I cleaned the pins and sockets for all the tubes before installing them, but I repeated the cleaning of the questionable 50A5 and cleaned the socket in the tube tester--600 grit sand paper and CRC Electronic Cleaner with a pipe cleaner for the tube sockets. Same result; the heater doesn’t heat even though the meter shows 30 ohms across the filament.
I took the tube apart. This what this beam pentode looks like inside.
I got down to the filament enveloped by the cathode, and it still showed 50 ohms across the filament. I couldn’t extract the filament because…
I don’t know what the white substance is, but it apparently coats the filament to insulate it from the cathode and the cathode is covered to insulate it from the screens. When I touched the white powdery stuff with an instrument, it released an invisible gas. Really! I could see nothing in the air, but it was extremely noxious--caused instant cough if I inhaled at all in the vicinity. I sealed it in a plastic bag, opened the windows and let the room ventilate for two hours before I reentered. I looked at a 50B5, and it has the same white coating on the filament. I waited 24 hours to be sure I was still alive before I posted this.