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- Hacker News
- Love this kind of fearless DIY. Keep at it.by egl2020
- Would you be able to reseal the cracked glass and regenerate the vacuum through the other end?
More glass, epoxy, or similar?
- An 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.by adrian_b
- Plastic generally isn't an air tight seal. The leaks may be slow, but generally we hope vacuum tubes last for years.by bluGill
- Hmmmm. Wonder if you could just induct through the glass with coils on each side? Seems perfect for high voltage applications?by smlacy
- Interesting idea! Wouldn’t have to be particularly high voltage either.by K0balt
- That 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.by bluGill
- If 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-...
But most hollow-state devices run on either DC or pulses, so coupled inductors wouldn't work.
by tliltocatl - I'm surprised there was no mention of Fernico or Dumet metal. they were designed for this application.by mmmlinux
- > 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.
by kazinator - 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?by SyzygyRhythm
- In this case a glassworking lathe is needed to align and bring together the copper and glass tubes while the seal is being made.by mgc_mgc
- 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.
by projektfu - > "good enough"
Probably not. The classic fix is a "getter".[1]
by Animats - > 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?" ..
by rigonkulous - Erm, don't the guys making homemade Nixie tubes have this figured out?by bsder
- 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.by tliltocatl
- 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.
by johnwalkr - As 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
by labcomputer - It 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 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
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-...
by MisterTea - 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.by tyingq
- 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)?by alister
- 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 tungstenby janez2
- I'm not sure what specific glass and metal are used in neon sign electrodes, but their definitely built to hold a higher vacuum under decades of use. Their relatively cheap and you can get them with small tubes on the end for pulling the vacuum.
- Vacuum tubes are still made today, so I'm sure the knowledge is not lost. I'm curious about the answer as well.by SoftTalker
- 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.
by fuzzfactor - The article pretty much tells you: "Copper's red oxide bonds very well to glass. In fact, the bond is stronger than the bulk glass: when it breaks, there's always a thin layer of glass left stuck to the metal. Along with its excellent electrical properties, it seems like an ideal electrode material." If you look at how vacuum tubes are constructed that's essentially what you see.
Tubes are evacuated through a hole created elsewhere, nowhere near any electrical connections. The getter is then flashed to clean up any gas molecules left over.
by CamperBob2 - As mentioned in TFA, the most important factor for successfully joining a metal and a glass is to match their thermal expansion coefficients.
Most pure metals have a much greater thermal expansion than any glass, which will cause cracks.
In the nineteenth century, the first successful joinings of metal with glass were done using platinum, but that is obviously too expensive for normal applications.
Eventually a special alloy of iron-nickel-cobalt was developed, which is named kovar and whose thermal expansion is matched to that of a certain type of borosilicate glass.
The use of kovar was widespread in electronics, starting with the vacuum tubes and gas tubes, and then continuing with the first generations of transistors and integrated circuits, which used metal packages.
All the old transistors and operational amplifiers that were packaged in metal cans had pins and package bases made of kovar.
When kovar had to be joined with a different kind of glass than the type with which it is matched in thermal expansion, that glass was coated in one or more layers of different kinds of glasses, with that matched to kovar in contact with the metal and the intermediate layers having intermediate thermal expansion coefficients, interpolating between the bulk glass and kovar.
Kovar is not a good thermal or electrical conductor, which is why the modern power transistors that use plastic packages (e.g. TO-247) and copper bases and pins (which are plated with nickel or tin, to avoid corrosion) can easily dissipate much greater powers than the old transistors in TO-3 metal cans, which had the same size. On the other hand, the old transistors in metal packages were pretty much immune of environmental influences.
by adrian_b - 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.
by tliltocatl