I've [just about] decided on a topology for my circuits for the neutron tube. I will make a shielded socket with short coax length to the preamp box. In the preamp box will be the feedback amp we've been discussing recently, signal conditioning that feeds the signal back onto an HV input, and a rechargeable 9V battery and recharging socket.
A longer length of coax will then run out of this, down to a box with the HV supply inverter in, along with some further signal conditioning to pluck the signal back off the HV line to feed to an internal counter display and out onto an audio line output. I've ordered some shv plugs for the preamp-to-HV supply connection, so that I can drive the preamp with either my box, or my crate, or, indeed, any future 'Geiger'-like supply that is looking for a units-volt pulse on the feed line.
I've decided to run the preamp and HV supply continuously at regulated volts supply, so as to be assured of consistent circuit behaviour while I am still trying to figure everything out. (I can save adventures into battery efficiency for later). This means I won't be introducing any energy-saving measures in this iteration (more to come I am sure, like Doug's scheme of starting the inverter up intermittently).
My options for powering the inverter; I've tests how I need to power it, and I need to feed it with 7V to maintain the correct voltage. I could use PP3 rechargeables. It's a 50mA draw, so two in parallel should last me 6 to 8 hours or so. That's fine, I've no issue with plugging in a charger at the end of a day's work (maybe it'd even be a good thing to go flat, limiting the time I spend on it and sending me to bed at a reasonable hour!!). I would like to use two pp3's as I can package them in one box. BUT, how do I get a reliably steady 7V from a PP3? As the voltage drops, so the voltage of the inverter drops markedly, so how to do with so little overhead for regulator voltage drop? I want dead steady volts so I know where I am at - I want assured repeatability for now. The option at the moment is that I house a 12V lead acid 800mA within the HV inverter supply box, so that I can regulate to 9V and then drop the volts with a plain resistance (prob adjustable, too).
But is there not a better way? In fact, is there a way to regulate the HV line voltage directly? The only scheme for that I could come up with though was to pulse a FET, enabled if the volts on a divider string are above a set point. The fet, via a large resistor, can then drain the capacitors in pulses if the value is reading a bit high. Or maybe I can do the same somehow with the power feed and end up implementing 'energy saving' as a natural consequence of trying to regulate the volts?
Or I could just regulate the input volts to the inverter and hope the output stays constant, and if I do that then how can I achieve a steady 7V for the whole discharge time of the pp3, or should I use 12V?
Summary of the two main questions;
1) How can I best regulate to exactly 7V (pref small adjustments too) from a 9V battery with so little overhead for voltage drop, or best just to go for 12v in that scenario?
2) Can I regulate the HV directly by monitoring a voltage divider potential, and if so how do I best do that (e.g., disable supply current, enable a discharge circuit, put a bipolar transistor somewhere directly into the HV line?)