The electrode is the cathode. The cathode may be directly heated, in which
case it's called a filament, or indirectly heated, when the cathode is the
electrode coated with emissive material (usually barium and strontium
oxides), and is heated by the heater. This is an indirectly heated
tube/valve/lamp (depending on language). The heater is insulated from the
cathode, usually for about 100 volts, although some tubes were insulated to
withstand several hundred volts. Typically, these were small rectifiers,
such as the 6X4 and 6X5, and the heater cathode insulation was rated for
around 300 to 350 volts. Some of the series heater tubes meant for series
heater string TV and 'universal' radios were similarly well insulated, such
as the 35L6. Some indirectly heated tubes had cathode and heater connected -
usually rectifiers such as the 5Z4 and 5V4, although one or two special
purpose military pentodes had such a connection.
Cathodes are generally essentially nickel tubes with the heater a hairpin of
tungsten wire, possibly coiled, inserted in them. (Imagine a long coil bent
back on itself a cfew times, and then stuffed in the tube.) Larger
transmitting tubes use various cathodes, some like the receiving tube (eg
6146) and the bigger ones, especially in microwave tubes, rather different
in shape.
Cathodes were used for tube run off AC as it was very difficult to avoid hum
modulation in low level stages with filaments run on AC. They also added a
lot of flexibility to circuit design without having to have isolated
filament supplies for each stage.
So 'filament' and 'heater' get used somewhat interchangeably, but those are
the meanings.
73
Peter G3RZP
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