Technical Notes:
Page 6
All Italics Mine
Kevin O'Neill
At the beginning of tube technology symbols were already completely in use, which remain today
nearly alike until:
< anode
< heater element, at the same time cathode
< anode
< cathode, indirectly heated by heater element
< two anodes
< In nearly all tubes the cathode is indirectly heated, so the connections for the heater can be omitted.
< two anodes
< two cathodes
In order to receive a stronger electron flow with certain tubes, (electric rectifier, thyratron),
they are filled with gas instead of being evacuated. Noble gases (neon, argon) or mercury Vapor are used.
The tube lights up during operation. Disadvantage: the electron flow will have carriers atoms,
so they can only be used for low frequencies.
In order to mark that a tube contains gas, a point or shading within the symbol is added.
