G3SEK published a very interesting screensupply in DUBUS 3/90. The idea is to use a current source and a shunt-regulator in series. This circuitry is able to sink and source current.
The circuitry survived some flash-over due to wrong tuning, but died two times. I think ist difficult to make everything "fool save" with that high voltages.
The PA is not easy to tune, you must follow the tips from OZ9ZI !
DL8MX and DB1NV published a very interesting idea (from AA5C) to regulate the HV-output of a transformator in UKW-Berichte 2/88.
The advantages of this circuitry is, that the anode-voltage remains within +/- 150V (at 3kV rating), even if the mains goes down due to the high load. Second, you do not need start-up sequence for the transformer (let the ref. voltage of the comparator rise slowly after switch on).
This circuitry works very nice.