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I have recently purchased an SBM-20 Soviet Geiger-Muller Tube, which I want to use in a Geiger Counter circuit. I powered it with a 12V to 400V DC-DC power supply, connected in series with a 5.6 megaohm anode resistor. However, when I connected the output of the Geiger tube to an LED, it stayed lit continuously. From what I understand, a Geiger-Muller tube should be off until an incident, in which case a short pulse is produced, then turns off again. I have barely used this tube, and always used a voltage within it's operating range, with a minimum 1MOhm resistor in series. I double-checked the polarity of the tube, and made sure the two terminals were not shorted. Is this tube defective, or am I doing something wrong?

leo
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  • There is an operational region (continuous discharge) that resides past the Geiger-Mueller region. Is it possible that your SBM-20 tube is operating there? Can you reduce the voltage a bit? (It might also be that the tube is malfunctioning, as well. Do you have several of them to try?) – periblepsis Jun 22 '23 at 04:50
  • How are you stepping up the voltage? Might it be a high frequency inverter? The tube may be acting like a capacitor and leaking some high frequency noise into the LED. If you're using a white LED with an internal phosphorous layer there are a lot of things that can start that glowing, (such as nearby high voltages, nearby radio-active materials, and so on). Always use extra caution when working with high voltages and especially with Soviet surplus components , good luck comrade Leo... – Nedd Jun 22 '23 at 05:50
  • I found this that can help ... https://www.ebay.com/itm/Russian-Geiger-Counter-SBM-20U-SBM-20-SBM-20Y-TESTED-NEW-/164034840109?_ul=IL , see "initial" and "operating" voltages. – Antonio51 Jun 22 '23 at 06:08
  • @periblepsis My power supply was tuned to just around 398 volts, and according to the datasheet, it should still be well within Geiger-Muller region (350-475). I only have one tube. I will try reducing the voltage next time I test it, to see if it makes a difference. If I can't figure this out, I will buy a second tube to compare it against – leo Jun 22 '23 at 13:26
  • @Nedd I am using this boost converter module https://www.amazon.com/gp/product/B07T6L61D9/ from amazon. From what I understand, it's generating a square wave then stepping it up through a transformer. I used a green LED, not a white LED, and the only radioactive material present was an Americium-241 source. However, I also tried the circuit while the Am241 source was not present, and it continued to exhibit the same problem. The only high voltage present was the 400V power supply, and some very-very small EMF from AC city power. – leo Jun 22 '23 at 13:31
  • @Antonio51 What is the difference between initial and operating voltage? Should I use low voltage to start, then switch to a higher voltage afterwards? – leo Jun 22 '23 at 13:33
  • @leo Seems to be so. The operating voltage is from 350 to 475 V (to be "well" chosen). See this also for +/- side https://mightyohm.com/blog/2014/11/a-spotters-guide-to-the-sbm-20-geiger-counter-tube/ – Antonio51 Jun 22 '23 at 16:17
  • @leo I'm going from memory (I designed and built a Geiger-Counter circa 1971.) But the 'initial voltage' shown on the ebay website is likely the `starting voltage`. This voltage is the point where you see "almost a cliff" (sudden change) in counts as the voltage slowly climbs before reaching the desired plateau. Most of these voltage supplies take a little time to reach their final potential (10s or 100s of milliseconds.) So far as I recall, I did nothing special about this starting voltage, other than to be vaguely aware of it. – periblepsis Jun 22 '23 at 19:47
  • @leo I've marked a few things on an [example curve](https://i.stack.imgur.com/MBeee.png). I think I've placed a red line about what I'd call the `starting voltage`. It's nothing special, per se. There are green lines to indicate the Geiger-Mueller region. The blue line at upper left shows the slope of this region. The `quality` (the value of something you are buying) of the tube is determined by how flat this curve is over a range of 100 V or so. (You should actually make such measurements to see for yourself.) If this is flat to within 3% over 100 V change, you've got a nice tube! – periblepsis Jun 22 '23 at 19:53
  • @leo But it should be flat to within 10%, under all circumstances, over a range of 100 V. If not, then my old memory tells me that the tube may be bad or have some problems. This is one reason why it is important to have a design where you can adjust the voltage and keep track of counts. (Another reason is that if you have a mica window tube -- more expensive -- then you may want to be able to reach down into the proportional counter region where you can discriminate between beta and alpha events. There's a lot of study fun to be had!) – periblepsis Jun 22 '23 at 20:01
  • @periblepsis Thanks for the explanation, I have ordered a new tube and a test source to use in experimentation. Is it safe for me to connect the tube negative terminal to my oscilloscope to measure any pulses? – leo Jun 22 '23 at 21:59
  • @leo Actually, it is safe *if and only if* you use a capacitor. Something [like this arrangement](http://i.stack.imgur.com/JPCKi.png), just shooting from the hip about it. – periblepsis Jun 22 '23 at 23:13
  • @leo - If you monitor the output voltage with a scope (with a cap in series as in a comment), you may see the 75khz switcher frequency or some harmonics of that getting through. That may be what was is making the LED glow. You may be able to filter out some of that with a few small caps. – Nedd Jun 23 '23 at 03:23

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Answer: The Geiger-Muller tube I was using was not operational. After purchasing a new tube, I reassembled the circuit and tried it again. The LED remained normally off, and briefly flicked on for each incident captured by the new Geiger Tube. I then rewired the circuit to use a small piezo speaker instead of the LED, which produced a very faint click for each incident. In order to amplify the noise, I added two BJT transistors in a Darlington Pair amplifier configuration, which worked flawlessly. Now, each incident was accompanied by a loud audible click of the piezo element! I took this opportunity to try out a radioactive test source (Th-232), which as expected, produced very rapid successive clicks when brought close to the tube.

Thank you very much, everyone who helped! I think the LED was continuously on with the old tube due to some high frequency harmonics from the 400V boost converter circuit, as @Nedd mentioned. Hopefully this experience and the answers provided can help a future noob troubleshoot their Geiger. Time to graduate to the nuclear physics StackExchange..

leo
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