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Making glass-to-metal seals for homemade vacuum tubes
- LgWoodenBadger 4mo agoWould you be able to reseal the cracked glass and regenerate the vacuum through the other end? More glass, epoxy, or similar?
- adrian_b 4mo agoAn accidental crack can be resealed, but if the crack had appeared because inappropriate materials were used, e.g. an unsuitable metal-glass pair, resealing is pointless, because cracks will appear again after the device is turned on and off several times, causing expansion-contraction cycles.
- bluGill 4mo agoPlastic generally isn't an air tight seal. The leaks may be slow, but generally we hope vacuum tubes last for years.
- Supernaut 4mo ago> generally we hope vacuum tubes last for years If I may offer an anecdote, the output stage in my guitar amplifier is powered by a GEC tube that is now 55 years old. It sounds great. When I found the tube, it had been rolling around for a couple of decades at the bottom of a wooden box. This could simply be survivorship bias, but it does appear that back in the day, they knew how to build these things to last.
- egl2020 4mo agoLove this kind of fearless DIY. Keep at it.
- projektfu 4mo agoI 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.
- rigonkulous 4mo ago> Is this really "good enough" for a triode? Let us not overlook that its also a lot of energy. Its not a matter of "good enough", I think in this case - more "can I?" ..
- Animats 4mo ago> "good enough" Probably not. The classic fix is a "getter".[1] [1] https://en.wikipedia.org/wiki/Getter https://en.wikipedia.org/wiki/Getter
- smlacy 4mo agoHmmmm. Wonder if you could just induct through the glass with coils on each side? Seems perfect for high voltage applications?
- K0balt 4mo agoInteresting idea! Wouldn’t have to be particularly high voltage either.
- bluGill 4mo agoThat depends. Often vacuum tubes are used with DC (that is a rectifier) in some form though, in which case you can't do this since induction depends on AC. I'm not sure what purpose the article had for a triode though, depending on their application this might work.
- tliltocatl 4mo agoIf you want a gas discharge tube and not vacuum, you can even drop the coil on the inside: https://hackaday.io/project/194683-plasma-toroid-sky-guided-pcb-edition https://hackaday.io/project/194683-plasma-toroid-sky-guided-... But most hollow-state devices run on either DC or pulses, so coupled inductors wouldn't work.
- smlacy 4mo agoWouldn't work ... without additional electronic components.
- tliltocatl 4mo agoThan you can skip the hollow state part altogether. And any plastic-package parts would screw your vacuum up badly - if it survives sealing and bakeout that is.
- alister 4mo agoWhat 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)?
- SoftTalker 4mo agoVacuum tubes are still made today, so I'm sure the knowledge is not lost. I'm curious about the answer as well.
- NoMoreNicksLeft 4mo agoI was under the impression that they were only made in eastern Europe at this point, former bloc nations. Even then, the demand must be microscopic at this point.
- drum55 4mo agoxray, photomultiplier and laser tubes are still SOTA. PM tubes in particular have a huge number of glass feed through for the intermediate plate voltages.
- SoftTalker 4mo agoMusical instrument amplifiers still use them as well, at least some of them.
- tyingq 4mo agoThe research here is clearly interesting, but if you just need to get something like this working, premade neon tube electrodes are plentiful and inexpensive.
- tliltocatl 4mo agoOne 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.
- fuzzfactor 4mo agoPure galium has such a low melting point that it liquefies and acts like mercury if you hold a few grams in your hand. Probably non-ideal for vacuum tubes which do run a lot hotter than most other components.
- tliltocatl 4mo agoJust keep it liquid, that's the point. If it freezes, it bursts the glass, as the OOP says.
- kazinator 4mo ago> 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.
- crispyambulance 4mo agoIf 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?
- MisterTea 4mo agoIt will not hold a vacuum. Gas can diffuse through rubber and rubber itself will out-gas in very low vacuums. Metal and glass seals are the only truly air tight sealing methods. Even then, remaining gas is trapped in tubes and a getter https://en.wikipedia.org/wiki/Getter https://en.wikipedia.org/wiki/Getter is activated at factory to trap this remaining gas on the tube wall in a thin layer of metal. That is why many tubes have a dark silver coating at the top or a band, that's the evaporated getter metal. See also: https://en.wikipedia.org/wiki/Materials_for_use_in_vacuum https://en.wikipedia.org/wiki/Materials_for_use_in_vacuum Also, don't use V grooves for O-rings. Read the Parker o-ring handbook for proper seal design: https://test.parker.com/content/dam/Parker-com/Literature/O-Ring-Division-Literature/ORD-5700.pdf https://test.parker.com/content/dam/Parker-com/Literature/O-...
- labcomputer 4mo agoAs the other replier mentioned, that won't work. What would work is to mill a knife edge into both the end plate and the can and use off-the-shelf copper Conflat ("CF") gaskets, which are available in a large number of standard sizes. You'd have to work out a way to hold a vacuum in the can while you're tightening the flange though
- tliltocatl 4mo agoTo 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.
- 4mo ago
- mmmlinux 4mo agoI'm surprised there was no mention of Fernico or Dumet metal. they were designed for this application. [0] https://en.wikipedia.org/wiki/Fernico https://en.wikipedia.org/wiki/Fernico
- bsder 4mo agoErm, don't the guys making homemade Nixie tubes have this figured out? https://www.youtube.com/watch?v=wxL4ElboiuA https://www.youtube.com/watch?v=wxL4ElboiuA
- SyzygyRhythm 4mo agoThe 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?
- mgc_mgc 4mo agoIn this case a glassworking lathe is needed to align and bring together the copper and glass tubes while the seal is being made.