Making glass-to-metal seals for homemade vacuum tubes
maurycyz.com
maurycyz.com
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.
Probably non-ideal for vacuum tubes which do run a lot hotter than most other components.
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.
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.
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.
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.
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.
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.
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. :)
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.
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.
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-...
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
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.
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.
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?" ..
Probably not. The classic fix is a "getter".[1]
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 most hollow-state devices run on either DC or pulses, so coupled inductors wouldn't work.
More glass, epoxy, or similar?
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.