Types of Cathodes

Author: J.B. Hoag

The cathode may be directly or indirectly heated, as shown in Fig. 10 C.

Fig. 10 C. Types of cathodes

In the latter type, a coated metal sleeve or thimble K surrounds an insulated heating wire. The sleeve is coated with the alkali earth metals (oxide-coated) so as to be an efficient emitter of electrons at a comparatively low temperature. K is sometimes called an equipotential cathode.

If a.c. is used to heat the simple, direct-type filament, electric and magnetic forces are set up which, in a receiver, cause a hum in the output. The hum may be greatly reduced by the use of a center-tap connection, as shown in Fig. 10 D.

Fig. 10 D. Methods of reducing hum when directly heated filaments are used

The small condensers are used to bypass alternating currents around the transformer secondary.