Twig - Geiger with Russian GM tube

I’ve had my Twig Geiger board for a while since July but couldn’t use it because no GM tube. Finally got a Russian gamma-only tube (SI-180G) on eBay (~$25). It arrived yesterday and I immediately hooked it to the Twig Geiger.

But it didn’t work. The LED lights up, but doesn’t blink, just a steady light.

Finally figured out that it is detecting the gamma OK, but then goes into ‘avalanche’ mode and the LED stays on. I figured this out because I can quench the tube by merely measuring the voltage with my Harbor Freight volt meter (in the 1000V setting of course). Just tapping the probes on the + and - terminals is enough to drain the ions and reset it for the next gamma event. Then it goes dark for a half-second or so and lights up again when another gamma arrives.

So it’s the tube that is the problem, not the circuit board. Which is really nicely designed and a bargain for $26!

Do you have any suggestions for operating with these old Russian tubes? The recommended voltages are in the 400 volt range, which suggests they are the halogen charged, self-quenching type. But I think the halogen gas may have dissipated leaving only Argon, which is not self-quenching.

Would lowering the voltage help? I could probably build a simple voltage divider with resistors and try to run at a lower threshold.

EDIT: … (after looking at schematic) or perhaps just putting a jumper across Zener diode D8 would be useful for lowering the voltage by 100v, or D7 for 150v voltage drop?

Or perhaps someone could recommend another cheap Russian model to try, perhaps the SI-3BG, which looks more like the J408Y.

Has anyone had any success with Russian tubes?


… while waiting for some replies to my post I tried increasing the final load resistance by adding a small 5 megohm potentionometer in series with R12 and the tube.

By adjusting the pot, I can make the circuit work at a setting of 3 megohms additional resistance. I.e. the LED blinks randomly at approx 150 cpm.

But it was very unstable, the plateau is very narrow and it only works for a few minutes. Then the circuit went into a ‘dead state’ where nothing worked at all. Finally recovered after a day of rest, so I removed the extra megohms.

I think the solution is to find the optimum resistance for the final two series resistors R11 and R12 to match the SI-180G load resistance, which is documented as 5-10 megohms.

Has anyone had any success in matching this Twig circuit to other tubes besides the J408Y?