Question

Which is a superior thermionic emitter?

• A. Directly heated thermionic emitter
• B. Indirectly heated thermionic emitter
• C. Both are comparably good emitters
• D. None of these

Answer 1

Answer: (B)

Indirectly heated thermionic emitter

Directly heated cathode :

In this type, the filament itself is the cathode and emits the electrons directly. Directly heated cathodes were used in the first vacuum tubes.

Indirectly heated cathode :

In this type, the filament is not the cathode but rather heats a separate cathode consisting of a sheet metal cylinder surrounding the filament, and the cylinder emits electrons. Indirectly heated cathodes are used in most low power vacuum tubes.

Why Indirectly heated cathode is better?

The main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament, allowing vacuum tubes to use alternating current to heat the filament.

In a tube in which the filament itself was the cathode, the alternating electric field from the filament surface would affect the movement of the electrons and introduce hum into the tube output. It also allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source, even though the cathodes they heat may be at different potentials.