[ionburn]

I have used MPSA42 and PHE13009 in a darlington configuration in a capacitance multiplier (simplistically transistor regulator with no voltage reference and a large (100uF or 200uF) capacitor to the base). I used to use them often when working with higher voltages (even sometimes lower ones). Transistors were from RS or Rapid I think. There are circuits around using modern Fets too.

Although I did develop PSU's for over 500V I never worked with more than about 250V per stage so the driver transistor was Ok. I used the darlington arrangement as HV bipolar transistors all tend to have low gain. I generally used voltage multiplication for the final voltage so could smooth and add voltages for the result but have found capacitance multipliers really do work well. Effective capacitance is base capacitance x transistor arrangement current gain.