Many types of vacuum tubes require a hot cathode, which typically is (or is built around) a tungsten filament. Therefore, before attempting to make a more complex type of tube, I built a small light bulb to get experience working with a hot filament in a vacuum. A light bulb is one step away from a diode, which will be my next project.

   Building this tube was not significantly more difficult than building the glow tube. I started with a 6 inch length of 6mm glass tubing, pinched off one end, and blew a bulb in it about an inch from the pinched end. I then broke off the pinch and inserted the electrode assembly through the broken end. Finally, with the electrodes in place, I thoroughly heated that end and pinched it again to seal the electrodes into the glass.

   The electrode assembly itself was very similar to the glow tube "hairpin" assembly, with two differences. First, I added a 0.1mm tungsten filament across the ends of the hairpin, which I crimped in place and spot welded to melt the titanium and tungsten together. Second, I used 0.5mm titanium wire instead of 1.0mm wire. This was to make the wire easier to crimp around the tungsten, as well as to test the current carrying capacity of the thinner wire. The filament requires around 2A of current, which produces a fair amount of heat in 0.5mm titanium wire. I wanted to test to see if the heat would cause the glass to crack, which it did not.

   To evacuate the tube, I attached the open end of the 6mm tubing (which the bulb was on the other end of) to my vacuum pump manifold using hot glue, just as I had with the glow tube. I then pumped the bulb down to 10 millitorr*, and tested its electrical characteristics. It glowed brightly at around 3V and 2A, but got hot quickly. I didn't feel comfortable running it for over a minute at a time. Regardless, it worked, so I heated the tubing between the bulb and the manifold until it collapsed and sealed the bulb off. The results can be seen above. On the left is the finished light bulb next to the glow tube; the hot glue at the bottom is for strain relief so the wires can be bent and moved around without cracking the glass. On the right is the light bulb glowing on a chair outside, running from a 6V battery (the internal resistance drops the voltage to the 3V required by the bulb).

 * I would have liked this to be lower, but the hot glue seal wasn't very good this time. If it continues to be unreliable, I may build a compression fitting using an O-ring and vacuum grease instead.