Take 3 small 230V primary to 6VAC secondary 2VA potted transformers, and use them in reverse. Feed the parallel secondaries with your adjustable 4-6VAC, and wire the primaries ( now your secondaries) in series, so you have an adjustable AC source, with a low power capacity, but able to drive a meter input. Just make sure you wire the secondaries so they add up in voltage.
Supply can be an audio amplifier fed from a signal generator, or simply a 10VA 8VAC transformer with the primary fed with a variac. Your transformer basic insulation for the potted types is easily capable of withstanding 600VAC, as they have to be able to withstand 2kV for flash over testing in manufacture.
I did a similar thing to make some vacuum tube power supplies for a Marconi moisture meter, using what I had on hand, 220VAC to 12VAC 2VA transformers, using 5 transformers to make a 1V4 filament supply ( LM317 regulator and 2 resistors) and a 60V supply, using a string of 12V 1W zener diodes and a resistor. It worked for years, and was a lot cheaper than the weekly buying of PM3 batteries and D alkaline cells.