GoKawiilAfter my presentation at Hackaday Superconference 2022 I was asked by my friend David to write a piece for Make: Magazine. I originally wrote the article around the diode tube that I presented at Supercon. However, because I had built that tube hastily in the days before the conference, I didn’t have a lot of good photos of the process. I decided the best thing to do was to make another tube specifically for the Make: article and take care to document the process this time.
The article ran in Make: Volume 86 but couldn’t contain all of the photos that I took at full resolution, so I thought it would be fun to go over them in more detail here.
The Ol’ Flare and Pinch
The hardest part of making a vacuum tube is fabricating the base, where the pins pass-through from atmosphere to the high-vacuum side. The easiest way to achieve this as a hobbyist is the pinch method. With the proper combination of metal and glass, a pinch seal can be extremely robust. Because I prefer to work with borosilicate glass, my feed-through metal needs to be Tungsten (although Molybdenum foil also works as well as specialty alloys such as Kovar). To make a vacuum-tight pinch, the Tungsten wire needs to be cleaned to remove any oils and other junk and then treated with a flame to apply a layer of oxide. The Tungsten oxide is what actually binds the metal to the glass so it’s important to have a uniform film. This sounds more difficult than it is. I’ve made good working seals by cleaning the wire with alcohol and then heptanes, passing it over a flame until white hot, then rubbing off the loose oxide with a cotton swab. This method is demonstrated by jdflyback in this YouTube video about building a handmade triode.
Because Tungsten is brittle, I first spot-weld short Tungsten pins onto longer pieces of Nickle wire. The Nickle is more mechanically forgiving and readily welds to the Tungsten, while the Tungsten portion wets to the glass and creates our seal.
Pins being prepared in my extremely high-tech clamping fixture
Once the pins are prepared, we need a piece of glass to pinch them in. The glass needs to be a smaller diameter than the tubing for the envelope of the tube so that once it’s pinched, it still fits inside. That’s why the end is flared, to make it easier to join with the envelope.
Behold. A bad, small, trumpet.
To make the flare, I simply chucked up the tubing in a power drill and heated the end with a MAPP gas torch while turning it. I nudged it gently with a tool to get the end to flare. Ideally, I would do this in the glass lathe, but I haven’t made a collet to hold these small parts. After the flare is made, I pre-flatten it just enough to support the wires before the pinch.
Here you can see the way that the pinch will fit in the envelope.
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