Trying to rig a variac switch to slowing raise the voltage on an old tube amp

DonL · Oct 19, 2013

John in herndon said:
I used to be in the audio repair business and used this technique all the time on both tube and solid state amps. It prevents doing a lot of damage if you didn't find all the blown components.

That said, you have to use a VARIACÂ® or variable autoformer to do this because this preserves the sine wave of the AC power. Solid state dimmers do not do this. They use SCRs and TRIACs to vary the voltage by delaying the turn on point of each individual cycle of the AC waveform. The result is a non sinusoidal waveform which would not work to power an amplifier or other electronic device.

If it doesn't weigh 10 pounds or more it is not a transformer and won't work.

You can use a variable high power resistor also, If you do not need isolation from the mains using a transformer.

I never have seen a DC transformer that lasted very long. They do not like chopped DC, unless they are designed for it.

I use a light bulb in series with the AC line and it gives a good indication of your situation.

Have Fun.

Wet_Boots · Oct 20, 2013

Besides seconding the "use a genuine variable transformer" advice, there is also a caution for some circuitry, where it can actually be advisable to construct a replacement for a rectifier tube, using solid-state components and a tube-base connector. I don't have more details at my fingertips, but I know the idea behind the temporary replacement was that the initial low voltage applied in a series of several steps was not going to be enough to make a rectifier tube functional, and the solid-state replacement would function with any input voltage applied to the tube device's power transformer.

Trying to rig a variac switch to slowing raise the voltage on an old tube amp

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