Quick read: The wattage stamped on the back of your amp tells you how much power it sends to the speaker, not how much it draws from the wall. To size a voltage converter, find the amp's mains draw in watts and pick a converter rated at about twice that in VA, because a tube amp pulls a big inrush surge at switch-on that an undersized converter cannot ride. Then get the direction right: Japan's 100 volts needs a step-up for a 120-volt amp, while the UK and Europe's 230 volts needs a step-down, and skipping the step-down kills the amp on contact.
A man called me last spring, a couple of days before a run of dates in England, asking why his brand-new voltage converter hummed and sagged the moment he switched his amp on. He had done his homework, or thought he had. His head was a 50-watt amp, so he bought a 100-VA converter, figuring double the wattage was plenty of margin. The converter was a tenth of the size it needed to be, and the amp barely lit up.
That mistake is the most common one in the whole business of getting a tube amp safely across an ocean, and it comes down to a single confusion — the number on the panel is not the number that matters.
Speaker Watts Are Not Wall Watts
Your amp has two completely different power figures, and only one of them sizes a converter.
The first is the output wattage, the one on the panel — 22 watts, 50 watts, 100 watts. That is the audio power the amp pushes into the speaker. It is a useful number for knowing how loud you will be and almost useless for anything to do with the wall.
The second is the mains draw, the power the amp pulls from the outlet to run everything inside it. Tube heaters glowing, the high-voltage plate supply feeding the tubes, the whole machine — all of that comes from the wall, and it adds up to far more than the speaker ever sees. My own '64 Deluxe Reverb, a 22-watt amp by the panel, draws something close to 120 watts from the wall when it is working. The output number and the draw number are not even in the same neighborhood.
So when you size a converter, you size it against the draw. If the back panel or the manual lists a current rating instead — say 1.5 amps at 120 volts — multiply volts times amps to get the draw in watts, and 1.5 times 120 gives you 180. Either way, you are after wall watts, not speaker watts.
I expected, the first time I sent an amp overseas decades ago, that the panel rating would be the figure to size against. I sized to it. The converter ran hot and the amp felt strangled, like it was playing through a wet sock, and it took an honest afternoon with a meter to understand that the amp was quietly demanding three or four times what I had budgeted for it. The panel had told me the truth about the speaker and nothing about the plug.
Why You Double It
Once you have the draw, double it to set the converter's VA rating. A converter rated in VA — volt-amps — wants to be roughly twice your amp's running draw.
The reason is the surge at switch-on. A tube amp does not sip power steadily from cold. The instant you flip it on, the filter capacitors charge from empty and the tube heaters are stone cold and pulling hard, and for a moment the amp gulps several times its running draw. A converter sized right at the running number sags under that surge, the voltage droops, and you get the hum and the brownout that nervous man heard on the phone. Size it at double and the surge has somewhere to go.
| Amp | Output (speaker) | Typical wall draw | Minimum converter |
|---|---|---|---|
| Blackface-style combo | 22 W | ~120 W | 250 VA |
| Mid-power head | 50 W | ~150 W | 300 VA |
| Full-power head | 100 W | ~250–300 W | 600 VA |
When in doubt, buy bigger. A converter with too much headroom costs a little more and weighs a little more and never causes a problem. A converter with too little ruins your night and possibly your amp.
Which Direction: Step-Up or Step-Down
The other half of the question is direction, and getting it backward ranges from disappointing to catastrophic.
| Destination | Wall voltage | Your 120 V amp needs |
|---|---|---|
| Japan | 100 V | Step-up to 120 V |
| UK / Europe | 230–240 V | Step-down to 120 V |
| Australia | 230–240 V | Step-down to 120 V |
Japan is the gentle case. At 100 volts, your amp is being underfed, so it goes soft — spongy low end, early breakup, less clean headroom. Some players chase exactly that and run their amps on low voltage on purpose. But if you want your amp to behave the way it does at home, a step-up converter from 100 to 120 hands it back its headroom.
The UK and Europe are the case with no margin for error. At 230 volts you are feeding roughly double what the amp was built for, and a 120-volt amp plugged straight into that will cook its power transformer in seconds, sometimes with smoke and always with a repair bill. A step-down converter is not optional there. It is the difference between a working amp and a doorstop. Check the voltage before the amp leaves the flight case, every single time, because this is the one mistake you do not get to take back.
The 50 Hz Footnote
One last thing, and it is a footnote, not an alarm. Most of the world outside the Americas runs 50-hertz mains where the US runs 60. The frequency mainly affects the power transformer, which runs a touch warmer on 50 hertz. A healthy amp does not care. A vintage amp with a tired old transformer can run warm enough over a long set to be worth a hand on the chassis at the break. It is a thing to watch, not a thing to fear.
For the full picture of how an amp's tone and feel actually shift on foreign power, the 100-volt touring guide covers what your ears will notice, and the Kill-A-Watt workflow covers how to read what a strange outlet is really giving you before you trust it. Measure first, convert with headroom, and your amp arrives on stage sounding like itself.
