sound and i could see that working for someone in an apartment or home situation.
i also cranked the voltage up to 135 volt for a minute, but really didn't notice any difference.
so is it safe for the amp or person playing? i'm not sure because i saw that post above about them being unregulated. mine is a good quailty one, its been rebuilt, and upgraded to have a grounded outlet. its rated for 7.5 amp and has a fuse. i put an amp on it that only draws an amp or less.
If you're wondering about a Variac because of the rumor that EVH used one early on for the brown sound, I believe that was Bullsh*t on his part.
Around then he also claimed that Cesar Diaz had specially modded his MArshall, but later on said all he did was keep it stock, that he spun the 'mod' thing to give Cesar some business.
I use a variac with my 69 plexi clone only to keep it running constantly from 230 to 240V since its wired for 240V and the voltage on the wall socket one day can be 220V and the other 250V in my area...
There is always a digital Volt meter connected at the variac's output (green arrow) which i have double checked that it shows the precise voltage so i can easily monitor whats going on and adjust it...