And don't forget the first generation of R/C receivers intended for use in model airplanes. They needed something small, lightweight, battery-powered and inexpensive, so voila - miniaturized valves. I can't claim to have seen one "in the flesh" yet, but I have a hobbyist book on what they used to call "micro-electronics" in the mid-50s (ooh, printed circuit boards instead of solder lug strips!) which shows a device about the size of a packet of cigarettes with several such valves directly soldered onto a board with the wires coming out of them. Anybody here been R/C-ing back then?