what is mean by direct &indirect heated emitters?

Question

what is mean by direct heated emitters & indirect heated emitters.how it is called when we combine both the direct & indirect heated emitters? & give me any more detail information about the both the emitters.

Answer 1

In vacuum tubes a direct heated emitter is the heater itself used as the anode or emitter. An indirect heated emitter encloses the heater with an "electron-rich" sheath which is the emitter, or anode. Direct heated emitters have the advantage of simplicity, and indirect heated emitters offer a choice of materials so the heater can be of a stronger material while the emitter need not have very good structural strength.