Geiger tube -defective?

Other Hardware Sensors
1

I received a SBM-20 GM tube a few days ago and decided to test it. I connected it to my Geiger counter prototype and set the voltage to about 400V. The anode resistor is 4,7Mohm and the cathode is connected to GND via 1.5K and 100K resistors (the middle point feeds a transistor and there is a 470pF capacitor across the resistors).

I have understood that a geiger tube conducts zero current while it's idle and it only causes a short pulse when some of the gas inside the tube is ionized. Is this correct?
However, my tube constantly conducts about 40uA of current, which results in the transistor being turned on continuously (Vbe=0.67V). This is not how it should work, right? >:(

Edit. I tried connecting only the anode to a variable voltage (via 4.7Mohm resistor) and measured the voltage at the anode using my multimeter (not sure about the impedance of the meter). There was no voltage at the anode until I reached about 250V. At 400V I measured 125V at the anode!

The meter shouldn't show any voltage at the anode except for the extremely short pulses, right?

2

The tube is undoubtedly defective.
It should not conduct at all unless a charged particle arrives, or you exceed the breakdown voltage.

Geiger tubes can easily be destroyed by operation without a high value anode resistor. The arcing causes the gas mixture to become conductive.

3

AFAIK DMMs have an input impedance of 10Megohm.
If you measure 400volt through/after a 4.7Meg resistor, the voltage on the other side of the resistor is ~600volt.

Test your meter with e.g. a 9volt battery and a 10Megohm resistor.
If the DMM's impedance is 10Megohm, you should read half the voltage with the resistor in series.

Common 4.7Megohm resistors are only good for 100volt.
Use special high voltage resistors, or five 100k resistors in series.
Leo..

4

Thanks for the help!

I have contacted the seller of the tube and described the problem.

@Wawa : The 4.7Mohm anode resistor is a special high voltage Royal Ohm resistor which is rated for a kilovolt or so.