Mislabeled Tube Misadventure

[Winky Dink]

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.

[jr_tech]

Originally Posted by Winky Dink

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.

Two different materials with two different purposes... the white coating on the heater is alumina, applied to insulate the heater from the cathode... the white material on the cathode is barium carbonate which is applied to emit electrons. I don't recall any particular noxious fumes from either substance, when taking apart cathode assemblies.

jr

note: during processing on the pumping station, co and co2 are given off and mostly barium oxide is left on the cathode nickel surface.