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?
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..