Many years ago I made output transformers for old Marshall Amps. I sold them via a website, and with an eye on community service I started adding other content like rare schematics and tech info.
One of the info articles I wrote back then was about the use of a device called a Variac to change the supply voltage to a Marshall amp. "Variac" is a trade name, owned by General Radio Corp, so only the ones they make should say Variac on them, but they are all functionally the same; they are variable transformers. You plug it into the wall, you plug your device into the Variac and you adjust the line voltage that device gets with the big knob. It's pretty basic.
A Hamfest find many years ago, I think for $40. This model with both voltage and current meters has been a mainstay of my repair bench as long as I have had one.
The reason these pieces of test gear snuck into rock and roll history is largely because of Eddie van Halen. His technique and sound were so revolutionary that there was a lot of speculation about how he achieved it. He of course did the guitar hero thing of misdirection, because why not. Just like Kurt Cobain claimed to use heavy gauge piano strings, Eddie claimed to use a Variac to change the voltage to his amp to get that magic sound. Legions of guitar players followed these rumors like lemmings over a cliff, not really understanding what would or could happen. Some turned the voltage up, some turned it down. All seemed to argue for the "right" way to use this technique. There seems to be some age old trope that gear sounds its best when it is about to melt or catch on fire. I guess that is the tech equivalent of the rock and roll lifestyle.
People often seem to have opinions and feelings but often won't do basic experiments to test their ideas. I will confess to not understanding this phenomena. If you run a good test and find you were mistaken, you learn something. It's always a win.
With that in mind, I put my own 1970 Marshall on a Variac and recorded the various voltage measurements in the circuit to see what would happen. I wrote an article (still available via the wayback machine) about it, and in that article I actually nailed what he did, but I did not assert it. But recently I came across a video with one of the engineers who was on the VH1 recording sessions and he confirmed what was done, and it is what I had essentially suggested. So I feel pretty good about that. Call this a pseudo victory lap.
Here is what I wrote at the time: (May, 2000)
"What (Variacs) do is allow you to alter the amount of AC voltage your amp uses as its power source. This can be very helpful as many vintage amps were designed for 110VAC, and modern voltages can exceed 125VAC. While at first glance you might think, "so what".. that extra 15VAC becomes higher heater voltages (bad) and much higher plate voltages (also bad, in general). You can use a Variac to feed your vintage amp 110V, as it was designed for, and get the "correct" operation and tone from it. (not to mention longer life!). Of course you should have your amp biased at whatever source (AC) voltage you plan to run the amp from. There is absolutely no harm done by operating your amp this way.
Taking voltage reduction still further, below vintage voltages, you can enter new tonal realms - which is at the heart of the "EVH Legend".
As the story goes (according to some accounts), EVH lowered his Variac to 90VAC and found sonic nirvana. What did this do to the amp? Rather than theorize, I subjected my own 50W Marshall head to a Variac test. The results are below:
"AC in" is the Variac voltage that the amp is running on. "B+" is the voltage at a power tube plate. "Bias V" is the bias voltage on pin 5 of the output tubes. "Idle current" is the plate+screen current measured across a 1-ohm resistor from cathode to ground on either power tube. "PIa" & "PIb" are the two phase inverter plate voltage readings. "Pre B+" is the source voltage for the first stage of the pre-amp. "Va Brite" is the voltage at the plate of the lead channel and "Va Nrm" is the voltage at the plate of the darker channel.
What happened: Lowering the voltage into the amp lowered all the voltages, including the filament, B+ and bias. The bias setting does not track linearly with B+ so as the B+ drops, so does the idle current and the amp is biased colder and colder as we drop the voltage from the Variac, this is normally not what we want. What we could do (what some DO do) is to find a setting, say 90VAC, and then re-bias the amp. Doing this allows you to take advantage of the lower plate voltage (B+) and run the output tubes at higher current levels."
And in fact, that is what he did according to the recording engineer in the video. Lowering the voltages gives you a slightly darker, more compressed tone with less headroom, and lets you run higher currents (better sensitivity/touch) if you re-bias since the plate voltage is lower. Plate dissipation = Plate volts X Plate current, so lowering volts means current can come up.
Running your filaments too low is not good for tube life, they can be run about 10% low safely as a rule of thumb. If the manufacture issues specifics, obviously adhere to them. 10% less than 6.3V is 5.67V, which is around 108V based on the chart above. The supply voltages all track linearly, the only real issue is the bias current. By 1970, my own amp was clearly set to run on 120V, the heater voltage is the giveaway. So Eddy's 90V was a little hard on the tubes, but hardly setting them to melt. The generally given safe range for filaments is +/- 10%, which is 5.7 to 6.9VAC.
Now we don't know how they were biased, and we don't know what his plate voltage was to start with. The older 100W heads have a reputation for high supply voltages. But if we use my head as a guide, lowering the supply voltage to 90V gave us 325V on the plates, and the EL34s are 25W tubes so our max current is 25W = 325 * I or around 77mA max. That is still class AB, but at the hot end of it, not unlike a Vox AC30 by way of comparison.
Epilogue
In fairness to Eddie and the others, finding that magic combination of gear that is your musical "voice" can be a complex and time consuming process. Giving that away may or may not be a good thing for other players - for example if it stunts their own journey because it is easier to just adopt your "answer". Eddie, like so many before him, experimented with his guitar, his pickups, his amps, his effects, his speaker cabinets and more. Perhaps no one since Les Paul had dug so hard to find their answer. No one thing is the answer, and the piece that will always be missing is him. But hopefully it is inspirational to someone to go look at their supply voltages and perhaps install a VVR or other adjustment as an approach to find their voice. For myself, I lucked out and Ted Weber found me a bench supply at the Indy Hamfest that I used to run tests on different amp circuits. It was an education.
References:
Wayback machine (OEI Website) from May 21st, 2000: https://web.archive.org/web/20000521045204/http://www.obsoleteelectronics.com/Marshall_Tech/marshall_tech.htm
YouTube Video with Engineer's discussion and cool outtakes:
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