GoKawiilMaking glass-to-metal seals for homemade vacuum tubes.
2026-06-13
This page discusses sealing metal through borosilicate/lab glass: other chemistries behave quite differently.
When making vacuum tubes, the glass is actually the easy part: premade tube stock of almost any size is easily available.
Heating the end of such a tube softens the glass and allows surface tension to close it off.
I used a rotary vane pump to remove all the air from the tube and heated the middle, which the atmosphere crushed to create a sealed-off ampule.
Because glass is practically impermeable, it will retain that vacuum for a very long time, which can be shown by bringing it close to high-voltage AC (like a tesla coil):
This glow is due to residual air being ionized, but the fuzzy appearance indicates that the vacuum is good enough to work in a triode or similar device.
For those, the capacitive coupling trick won't work: I'll need to make electrodes that pass through the glass without letting air in.
This is a lot harder than it might appear.
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