(02-05-2020, 08:43 AM)ppppenguin Wrote: I still wouldn't bother with a dummy load for the filament.
Dan
Things are complicated a bit by the fact that the AC044 / PX4 is directly heated so effectively the electrical centre of the heater is the valve's cathode. The SAC35 has a centre tapped heater supply with the centre tap tied to chassis. Also, the SAC35 is one of those sets where valve bias is derived from resistors in the negative HT line and fixed bias is applied to the AC044 grid.
I think you'll be OK to tie the degeneration resistor in a MOSFET source ("cathode") to chassis, but watch the voltage that is applied to the AC044 grid - this is likely to be greater than the maximum Vgs of your MOSFET which is typically +/- 20V. You'll have to perform a bit of maths to determine the value of "cathode" resistor that reduces the gain of the MOSFET sufficiently while keeping within a safe gate to source voltage range. I haven't done the maths, but it may be that this isn't practical without changing the existing bias arrangements for the output valve.
John
