Isolation transformer upgrade for old guitar amps

Step 6 Choosing an isolation transformer

Unlike many power transformers, isolation transformers have a 1:1 voltage ratio. The output voltage is (for practical purposes) identical to the input voltage. They serve only to "isolate" the device from the high-current potential of the mains. DON'T use an auto-transformer--they don't isolate.

Transformers also have a Volt-Ampere or  VA rating. VA is roughly analogous to wattage (remember, wattage = voltage * amperage, or wattage = V * A.) for resistive circuits, but not for inductive loads. For inductive load, you can "guesstimate" wattage capacity = VA * 0.7, or the wattage of an inductive load is ~70% of the VA.

Wiki page on the Volt-Ampere.

So the first question is:
What is the total power consumption of the amplifier?

I.E., NOT the output wattage, it's only a fraction of the total wattage it takes to run small amps.

Most amplifiers have a power consumption rating on the back. My Gregory doesn't, but it's safe to compare it to other three-tube amps. My little Kay amp consumes 28 watts. My Danelectro DM-10 (4 tubes) is closer to 40 watts.

It's a safe guess that most three-tube amps don't consume anywhere near 40 watts of power, and probably not 30 watts. Since more than half the load of a small amp is resistive (the tube filaments), and 70% of 50VA is 35 watts, then a 50 VA rated transformer should be fine.

So we're going with a Triad N68-X isolation transformer, with a 50 VA rating. Good stuff.

The N-68X is inexpensive, and can be purchased at various online electronics stores. One example:
Allied Electronics (for $11.41 USD.)

Mouser has it, and Digikey probably does, too.

If your amp requires more than 50 VA, Triad also makes larger transformers. Of course, isolation transformers from other manufacturers will work just as well...