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.
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)?
You may still be able to buy the prefabricated mass-produced tube bases for regular 9-pin miniature tubes.
These are just "wafers" of glass a few mm thick with the 9 metal pins through them. Both having matched expansion characteristics that were improved over a period of decades.
The base is held in a jig, the inner electrode assembly is tacked onto the proper pins at each point. Then the tough borosilicate glass tubing is lowered and sealed to the wafer.
Evacuation is from the top, and then that is sealed to a point like you see on any ordinary 12AX7-sized tube.
not sure if the thermal expansion of pyrex is much different from just borosilicate glass, but https://simplifier.neocities.org/diode3 seems to have worked something out with tungsten
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.
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.
> 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.
If we're talking homemade vacuum tubes... I wonder if it wouldn't be easier to just use metal endplates with feedthroughs for electrical (like spark-plugs) and with v-grooves for o-rings or some other gasket material. this kind of construction can handle vacuum easily, I think?
To everyone screaming "this would not work": it would work (with right kind of fluorine rubber) as long as the lamp is attached to the pump. Thermionics (but not CRTs) don't actually need UHV, HV is good enough, so no copper conflat necessary. If you want to seal the tube off - then yes, the tube needs a proper seal and a getter. Afaik, the getter is there to counter slow outgassing of tube internals (virtual leaks) rather than any external leaks.
In addition to the other comments, in general o-rings are actually very difficult to use for vacuum sealing. Unintuitively, at least until you have some experience in seal design, lower pressure differentials are much harder to seal than large ones. They depend on the pressure differential itself to form the seal, they don't work like a gasket between 2 faces. This is also why compressing in a v-groove doesn't make them perform better.
Even if you can get an o-ring to seal a vacuum, it will likely only work in the context of applying a vacuum to a system to a desired level, not as a permanent seal.
The article mentions that thin-walled copper tube can be used, but then says a precision lathe is required. But you can easily buy off-the-shelf copper tube with 1.6 or 2.0 mm diameter and 0.5 mm wall thickness. Is that not thin enough? If not, could one chemically thin the walls (acid, electrolysis) until it is enough?
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[ 3.2 ms ] story [ 43.3 ms ] threadI 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.
These are just "wafers" of glass a few mm thick with the 9 metal pins through them. Both having matched expansion characteristics that were improved over a period of decades.
The base is held in a jig, the inner electrode assembly is tacked onto the proper pins at each point. Then the tough borosilicate glass tubing is lowered and sealed to the wafer.
Evacuation is from the top, and then that is sealed to a point like you see on any ordinary 12AX7-sized tube.
More glass, epoxy, or similar?
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.
Probably non-ideal for vacuum tubes which do run a lot hotter than most other components.
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.
Even if you can get an o-ring to seal a vacuum, it will likely only work in the context of applying a vacuum to a system to a desired level, not as a permanent seal.
[0] https://en.wikipedia.org/wiki/Fernico
https://www.youtube.com/watch?v=wxL4ElboiuA