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- Dec 18, 2019
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So if the PMT device consumes no current, the string of resistors has about 5.4uA through them at 650V. The behavior you are seeing is like a cap is charging. Maybe the resistors with the caps are open or a wrong value? For a clue, you could plot current vs time or something like that? What are the resistors mounted to? Standoffs or a PCB? Is it possible that there are leakage paths that are interfering?The operating voltage for PMTs varies depending on the application and specific tube. One website that uses the same setup I got specifies 650V, which is on the low side -- but has the advantage of lower noise. The current each dynode requires depends on the dynode, due to the gain. It increases for each successive dynode, so the divider chain also has capacitors across the last few resistors in the divider chain to reduce the voltage drop.
The Theremino approach recommends 10 megohms for each resistor in the chain (except for a 20 Meg at the cathode end) for a total of 120M. This is to permit the use of a HV supply that only needs to provide a few micro-amps -- it reduces the cost, and also is a lot safer.
I have attached a drawing I found online that shows how the PMT is wired.
View attachment 491013
It's dirt simple compared to the TIA schemes we've talked about using for the silicon PIN diode detector, but that's mostly due to the high gain of the PMT.
Kind of tough to measure 120Mohms. But can you measure the 10 and 20 Mohm resistors insitu? Remove the supply and short out the caps first! I know you know that, but this is in case someone reads this in the future.
HV is strange stuff, have to be careful, or you can get bitten badly. HV dielectrics can have some memory. Keep shorts across the terminals until use. A shorted cap can appear to spontaneously have a voltage on it after being discharged. I have some energy storage caps, they are always shorted because I've seen this behavior. There can be enough residual charge to fry you in some cases. If I recall correctly, 100mJ can stop your heart. I have a 3 200J caps waiting for a whacko project of mine. I always run the calculations on caps to see how much energy they store, and behave accordingly. Energy = 1/2 * C * V*V, where C is in Farads and V is in Volts. Units of Energy are Joules or watt-seconds. If you have lots of volts, it doesn't take much capacitance to become a problem.
That was a public service announcement. I get the feeling that you've dabbled in this stuff before, so my comments aren't directed to you. If I had my act more together, I'd be playing with HV again. I had made an HV supply and multiplier but it appears that it's lost to the ages. Pity, since it had a tube color TV flyback transformer in it. Those guys are practically impossible to find any more. I used a 1B3GT vacuum tube for the rectifier. The filament was run by 2 turns on the flyback transformer. Somehow it would self start. Everything was submerged in oil to prevent corona discharge. Got about 50kV out of it. It could go higher, but the rectifier started to produce X-rays. I could detect the onset by the fact that the inner surface of the glass was starting to fluoresce. At that point I stopped and waited for HV solid state rectifiers to come down in price. Eventually I got some, but they leaked too much, which makes your power supply bigger. Maybe someday I'll play with that some more.