> For the seal, this is a good thing: less metal means less expansion

But power tubes need to pass some decent amounts of plate current through some of the pins. Even small signal tubes have considerable current going through the heater filaments; you don't want hookup wires for that which are like metallic spider silk.

One thing about gallium/galinstan - it would actually make a descent high vacuum seal as it has lowest vapor pressure of all elements - so it doesn't evaporate. The problem is that it sticks to just about everything that isn't PE/PTFE. Galinstan thermometers use some proprietary coating to make glass repel it.

I was once entertaining the idea of using gallium for an electrostatically or MHD boosted Sprengel pump, but figured out sticking would make it infeasible. And now it's unobitanium too.

What was the large-scale commercial procedure for making electrodes that pass through the glass without letting air in? I assume that electronics manufacturers must have been making millions of such vacuum tubes in the past. Is the knowledge lost (or not practical for hobby use)?

The research here is clearly interesting, but if you just need to get something like this working, premade neon tube electrodes are plentiful and inexpensive.

I was wondering about the feasibilty of this, but I thought that useful tubes needed a harder vacuum than that. Is this really "good enough" for a triode?

I figured the wire-holding/element-holding aspect of a standard tube was in the base, and the glass-to-base seal is the important part. You can have a less-hot metal holding the filament and penetrating through the base. But I haven't looked carefully. These are my off-the-top-of-my-head thoughts about it.

Would you be able to reseal the cracked glass and regenerate the vacuum through the other end?

More glass, epoxy, or similar?

Hmmmm. Wonder if you could just induct through the glass with coils on each side? Seems perfect for high voltage applications?