RadMon Plus Geiger Kit counts without GM tube

For testing purposes I have connected a 5 Megaohm resistor instead of the tube.
And look, the kit keeps on going and count.
How can that be?
Is that the same with you?

What else did you expect?

The tube normally isolates the high voltage. If radiation passes the tube it activates kind of tunnel for the electrons, so the tube gets conductive for a short time. This activates an interrupt on the controlling microprocessor. As the connection is made over a capacitor (C6 in your case) that generates an interrupt from time to time if you connect the tube pins by a high resistance.

Interrupt with tube, ok? Without tube and with resistor nok!

If there is connect the tube pins by a high resistance, there is a constant voltage across the resistor.
C6 is connected to an input of the mcu.
Why is there an interrupt without a tube and what intervals do you think?

Why do you expect the circuit to work with a 5MΩ resistor instead of the tube?

The high voltage generator doesn't provide a continuous high voltage signal and some spikes in this signal may trigger the transistor that will trigger the interrupt (I found only the schematics of the Geiger Kit Plus but I guess the radmon kit is similar). The circuit doesn't work correctly if you replace the tube by a resistor.

Did you measure the high voltage by scope? Do you have a constant voltage? You cannot measure that with a multimeter and expect to see alternating values if the signal has a frequency of over 3kHz.

What do you want to show by connecting the resistor instead of the tube?

Why am I asking about the behavior? My GK does not work anymore.
Now I try to find the mistake. Is it the tube, is it the electronics?
How can you test the circuit without a tube?
That's why I would like to compare with your results.

Meanwhile I know more. Now you can test the circuit without a tube!
You can measure the voltage at the cathode of D2 against ground with a DVM or a scope.

Without a tube, there are no interrupts but you have to use the right resistor.
If you look with a scope, the voltage is calm and constant, there are definitely no interrupts!

The error is not measurable but in the end it was the diode D2.
Because of the defective diode the internal resistor was too big and the tube did not count anymore.

Thank you pylon for explaining how the tube works but usually interrupts are definitely not with the right resistor instead of tube.
And thank you mw0uzo for your explanation.

And if john reads that, respect you are the best