A variac is a variable transformer used to supply adjustable power for many projects and is invaluable. They are also expensive as they are large and heavy. Here, I explore the solid state option which may be preferable in some situations.
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This Solid State Variac I lashed up uses an IGBT out of an inverter microwave run by a TL494 at 2kHz. The tricky part is that it is “inside” a bridge rectifier so that it controls AC. This is inserted between the mains and the NST. There is also a snubber of 0.1uF and 400 that gets pretty hot. I haven’t tuned this for long term runs yet as a lot of power is still being dumped into a 230V MOV across the IGBT. (Fortunately I have 300 of these).

Pic shown full on running one half of an NST. You should be able to follow the wiring setup from the photo Yes I am turning an “old fashioned” variac but just to supply power to the setup.

I don’t really think that it would be more expensive than a variac for NST use of less than 5A 240V. The main rectifier could be 3A diodes, and the IGBT/Mosfet could be almost any one with a 500V rating at 10A. The TL494 circuit could be a 555 in this situation. The snubber may generate some heat and a mains filter is probably wise. A small 12V transformer or even old 9V plugpack could supply power. As ususal it depends how much can be scrounged up.
If this can be made to work reliably and effectively then it may be of interest to those who use NST’s.
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Photo Date: 2005