THE MULLARD CATHODE RAY TUBE ELECTRON GUN

 Well, this is what an electron gun looks like: -


It comprises of an indirectly heated cathode, a grid and two anodes.  The inner anode is maintained at a positive potential of up to 7kV on these early electron guns, yes, really!   Let's now look at the bits and pieces individually but first, an exploded diagram: -

The cathode assembly - this comprises of a short nickel tube, one end of which is closed by a disc coated with an emissive admixture of barium and strontium oxide. The heater, which comprises of a spiral tungsten wire coated with an insulating film of alumina was inserted into the open end of the cathode tube.  The cathode was mounted onto the centre of a mica mounting disc which carried connecting lugs for both the cathode and heater.

The grid assembly - this comprises of an inverted metal cup, the solid end of which is pierced.  The mica disc carrying the cathode was inserted into the grid cup and the electrons emitted by the heated cathode emerge through the grid piercing.

The anode assembly - this comprises of two anodes.  The first anode is a nickel cup with a central aperture on its closed end and a wide flange at the open end.  The final anode is a bent nickel tube with a flange at the outer end which is closed with a metal disc having a central piercing through which the electron strem emerges.  The final anode carries three springs which allow a path for EHT and also to allow centering of the gun when fitted to the tube neck.

Each of these individual assemblies were mounted onto two steatite ceramic support rods whilst aligning essential clearances with the aid of a mounting jig and distance pieces.  The electrode system was then mounted onto a glass base 'button' which was fitted with five connecting wires and a vacuum takeoff spigot for final pumping.   A final 100% inspection was made of the assembled electron gun using a projection shadowgraph and remedial tweaking to fine tune alignment was made at this stage.  The electron gun assembly was then passed for incorporation into the cathode ray tube - more on this in a later blog entry.