AT/ATX psu based on tl494 main keys switch form mje13007 to bu508d

[slimax]

Hi,
I want to convert AT/ATX power supply to work with higher voltages 450-550V DC.
I replaced two main input capacitors from 200V to 400V and main keys form mje13007 to bu508d.
It works fine but up to around 20% of designed power capabilities.
If I try to get more power from it - these transistors start heat-up really bad.
Are these transistors too slow for that application (fall-time)?
BU508D:
Storage Time: 7ms
Fall Time: 550ns

MJE13007:
Storage Time: 2ms
Fall Time: 100ns
?
Heating is a little lower with higher input voltage (~500v), so I'm assuming that probably all other components are ok with that sort of voltages.
Is there any way to make it working? I have 3-4 psu to convert and I already have these bu508d transistors which are also cheap.
Or maybe someone know any other fast-switching high voltage (>=700v) npn transistor to be suitable for this application?

The only difference in below schema are resistors  instead of 330k I have 220k.


Unfortunately I have no oscilloscope to make sure what's going on there.

[eblc1388]

The only difference in below schema are resistors  instead of 330k I have 220k.

These resistors are there for self starting. You have increased the supply voltage so you definitely should increase the value of these resistors, as long as the power supply can reliably startup. Maybe the switching transistor does not fully turn off in operation causing possible heating problem.

I would try 470K or higher until the power supply don't start. Then reduce the value a little.