Another archaic technology is magnetic amplifiers. These were more used in Germany (in the Kriegsmarine and also in the V-2), but got some more play in the US immediately after the war, before being largely supplanted by transistors.

The idea here was to use magnetic saturation to modulate the behavior of a transformer, allowing a small control current to modulate larger AC currents.

https://en.wikipedia.org/wiki/Magnetic_amplifier