This is where you tube gurus can tell me if this concept works. The zener would probably be just under the max differential voltage. (35V???). The top pot would be a high R, just to set the biasing for the control grid for optimal performance. 24V/Rtop sets the ref. current(Iref). (Vmax-24)/Iref=(R fixed + R adj.) The output cap is there for stability and should be >= 1uF. The adjustment filtering cap might be more than 1 cap. If you use a large capacitance for a slower ramp-up, you would probably use a small disk for RF bypassing. The zener should double as the I/O protection diode and the diode will discharge the filter capacitance.
This circuit puts the adjustment pot closest to circuit ground. It works with the assumption that you would want a given high voltage +/- a certain amount. You would take your voltage range / I ref to determine the closest linear pot value and make up the rest of the total with the fixed resistor. You could also use a multi-turn pot and no fixed R or a low R "FINE" adjustment in series with a high R "COARSE" adjustment.
So this is where Paul and John and Mike should chime in with triode suggestions (cuz I'm clueless!) The last tubes I worked with were 1KW broadcast tetrodes and 55KW klystrons, and I'm powerless here. I'll vouch for the part past the tube. Even if this circuit doesn't fit your pre-amp, it wouldn't hurt to have it around to tweak your other tube circuits.
As always, HAVE FUN!!!
Joe
