In the early days of transistors, RCA and GE were battling against silicon with ever smaller vacuum tubes. These tubes – Nuvistors, Compactrons, and some extremely small JAN triodes were some of the tiniest tubes to ever be created. [glasslinger], YouTube’s expert on DIY valves, is pretty close to beating the tiniest tubes that were ever manufactured. He’s created a miniature diode and triode that are about 1/4″ in diameter and 1″ long.
The most difficult part of making a vacuum tube is getting a perfect glass seal around the pins. For this, [glasslinger] is using very fine tungsten wire and glass beads. A bead is placed around each wire, mounted in a stand, and melted together with a torch.
A diode is simple as far as tubes go, requiring only a filament between two pins. [glasslinger] is just stringing a fine piece of wire between two pins and welding them on with a miniature spot welder. After that, it’s just an issue of melting a 1/4″ glass tube to the base of the tube, putting it under vacuum overnight, and sealing it shut.