The output of the inverter is rectified and filtered and and then feed back into the feedback of pin 4 on the LM2576 through a voltage divider. The output of the LM2576 is then used to supply of the inverter.
The LM2576 maybe seem like over-kill but my aim is to be both accessible and cost effective, and although there are better chips available unless your building a few hundred of these, it would be harder to choose a cheaper and more accessible alternative. The spin-off of cause is you can use this circuit with higher wattage inverters for other applications, and the LM2576 is fairly indestructible with thermal overload and short circuit protection built in.
**note The component values in the circuit below maybe changed to achieve higher voltages by changing the value of the 1M resistor or R fb, the basic rule of thumb is for every 1M added the voltage is increase by 600V. So if you require 1.2Kv for a PMT, then the value would be 2M. Where the 2k2 trimpot offers the ability to adjust the output by 100V. The IN4007 is rated for 1000V so two diodes in series will be quite ok up to 2Kv. The capacitors in the HV filter are wired in series for higher voltages, although this does decrease capacitance and so increases ripple these inverters operate at a high frequency >20khz - 50Khz and so this is really not a big problem.