Re: PX4 substitutions
A triode-connected 6L6 (or G/GT) and a semiconductor diode voltage-doubler for the heater (could someone here kindly model silicon versus shottky and different capacitor values for 900mA to get somewhere near enough?) and the cathode connection brought out separately on a wire to go to the centre tap of the original 4-volt winding should do the job.
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