Gary,
A quick and easy way to get an isolation transformer of low to moderate power say 100 watts or less is to use a low voltage transformer,
lets say out of an older mobile radio supply ( 12 volts at 10 amps ), and connect it backwards.
You would use your variac to supply the low voltage, say 15 to 18 volts, and connect that to the transformer’s low voltage secondary.
Adjust the variac to get the 110 – 120 volts on the primary.
A 10 amp supply of 12 volts will probably get you a 100 to 120 watt isolation transformer.
That’s enough to run virtually almost any smaller tube radio.
For example An all american five with miniature tubes requires around 50 watts.
This assumes your variac can deliver about 9 to 10 amps.
Just an idea from an old scrounger !
