Well, today at 15:50 - conveniently as I was acceptance testing the Owon oscilloscope, using the Geiger detector circuit as a signal source - the replacement tube was delivered.
After visually inspecting it and being happy it was undamaged physically, I connected it in place of the BOI-33 tube - and nothing happened! At this point I was a little miffed, but, I had a shiny new oscilloscope in front of me monitoring the cathode port - and yes, there were very faint, low level pulses! In fact, looking at the indicator LED of the Geiger circuit in the dark, I could see it occasionally flash dimly.
Ah-ha! So, this tube is doing something! So, what is amiss that prevents a good count pulse? The first thing to eliminate is the additional capacitance and resistance of the croc-clip leads used to connect the tube. With the first, faulty tube, I had an anode 4M7 resistor connected right to the tubes short anode lead, but this time, I was using the boards anode resistor, and a 30cm long croc-clip lead. So, I moved the anode lead of the tube to the anode clip on the board, and just used one croc-clip to secure it - and yes! Good, strong pulses and indications!
Of course, being such a tiny tube, its sensitivity is nothing like that of the BOI-33. Its background count is around 2cpm, compared with the BOI-33's 20cpm, but, it is a fraction of the size! It is also an end-window device, so is most sensitive from that end and little from around the body. This is fine though for a sub-miniature detector intended for sniffing out sources at charity shops and flea-markets! I think this tube will go into a circuit using 555 timers as HV generator and monostable indicator, but that will need to wait until I receive the project boxes from China.
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