A Do-It-Yourself Vacuum Tube

Mark Valentine

I loved Forrest Mims' article about vacuum tubes.

Readers can make their own diode vacuum tubes with a dual-filament automotive tail light. These lights have two filaments and three connecting terminals. One end of each filament shares an electrical terminal with the other; the other end of each filament has its own unique terminal.

If you blow out the weaker filament, a free-standing electrode remains inside the light bulb, creating an arrangement very similar to what Edison first used to demonstrate the "Edison effect." Long ago I created a demonstration platform based on a bulb prepared in this manner (and an appropriate lamp socket from an automotive supply store). I powered the intact filament in the bulb with eight "C" cells connected in series to supply 12 volts at high current and used two 9-volt batteries in series the develop 18 V between the common terminal and the now free-standing terminal that once connected the burned-out filament of the bulb.

When the 18 volts is applied so that the free-standing terminal is positive, a small current flows that can be measured on a sensitive analog meter. The current is on the order of 10 microamps. When the polarity of the 18 volts is reversed, no current flows, because electrons can only hop off the hot filament, not the cold, free-standing electrode.

I'm not sure, but I think there would probably be more current if the "C" battery stack was connected so that its + side connects to the common terminal of the bulb (creating a potential of up to 30 volts between the most negative part of the filament and the free-standing electrode. Also, it would be interesting to see whether leaving the larger or smaller filament intact produces the most current, since one probably gets hotter, while the other probably has a larger surface area.

One more thing: Even though it's powered by "C" batteries, the bulb still gets quite hot, so please be careful!

There's also this great kit where readers can make a true tube radio: http://www.thamesandkosmos.com/products/ra/ra2.htm