Solid state output valve

[ppppenguin]

What I was wondering is to modify the circuit above, would it be as simple as having 2x approx. 3R resistors across the filament pins on the socket & connecting the cathode side of the FET circuit and omitting the G1 connection or is there more to it?

Using resistors to replace the heater/filament is only required in a series heater chain. I can't imagine a PX4 ever being used like this. Just ignore the filament pins, except that one must be used as the cathode.