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.
