"Muddy Waters invented electricity." One of my favorite lines from 'Crossroads'. Yes, there were guitar players long before Edison lit up our lives, arguably some of the best ever. But once electricity made the scene it changed us for good. Hell the whole reason for amplifying a guitar in the first place was so more people could hear better.
Sometime in the mid 70's Jose Arrendo hooked up a Variac to Eddie Van Halen's Marshall and the sound of rock and roll changed forever. The 'brown sound' as Eddie called it has proven to be one of the most discussed tones ever. There have been a lot of great sounding amps made over the years. Some modern rigs have a built-in Variac mode. In my opinion there is absolutely no substitute for the real thing.
First off, a word of warning. As long as you use a Variac correctly there is virtually no chance of damaging your amp. That being said I take no responsibility for your personal results. If you crank more juice to your rig than it can handle it will blow. Not if, but when. If you have any reservations about doing this, don't do it. Any modifications to a stock amp, even something like this, need to be done by and supervised by a professional. Got it? Good. Now that we have the legal mumbo jumbo out of the way lets get started.
A Variac is a variable voltage transformer. It's designed to reduce or increase the voltage coming out of the wall outlet. Make sure that you check the voltage with a volt meter. Even if your Variac has a built in meter (which I suggest) spend the small amount of money and get a volt meter. It's a lot cheaper than the repair bill for your rig.
Electrical current in the U.S. nowadays runs around 120 v.a.c. Back in the day house current was as low as 110. Over the years it has slowly increased to 115 and now 120. Here's the rub. 120 doesn't necessarily mean 120. Power companies operate within an acceptable range of 114 to 126 volts. Now I know my coffee maker isn't going to act differently, but my rig notices the difference big time. Depending on where you are you could have as much as 10% variance in current. Tubes are very sensitive to electrical current. So are all the other components in there. A 10% variance in electrical current totally changes how your rig sounds.
Now the big secret of using a Variac is to REDUCE the voltage to your rig. Start with the voltage at 120 and slowly turn it down in increments. Move it down maybe 5 volts at a time and play for a few minutes. Let your ears be your guide. You will be able to hear and feel a difference in the response of your rig. Just be aware that if you lower the voltage to much the rig will eventually shut down. The tubes must have a certain amount of juice to function and once they drop below that level the rig shuts down. Reducing the voltage can wear your tubes out quicker because super low filament voltage can strip the chemical plating off the filament. My rigs sound best to me between 90 and 95 volts.
There is a lot of debate as to what this does. Some say by reducing the plate voltage to the power tubes you are essentially 'starving' the amp. This allows you to run the amp harder with a reduced output. The volume level is reduced but not significantly, however the natural harmonics are increased which gives it a smoother tone. The other thought is by lowering the B+ voltage in the preamp tubes you increase the compression and high end roll-off of the distortion again producing a smoother tone. All I have to say is whatever happens, it's incredible. The amp takes on a whole new texture. My clean channel breaks up sooner and every channel is more touch sensitive. My drive channels seem more compressed and have a bit more sustain.
If you have a vintage amp you probably need a variac just to make it perform properly. When you fire up an amp it sends a large load of electricity to the components. Newer rigs are better prepared for this but every rig that gets punched like this strains the tubes a bit. Thats the reason you see vintage preamp tubes 'flash'. I reduce the voltage down to 60 on startup and roll the power up to operating level in about 5 seconds. It eliminates the flash and protects the circuitry. With vintage amps you may want to roll the voltage up even slower. I've read where audiophiles working with antique radios have a long power-up sequence they use on radios that haven't seen juice in years. The reduced voltage at the very least should help the older components to run a little easier.
I hope this helps you. I highly recommend you give a Variac a try. Just follow the rules and see what you find. Talk to your amp tech guy or a qualified electrician.