More electronics from behind the curtain

I'm old enough to remember the fall of the Berlin wall (though not as many would have it the abandonment of Hadrians!) and it's because of that moment in history that today I could take delivery of a Soviet era SI-19BG Geiger tube.

This tube really is tiny! I would never have thought that a G-M tube could come through the post in a normal envelope! And that's wrapped up in bubble wrap!

I've done some tests on the breadboarded HV circuit now I have the 1Gohm resistor. The 3V circuit has trouble cutting back the current if final output regulation is used, but seems to happily give around 400V with the regulator chain connected at the first multiplier, dropping the current from 4.5mA to about 1.5mA.

I'm sure I can get this lower, with a bit of circuit tweaking. Right now though, the daft 11M impedance of my DMM means that the voltage reading on the LCD is not quite accurate,  so I hope to work out a way you correct this using parallel resistance to get a true 100:1 divider chain. I might check the impedance of some of my older less useful DMMs - I might be able to make a dedicated HV probe meter.

Edit - Well Doh! It isn't a parallel resistance I need! Its more series resistance of course! 1089MΩ in fact.  With the 1GΩ resistor this means I can make up the extra using standard low cost, low voltage resistors, plus a preset to allow precise calibration.