I got a Marshall JCM800 2203X about a year ago and on the back of the amp it has selectable input voltage settings for 110V, 220V and 230V. Since I live in the US, the voltage that comes out of the wall here is around 120V.

Back of the amplifier:

enter image description here

The amp runs okay on 120V, and no tech that I have brought the amp to has told me that I need to lower the voltage or I'd risk damaging the amp, but I was wondering if running this amp at 120V is wreaking havoc on my tone... I've noticed that if I turn the mids up to any sane level (above 3) my amp begins to sound very harsh.

1. Would lowering the input voltage using something like a variac to the 110V my amp is "expecting" to see make my amp sound less harsh?

2. Could further lowering the voltage ...to say 90V?... improve the tone even more?? Or will i just start breaking stuff? (The internet seems very divided on this topic.)

I know Eddie Van Halen used to use a variac set as low as 90V to get the "brown sound" he was looking for, but I was wondering if there was any real merit to this claim and if it could solve "harshness" that I am hearing in my own amp.

  • On a side note, I recently ordered a variac online, so I plan on posting my own answer to this in a few days when it arrives. :) – tjwrona1992 Dec 5 '16 at 22:56
  • fyi, house voltage can vary by +/- 5% or so, and it is not unlikely that your wall votlage is close to 115-117. Devices typically can handle +/-5% of their rating also. – Yorik Dec 6 '16 at 15:42
  • @Yorik, I measured my wall voltage and it seems to be consistently around 121V. So it is running hotter than I would like when the amp is expecting 110V. – tjwrona1992 Dec 6 '16 at 16:14
  • Mine are closer to 117, but chords and length of outlet runs can affect the measurement. Most devices list the lowest spec they are designed for, but amplifiers seem to be a special category. There never was a 110v standard in the US from my understanding. My understanding of tube amps is that lower voltage acts as compression and also a high-cut filter. The sweet spot for tube life is around 90-110 I think. – Yorik Dec 6 '16 at 16:21
  • @Yorik, Compression and high-cut filter sounds like it might solve my harshness problem. There's people all over the internet saying that using a variac is great, but no one actually describes what it really does to your tone. I will let you know what happens when I get mine. It should be arriving tomorrow! :) – tjwrona1992 Dec 6 '16 at 16:31

Better is very subjective, but the simple thing to acknowledge is that you are operating an electronic device outwith the parameters for which it was designed. Therefore you cannot assume it will sound 'better' as much as 'different'.

Eddie Van Halen is possibly the source of this (mis)adventure, as he commented, erroneously, in early interviews that some of his tone was derived from stepping up the voltage to his Marshall amps. It seems to be the case that he was actually lowering the voltage to his Marshalls, to give him that slushy-yet-tight overdrive you hear on Van Halen I. Sure enough a Variac is visible in some contemporary photographs of Van Halen onstage and in the studio. Of course that tone is also in part due to the plate reverb used in the studio and the modest array of stomp boxes Eddie was using back then, so you cannot simply get his 'brown' tone by starving a Marshall head and calling it a day.

Eddie also dialled in his amps by ear, starting off at 120 volts and bringing it down until it sounded good to him. Simply running your amp at 110 volts might not be optimal, and there is a chance that your amp sounds pretty nasty when starved of power.

  • 2
    Is that the good "nasty" or bad "nasty"? hahaha – tjwrona1992 Dec 6 '16 at 13:35
  • Heh! Again, subjective. Eddie Van Halen's tone might not have worked in another context and be, accordingly, viewed as 'bad' nasty in some circles. Likewise had somebody else produced and mixed VH1 then perhaps nobody would even entertain the notion of starving a Marshall-type amp today. – ABragg Dec 6 '16 at 13:47

My variac arrived today and I have to say I'm very happy with it. My wall voltage seems to run a little high usually between 120-123V so the knob on the variac was a little off from the actual voltage reading. (The actual voltage coming out of the variac seemed to be about 5V higher than what it showed, but from what I've read this was to be expected). Below is a picture of the variac that I purchased. It's a "Tenma" brand variac that I don't think they technically make anymore, but it seems to be very high quality for the price.

enter image description here

Lowering the voltage to 110V seemed to definitely improve my tone. It made my amp sound less harsh and cut some of the exessive high frequencies I was hearing. Overall it has a smoother tone. (This description is really what I was trying to find on the internet... I've read that vintage tube amps sound better with a variac, but no one would really say how they sounded better.)

I will have to play around with it a little and possibly rebias my amp to get the full effect, but i am glad I gave it a try. Lowering the voltage also seemed to noticeably lower the volume which is definitely a plus with a 100W beast like a JCM800. The effects are pretty subtle, but they are definitely noticable.

Also as a disclaimer I would like to mention that I have not tried lowering the voltage below 100V, nor have I tried going above 120V. I'm sure the effects would be more noticeable below 100V, but I personally don't feel comfortable risking the health of such an expensive amp to experiment with different tone possibilities. I measured the output of the variac with a voltmeter to ensure I was using a safe voltage at all times and I would suggest that anyone else trying a variac out for the first time should do the same.


The simplest answer is no...and yes!

It will sound different, and you may find a voltage that changes the valve bias to a level that you like the sound of. But you may not.

There are plenty of amps that have variable voltage control for valve bias and other sections, or variacs. Play with them (or with simulators because at the end of the day you just need to try ones until you find sounds you like.


A "genuine" variac (like the picture in the OP's link) is simply another transformer, so using it for a small variation in supply voltage (10% or less) is unlikely to harm anything.

A big increase the amp's supply voltage above the specified value is (obviously) not a good idea.

A big reduction below the recommended voltage may also be a bad idea, because it might also reduce the heater voltage for the valves. That could reduce their life - there is a good reason why you are supposed to let valve amps warm up in "standby mode" before you turn the volume up to 11 and start playing! But without seeing a circuit diagram for the particular amp, there is no way to tell - and in any case Eddie Van Halen could probably afford to replace all the tubes in the amp every 10 hours, if that made the sound he wanted.

On the other hand, if you get an "all-electronic" variable voltage power supply, or a device to run mains-powered electrical equipment from a car battery, all bets are off about what it actually outputs.

As for the harsh tone, bear in mind that the values quoted for loudspeaker impedances are often very "approximate," and the value is not constant across the whole frequency range. Different speaker cabinets will sound very different with the same amp. Compared with transistor amps, valves are much more sensitive to variations in speaker impedance.


You are better off with 10% too much voltage than 10% too little. Higher voltage stresses condensers and some other parts, but their tolerances are pretty high. Lower voltage, however, leaves the tube cathodes too cold which means that they are bleeding metal vapor in addition to just electrons. The metal resublimates in places where you don't really want it, building dendrites and worse.

  • That may be true, but at a minimum changing the voltage to 110V which my amp is technically designed for would probably be better for the amp than continuing to run it at 120V. I've heard that as little as a 10V difference can have a substantial tone difference as well. – tjwrona1992 Dec 6 '16 at 17:59
  • This is called "cathode stripping" and it is definitely a problem both for the life of the tubes and the grids – Yorik Dec 7 '16 at 18:28

I once plugged a modern PA amp into a stage socket that proved to be on a dimmer circuit (so it shouldn't have been on a British 13A outlet, according to code, but that's another matter). The amp got very hot.

So be careful. Experiment, but be wary. Tubes can sound different when running at different voltages, certainly. An old amp probably won't have a stabilised power supply. That might actually be an advantage! But there are some eccentric designs out there. No rules. Be careful.

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