Making glass-to-metal seals for homemade vacuum tubes(maurycyz.com)
maurycyz.com
Making glass-to-metal seals for homemade vacuum tubes
https://maurycyz.com/projects/glass/1/
50 comments
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.
Probably non-ideal for vacuum tubes which do run a lot hotter than most other components.
Just keep it liquid, that's the point. If it freezes, it bursts the glass, as the OOP says.
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)?
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.
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.
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.
As you say, incandescent bulbs are less demanding because they do not use high vacuum, but they have the additional requirement that the pins that support the tungsten filament must resist to very high temperatures, because some heat is conducted through the filament into its support.
This is why the pins that support the filament are typically made of molybdenum. Molybdenum has a relatively low thermal expansion coefficient in comparison with most metals, so there are certain glass compositions that can match its TCE. The glass through which the pins pass is not of the same type as the bulb, which is made of cheaper glass, but it is of the type matched in TCE with molybdenum.
This is why the pins that support the filament are typically made of molybdenum. Molybdenum has a relatively low thermal expansion coefficient in comparison with most metals, so there are certain glass compositions that can match its TCE. The glass through which the pins pass is not of the same type as the bulb, which is made of cheaper glass, but it is of the type matched in TCE with molybdenum.
AFAIK, in in the US: Modern high-efficiency incandescent bulbs are exactly like the halogen bulbs of yore, but with the small quartz envelope wrapped in a familiar light-bulb-shaped shell, and with a base that matches. It's like a light bulb within a light bulb.
See this random example of a GE bulb (which I selected just because it includes the first picture I could find of a modern bulb made with clear glass): https://www.toolboxsupply.com/products/ge-lighting-62616-ene...
Except for all the ones that aren't modern or efficient. Common 40-Watt appliance bulbs, for instance: Those are still built using the old methods. They never changed. This strongly suggests that we never forgot how to seal metal wires into a glass bottle full of nothing.
But this article isn't about industrial processes. It's about rediscovering things at home, and that stands on its own merits. :)
See this random example of a GE bulb (which I selected just because it includes the first picture I could find of a modern bulb made with clear glass): https://www.toolboxsupply.com/products/ge-lighting-62616-ene...
Except for all the ones that aren't modern or efficient. Common 40-Watt appliance bulbs, for instance: Those are still built using the old methods. They never changed. This strongly suggests that we never forgot how to seal metal wires into a glass bottle full of nothing.
But this article isn't about industrial processes. It's about rediscovering things at home, and that stands on its own merits. :)
Incandescent bulbs used dumet/platinite, which is an nickel-iron alloy like kovar except it's turned for a different CTE. This stuff isn't that expensive when mass produced - it's just those who can afford usually borosilicate can afford paying a premium for kovar.
Incandescent bulbs are 0.8 atm (equivalent to 15,000 ft altitude) when off, and when they're on, they're actually slightly pressurized.
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.
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.
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.
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.
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.
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.
Right, point being that metal-glass seals are very effective given the right materials. This has nothing to do with how the tube is evacuated, and there is never a point where the wires have to be "passed through the glass without letting air in."
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.
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.
Vacuum tubes are still made today, so I'm sure the knowledge is not lost. I'm curious about the answer as well.
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.
xray, photomultiplier and laser tubes are still SOTA. PM tubes in particular have a huge number of glass feed through for the intermediate plate voltages.
Musical instrument amplifiers still use them as well, at least some of them.
A lot of the demand is from guitar amps, while you can just as well simulate the behaviour of a valve amp these days there's still plenty of demand for the real deal. As long as the likes of Fender, Marshall and so on want to supply valve amps to the mass market there'll probably be factories producing ECC83s, EL34s and the other common audio valves. In a lot of ways the combination of electric guitar + valve amp basically is the instrument in some styles.
There's also a niche HiFi market, my daily driver audio amp is a 1960s Leak EL84 amp. It's a cool bit of living history and it's very non-fatiguing to listen to for long periods.
There's also a niche HiFi market, my daily driver audio amp is a 1960s Leak EL84 amp. It's a cool bit of living history and it's very non-fatiguing to listen to for long periods.
They are also made in China. There is also one American company that makes a single audio tube, the 300b. They bought the Western Electric tooling, materials, and trade dress from whoever had bought it from AT&T.
I’m not sure where non audio vacuum tubes are made. I’m sure there’s a variety of companies around the world making X-ray, transmitter, laser, etc. tubes.
I’m not sure where non audio vacuum tubes are made. I’m sure there’s a variety of companies around the world making X-ray, transmitter, laser, etc. tubes.
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.
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.
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?
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-...
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-...
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
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
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.
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.
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.
Erm, 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
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.
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.
> 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?" ..
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?" ..
> "good enough"
Probably not. The classic fix is a "getter".[1]
[1] https://en.wikipedia.org/wiki/Getter
Probably not. The classic fix is a "getter".[1]
[1] https://en.wikipedia.org/wiki/Getter
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?
In this case a glassworking lathe is needed to align and bring together the copper and glass tubes while the seal is being made.
> 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.
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.
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
[0] https://en.wikipedia.org/wiki/Fernico
Hmmmm. Wonder if you could just induct through the glass with coils on each side? Seems perfect for high voltage applications?
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.
But most hollow-state devices run on either DC or pulses, so coupled inductors wouldn't work.
Wouldn't work ... without additional electronic components.
Than 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.
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.
Interesting idea! Wouldn’t have to be particularly high voltage either.
Would you be able to reseal the cracked glass and regenerate the vacuum through the other end?
More glass, epoxy, or similar?
More glass, epoxy, or similar?
Plastic generally isn't an air tight seal. The leaks may be slow, but generally we hope vacuum tubes last for years.
> 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.
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.
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.
Love this kind of fearless DIY. Keep at 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.