Last time, we only covered half the story about the CATHODE and today we'll look at the indirectly heated type. These have a construction in which a pure metallic nickel tube is sprayed with an emissive coating admixture comprising of barium and strontium carbonate. In the picture below, you can see the various stages taken to produce an indirectly heated cathode tube, and working from L to R these are: - cut; reamed; end swaged; pointing for mica location, flattened; lower end pointed; connecting strip welded; etched to allow emissive coating adhesion.
Once the cathode tube is coated, a heating filament which is insulated from the cathode tube is inserted into it. the filament is made of pure tungsten wire which is coated with magnesium oxide which acts as an insulation barrier between heater and cathode. In the picture below, working from L to R you can see in turn, the coated cathode tube; the insulated heater filament; the assembled indirectly heated cathode.
Typically, the indirect cathode type is used in amplification and other mains powered equipment as this method of cathode construction reduces the risk of mains borne 50/60Hz hum.