So, there is my first nixie tube. I was really eager to make anything sealed, so I didn’t pay much attention to details and lifetime of the tube. There are two (stainless steel) cathodes – “0” and “7”, a small piece of stainless steel sheet is used as the anode. Distance of the cathodes to the anode is different, so behaviour (breakdown voltage, current needed) is different – it is easier to light the “7”, the “0” gives uneven glow. However, when I fill it on the manifold, both striked nicely, I think that some moisture from the glass was evaporated into tube during the tip-off.
Finally! I saw a neon glowing today, first time in my system! I filled the tube first on 1 torr with argon, then to 32 torr with neon, so the ratio is 1:31 = 97% neon and 3% argon. The glow doesn’t cover whole digit, the shape of electrodes is not perfectly parallel and there is just a back plate as the anode, so the electric field is not homogenous. So I have now reliable gas filling manifold. From now on, I am going to play with glass and make some “lab samples” 😉
I unfortunately drop my glow tube on the ground and the valve broke out.. So I just weld a tube onto it and use it without it, what a pitty 🙁
The last thing I had to make was a connector to neon bottle. This is a low pressure (12 bar) disposable aluminum bottle used in neon sign making. There is 7/16 UNEF thread, it uses the same standard as cans for camping stoves. I machined it from brass so I would be able to solder the copper capillary into it directly without playing with teflon washer. After screwing the connector to the bottle, there is no flow from of the gas. You have to turn down the stem where is a thorn, this thorn opens the seal in the bottle and gas is released out. The gas fills the inside space of the connector and it could be closed again. Now there is a few cm3 filled with neon of pressure at 12 bar, plenty for an experiment 😉
I have been looking for reliable vacuum needle valve for a long time. It should not be to expensive, it should allow me to control the flow fine and not cost much. I tried glass system, cheap valves from China and more, but I was not satisfied with any of these. But as usually, Ron came out with great design of a needle valve what could be made on a lathe easily. Just a aluminum bar, some common fittings, teflon rod and a piece of a mild steel bar. Easy! I decided to take some photos during machining so that anyone could make his own!
The last part of the manifold was the pressure gauge, I had the 0-100 torr baratron, but no controller. The baratron needs supply of +15V and -15V. It gives linear output of 0-10V for 0-100 torr. So I build a simple power supply using two zener diodes (15V) and two 15Ω resistors. If You are interested in schematics, just write a comment. The input power is 40V, output ±15V. A cheap voltmeter from Ebay was used to display the output:
I finally made a progress in making my new gas filling manifold. I finally made it modular instead of brazing it together. I had really big problems with brazing the aluminum, I use the “Castolin 190 FB” solder and had to learn how to use it.. It is necessary to heat the aluminum sufficiently (approx. 600°C) so that the solder melts immediately after touching the joint. When the solder cools down, it makes white layer of flux what have to be filed off before another soldering – when You want to repair it. I initially used just gas-air torch, but it was not able to heat it enough, the solder stuck to the aluminum and formed a sphere instead of melting and flowing into every small cavity (it is a capillary type solder). Then I took the oxy-gas torch and the success came immediately! It was able to heat all the brazed part so that the solder made really nice and uniform weld (see the last picture)!
I have just made first test with the glow tube I made yesterday.. It works, all seals seems to be vacuum tight, or at least good for my purposes (short-term test). It turned out that making the glow even along all the number is pretty difficult, I will have to form the anode grid flat and probably add a back plate. Or just add the back plate and remove the anode grid.
The gas used is argon, striking voltage around 350V.
I spent some time on the leak detector recently. There are two fans, AC 230V, one for the diffusion pump, one for cooling the electronic box. Both started to make loud noise, so I had to replace them. I was surprised that the new fans are absolutely identical to these original 30 (or how many) years old ones! The same faston connectors, threaded holes for ground of the same diameter.. I would not believe something on that old machine could turn out to be so easy 😉
This machine has two vacuum pumps, one for making rough vacuum in inlet port, one for making vacuum in detection section (together with diffusion pump). When You want to test some tube for leaks, the detection section is started first – it has to pump itself down to 5×10-5 torr, the filament could be switched on only around 10-5 and 5×10-4. It would react with oxygen at higher pressure and would gradually burn off. When the detection section is ready, the roughing pump is started, it pumps the inlet port with tested tube down to 10-2 torr. Than, with roughing pump still running, the manual valve between detection section and inlet port is slowly slightly opened until the pressure in detection section rises back to 5×10-5 torr, than the helium could be sprayed onto the tested tube to detect leaks. In this moment, the pressure in the inlet port is still on 10-2 torr despite the fact that it is connected to detection section where is much lower pressure – the manual valve is opened very slightly. All the process is automated and controlled by solenoid valves, one opens detection section, one allows the air go inside (to atmosphere pressure) and one opens the valve with roughing pump.
A little bit longer introduction 😉 But.. The machine in this form doesn’t allow me to pump the inlet port down to high vacuum because when the detection solenoid is opened also the roughing port is opened.. And I would like to use this machine to pump tubes down to high vacuum.. So I decided to add some level of user-control 😉 So I cut the wires going to solenoid valves and brought them to a little box with three switches (guitar 3way selector switch).