I was recently at the Central Coast Amateur Radio Club’s annual get together. And I saw this beautiful glass vacuum tube there.

It’s a vintage Russian KP1-4 high voltage capacitor. Basically two metal plates in a vacuum separated from each other by a small distance. What makes this unit cool is that it has a dial on the top to allow you to vary the spacing, and hence the capacitance. There’s some flexible vacuum bellows, and a leadscrew to allow you to change what’s inside the high vacuum glasswork without breaking the seal.

I figured the unit was going to be a bit fragile to have on the shelf by itself, and also tricky to connect to if I wanted to experiment. So I made this lasercut plywood box to hold it, and have acrylic windows to stop prying fingers getting near the HV. There’s a pair of connectors on the side so it is somewhat functional. (Although I’m sure the bandwidth of my banana plugs would be terrible if I actually tried to use this for radio stuff).

Interestingly I can’t see any “getters” inside the vacuum enclosure. Perhaps there is a hidden one, or maybe it’s just that as the whole assembly stays cold (no thermionic emission), it is less sensitive to bad vacuum than other tubes are.