Improved Heater Coating
The incidence of heater-to-cathode breakdown in a valve and the susceptibility of a valve to hum modulation depend to a large extent on the purity and texture of the alundum coating on the heater.
Recent innovations have enabled Mullard to produce alundum with an extremely low concentration of certain impurities (some of which are known to be troublesome) and research has led to modifications to the texture of the purified powder. Consequently the alundum coating now used in Mullard valves ensures improved hum performance and better heater-to-cathode insulation.
Stranded Heater Spirals
Conventional valve heaters have to fit within the cathode and must therefore be as compact as possible. However, the heater wire must be thick enough to carry the current to heat the cathode adequately, and thick wire is subject to large stresses when wound in a tight spiral. In those types where the dimensions are such that the heater is likely to be a potential source of trouble during the life of the valve, Mullard are now able to replace the single strand of wire by two or three wires wound together without reducing the rating of the heater. The emissive property of the valves is thus unaffected, but a marked improvement in heater reliability during life is ensured.