Size matters. Last night I swapped a different step-up transformer into my very anemic Geiger counter high voltage generator: a five-pound brick of an aviation power transformer from the early 1960s, with a 465V secondary. I put the interrupted DC into the 5V rectifier filament winding for maximum turns ratio, and then started pumping the buttom.
Way better! Thirty or forty presses got spark every time, and took the voltage across the accumulator capacitor (.5 MFD @ 600V) up to about 620V. That's not the 700V called out for the Geiger tubes I have, but I think it'll be plenty to detect the occasional hapless gamma. One problem I knew I would have is that the capacitor leaks charge far too quickly: In about six seconds, the voltage goes down to 300V. Without more stored charge, I'd be pumping the button (or spinning the rotary interrupter, assuming it works) pretty much constantly.
I ducked over to OEM Parts on my Monday errands wander to peruse their capacitor collection, and picked up a couple of new 1.5 MFD @ 630V caps. Two in parallel provide 3 MFD, which keeps its charge long enough to be useful, assuming the Geiger tube will still conduct with only 400 volts on it. I also turned one of the spark gap electrodes around (see macro shot above) so that the gap is between a point and a flat face. The gap became reliably unidirectional after that, and there were no more sparks on make, but only on break.
Next I tried lashing up the full Geiger counter circuit, with the signal from the tube going into a 2-stage tube speaker amp that I built fifteen years ago. Problems came up immediately:
- My 525V DC supply (which I haven't powered up in almost 20 years) has a bad filter cap, so if there are detector pulses, they're drowned in the AC buzz. No huge problem, as I can now run the Geiger tube off the pure DC in the larger accumulator cap. (That's tomorrow night's project.)
- I don't have a reliable pinout for the Geiger tube. Weirdly, none of the articles in the old magazines show a basing diagram, which is three pins in an odd arrangement on a 5/8" base. One of the three pins goes to the metal shell, as determined via ohmmeter. I'm assuming that the center electrode is the "lonely" pin, and the third pin goes to the conductive inner surface of the tube. Interestingly, in the junkbox socket I found, the two "close" pins were wired together. I sense some cut-and-try in my immediate future.
- I may or may not have a radioactive sample to test it with. I dug my grandfather's 1953 gold retirement watch out of the curio cabinet, only to find (in defiance of memory) that it did not have a radium dial. My other possible sample is an 0A2WA gas regulator tube, which is salted with .03 microcurie of Krypton-85 to ensure immediate startup. Sounds great--except that the half-life of Kr-85 is 10.7 years. The tube was manufactured in 1962, which is 4.5 half lives ago. Unless I'm doing the math wrong, that means that only 4% or so of the Kr-85 is still in there throwing particles.
Of course, I can pull one of the smoke detectors off the ceiling and try that, but there's a more intriguing possibility: WWII aircraft equipment meter faces often had radium markings, which are still radioactive even if they no longer glow in the dark. I have three or four old military panel meters from that era, and if I can find them, they may still be active enough to come up out of the background noise.
Assuming that at least one of my two Geiger tubes is good, I'd say we're getting close.