Making glass-to-metal seals for homemade vacuum tubes

by zdw • 1 month ago

147 50 1 month ago

projektfu• 1 month ago

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.

rigonkulous• 1 month 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• 1 month ago

"good enough"

Probably not. The classic fix is a "getter".[1]

[1] https://en.wikipedia.org/wiki/Getter

smlacy2• 1 month ago

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

K0balt• 1 month ago

Interesting idea! Wouldn’t have to be particularly high voltage either.

bluGill2• 1 month ago

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.

tliltocatl6• 1 month ago

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.

smlacy2• 1 month ago

Wouldn't work ... without additional electronic components.

alister• 1 month ago

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)?

SoftTalker3• 1 month ago

Vacuum tubes are still made today, so I'm sure the knowledge is not lost. I'm curious about the answer as well.

NoMoreNicksLeft• 1 month ago

I 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.

ludicrousdispla• 1 month ago

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.

CamperBob22• 1 month ago

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.

adrian_b4• 1 month ago

Nope, as also mentioned in TFA, copper has a too great thermal expansion coefficient in comparison with glass.

If vacuum tubes had pins of copper, the glass-metal joining would have cracked very soon during normal usage cycles, and there would have been no vacuum left in the tube.

Real vacuum tubes and gas tubes had pins made of kovar, which is a Fe-Ni-Co alloy with a TCE matched to a certain composition of borosilicate glass.

The kovar pins were normally plated with nickel on their external parts, to enable soldering, because molten solder does not wet kovar.

adrian_b4• 1 month ago

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.

SoftTalker3• 1 month ago

Ordinary incandescent bulbs must have similar sealing requirements, but they probably mostly rely on using a thin conductor that doesn't contract much when it cools. Also IIRC modern incandescent bulbs do not use a high vacuum but contain a low pressure inert gas so leakage would be slower if it occurs at all.

phendrenad2• 1 month ago

A lot of thought went into selecting the correct metal and glass for the application.

Kovar[1] was typically used in commercial applications where tubes were constructed from hard (borosilicate) glass. In fact, there were special formulations of borosilicate, such as Corning 7052, and later 7073, which were designed to match with Kovar. So both the metal and the glass were designed to work together. This involves engineering the metal and glass such that they shrink at about the same rate from the glass's setting point (temperature where the glass's internal stresses start to align, but before the glass solidifies) down to room temperature.

An aside on how Kovar works, because it's neat: Kovar is ferromagnetic, and the mixture of metals changes the Curie Point - the temperature above which which a ferromagnetic material stops being magnetic due to the atoms being too energetic. The Curie Point isn't a single point, it's a region. Ferromagnetic materials' lattices actually expand as they become more magnetic - this is the Magnetovolume Effect. So by adjusting the ratios of materials, Westinghouse was able to balance the Magnetovolume Effect (materials wants to expand as it cools and regains its magnetism) with the natural lattice shrinking due to cooling, and create a region where the metal matches the shrink rate of glass.

Conversely, consumer-grade vacuum tubes, such as the ones in radios, guitar amplifiers, incandescent bulbs, and televisions, typically use cheaper soda-lime or lead-alkalai silicate glass[2]. This glass had completely different thermal expansion characteristics, so different materials for leads were required. For thin leads, what they typically[3] did is use a Dumet (42/58 Nickel/Iron) wire clad in a copper sheath and coated with borax. The bonded dumet-copper (about 80/20 by weight) expands at a compromise between the two, so it can be matched to the thermal expansion of glass. The borax aided in oxide control and bonding to the glass (this is copper's "red oxide" as mentioned in the article). But this format only works for thin wires, because as we accumulate surface area we start to have to worry about axial stress from the wire expanding along its axis. So for larger leads, a (more expensive, less conductive) one-piece alloy of 52/48 Nickel/Iron had to be used instead[4].

The anodes of CRTs used yet another alloy, designed for higher expansion volume, known as "Glass-Sealing 42-6", and standardized as ASTM F31. These are 42/6/52 Ni/Cr/Fe alloys.

Lastly, to bring it all back home, the glass matters as much as the metal, and the author of this article is using an exceptionally poor glass for vacuum tube work. It seems like they are using regular Pyrex, which has a much lower expansion coefficient than most vacuum tube glass, and in fact, most metals.

[1] - The generic term for Kovar is Fernico (from Iron-Nickel-Cobalt, Fe-Ni-Co). It was invented by Westinghouse in the 1930s. Other names for Kovar are: ASTM F-15, NILO K, Pernifer 2918, Rodar, and Dilvar P1.

[2] - An exception is tubes that experienced high temperatures that might melt normal glass - such as Xenon flash tubes. Another exception is metal vacuum tubes which had small glass borosilicate beads around each lead wire, bonded to both the wire and the surrounding metal. These were common in 1940s radios.

[3] - US Patent 4824459 - Marker Pin for a Universal Stem Mold - https://image-ppubs.uspto.gov/dirsearch-public/print/downloa...

[4] - Interesting footnote: Platinum also works great as a soft-glass seal wire, if you have the $$$$. Dumet was originally marketed as "platinite" - a platinum substitute.

fuzzfactor2• 1 month ago

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.

janez2• 1 month ago

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

LgWoodenBadger• 1 month ago

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

More glass, epoxy, or similar?

adrian_b4• 1 month ago

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.

bluGill2• 1 month ago

Plastic generally isn't an air tight seal. The leaks may be slow, but generally we hope vacuum tubes last for years.

Supernaut• 1 month 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.

tyingq• 1 month ago

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.

tliltocatl6• 1 month ago

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.

fuzzfactor2• 1 month ago

Pure 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.

tliltocatl6• 1 month ago

Just keep it liquid, that's the point. If it freezes, it bursts the glass, as the OOP says.

kazinator• 1 month 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• 1 month ago

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?

MisterTea• 1 month ago

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.

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-...

labcomputer• 1 month ago

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

tliltocatl6• 1 month ago

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.

johnwalkr• 1 month ago

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.

mmmlinux• 1 month ago

I'm surprised there was no mention of Fernico or Dumet metal. they were designed for this application.

[0] https://en.wikipedia.org/wiki/Fernico

bsder• 1 month ago

Erm, don't the guys making homemade Nixie tubes have this figured out?

https://www.youtube.com/watch?v=wxL4ElboiuA

SyzygyRhythm• 1 month ago

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?

mgc_mgc• 1 month ago

In this case a glassworking lathe is needed to align and bring together the copper and glass tubes while the seal is being made.

egl2020• 1 month ago

Love this kind of fearless DIY. Keep at it.

Making glass-to-metal seals for home­made vacuum tubes

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Making glass-to-metal seals for homemade vacuum tubes