Danth A Variac will work but I can remember I think it slows down the motor that spins the drum as well (which runs of rectified AC via a transformer). The slowing of the motor is a problem. I have a Variac (Zenith/Claude Lyons) they are expensive, heavy and for the Gene the cheap Chinese ones won’t do. They may say 8 or 10 amp, but they are not. I have one of the Chinese units as well and it weighs half what mine does and my big Variac is 8 amp.
The gene draws about 5 amp…
The dimmer mod is the easiest way to do it and I came up with it 15 years ago, just because it doesn’t affect motor speed…only the heating element power. It also allows you to use a 230V element if your in the UK. There is a wide range of voltage control units on ebay now instead of modding a light dimmer, and they work very well, especially with a little heat sink and the correct value potentiometer.
In fact this would be ideal wired only to the heating element….https://tinyurl.com/25twjjdr
And of course you need a power measuring wall plug