The expression "exclusive of filament or heater power" as stated in 15.219 has never become clear to my understanding. Obviously it refers to electron tubes, which contain filaments. "Heater" is another word for filament, owing to their contribution to heat within the tube. But in determining the input to a final electron tube stage the power drawn by the filament is clearly separate from input or output power from that tube, so there seems no reason to mention filament power at all. Why are filaments mentioned in the rule?