Proximity fuzes for bombs in WWII were done with thyratron tube switches (or at least were done this way by the end of the war).
There is an oscillator in the circuit emitting RF and using the ground as it rushes up as a "plate" in a capacitor, it can trip the switch to explode. I wonder if thyratron tube switches could be use for Theremin design? (Or at least you could make the bomb "sing!")
Likewise, Thyratron tubes were replaced in Thyristors in the 1950's. It's a transistor that's a PNPN thingy. Sort of like putting two transistors end to end. I wonder if anyone has ever used a Thyristor in a Theremin?
Inquiring minds want to know....