Something akin to these is what I use.
https://www.alliedelec.com/circuit-p...20Stock%20Only
The way I understand it there is a motor run cap inside that regulates the power through a resonance effect. They can hold outut voltage rock stable with anything between 90-140 input...Heck they can even hold stable output voltage through a missing cycle or two of input.
I see new ones are stupid expensive...I got all of mine by haunting Hamfests and antique radio swapmets...I don't think I paid over $25 a pop for mine. Every one of the 4 I've got was a good purchase, and I'm still lookin for more.
The advantage of a CVT is there is no switch mode supply to potentially generate RF hash on the line...Also fewer parts means less to go wrong.