September, 2010:

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:

1. 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.)
2. 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.
3. 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.

Sparks. Let’s talk about sparks. Last night I finally got things lashed up sufficiently to see whether I could translate three volts–a pair of C cells–to the neighborhood of 600 volts, using an old 25,000 ohm : 3.2 ohm output transformer and a spark gap. Got sparks. Didn’t get 600V. (Got about 350 at best) Drained the batteries pretty quickly.

Nonetheless, it was a fascinating experiment, in a technological backwater I’ve never really messed with before. In summary: Put a pulse of current through the low-impedence winding of an output transformer, and a pulse of high voltage (compared to the input voltage) will appear across the transformer’s high-impedence winding. Rectify the pulses, and you can accumulate voltage in a good, high-value low-leakage capacitor.

One way to rectify the pulses is to send them through a spark gap. The air gap breaks down against sufficiently high voltage and current passes one way across the gap. Put a cap in series with the spark gap and it will store a certain amount of charge each time the spark jumps.

At least, that’s how it works in theory. In practice, with a very high resistance voltmeter across the capacitor, I saw two phenomena I wasn’t expecting:

1. About half the time, I get sparks on both a make pulse and a break pulse. (Ordinarily you only expect a spark on the break pulse.) If both make and break generate a spark, a pulse jumps the gap in the opposite direction as the pulse that preceded it. This means that the charge placed across the capacitor is then of the opposite polarity, which drains the cap by about as much energy as the previous pulse placed in it. Tinkering with the gap spacing didn’t help, though the effect happened more often with a higher voltage (>6VDC) into the transformer.
2. Eventually, the spark refuses to jump. It looks to me like accumulating a certain voltage on the cap bucks the spark gap and makes it harder to jump with the same pulse from the output transformer. And of course, once the spark ceases to jump, voltage on the capacitor ceases to rise.

With my lashup, once voltage got to about 320, there were no more sparks, with about .003″ across the gap. Putting a stronger current source across the input didn’t help. I was eventually pulsing 12.6VDC from my 30 amp linear bench supply, which heated up the poor transformer pretty badly but didn’t give me any more voltage across the cap. Now, 320V may be enough to get conduction through a Geiger tube (I’ll find out shortly) but the articles I’ve read suggest 600-900V, and seem to think that this can be had from a couple of C cells and a spark gap.

I did better placing a husky 1000 PIV 1N5408 silicon rectifier diode across the spark gap. The charge went up and only up (because current reliably passes only one way through a rectifier diode) but it still topped out at about 350V. I suspect that that limit may be inherent in the relatively small output transformer I’m using, and when time allows I’m going to troll the collection for the largest one I have and swap it in.

Now, a steampunk mad scientist never runs out of #40 copper wire and thinks nothing of winding his own transformers, so if that’s the secret, a steampunk Geiger counter remains a possibility. However, I’m beginning to wonder how well I can achieve the steampunk ideal (no active devices) with only what I have lying around. Winding my own step-up transformer is just on the other side of what I’m willing to do.

The next step is making sure my two Geiger tubes are good by lashing them up to my 525V DC supply and exposing them to a (mildly) radioactive gas rectifier tube. Don’t know yet when I’ll be able to do that (real work has been piling up this week with the two of us trying to recuperate) but I’ll continue the series here as time permits.

With the rash fading and the nerve pain lessening (and Carol safe home!) I stole an hour or so downstairs last night to see what could be done in an hour or so downstairs. And I did well: I got a spark gap together, all made out of junk I’ve had lying around here forever. Not much to it, really: Some brazing rod, two husky black 5-way binding posts, and a battered 4-row barrier terminal strip. I had to drill out two of the holes in the barrier strip to pass a 10-32 thread, but apart from that, the only work was grinding points on two pieces of brazing rod and adjusting the binding posts so that the brazing rod pieces met point-to-point.

Done. Didn’t test it; ran out of evening. However, I did put 450V on my capacitor bank to make sure the caps were good, and it took a right-fine charge. It was interesting to watch how quickly the 450V bled off into the probes of a 50,000 ohm/volt VOM. (I miss my old Heathkit VTVM, may have to pick up another.) The more capacitance, the more energy is stored from the spark rectifier, and the longer the counter will run without another crank on the interrupter. So as time allows I’m going to put together a second plug-in cap bank.

The next step is to lash up the interrupter circuit to see if it can charge the caps and make sparks on the above rig. The rotary interrupter still needs some work, but I can clip-lead in an SPST pushbutton switch to give it a go.

Damn, but it feels good to get my hands dirty again. And let me tell you, boys and girls, there is nothing quite like the smell of a gallon milk jug packed full of 40-year-old black Bakelite barrier terminal strips!