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?
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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."
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?
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
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.
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.
> 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.
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.
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.
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.
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?
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.
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.
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.
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. :)
Incandescent bulbs are 0.8 atm (equivalent to 15,000 ft altitude) when off, and when they're on, they're actually slightly pressurized.
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.
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.
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.
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.
Those are Not where the tubes are SOTA.
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 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.
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.
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.
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."
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?
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
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-...
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.
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
> 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.
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.
> 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?" ..
> "good enough"
Probably not. The classic fix is a "getter".[1]
[1] https://en.wikipedia.org/wiki/Getter
Would you be able to reseal the cracked glass and regenerate the vacuum through the other end?
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.
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.
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.
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.
Erm, don't the guys making homemade Nixie tubes have this figured out?
https://www.youtube.com/watch?v=wxL4ElboiuA