Heater/cathode insulation in indirectly heated valves is considered as a two-part phenomenon: the comparatively slow deterioration during operational conditions followed by a rapid thermal breakdown, which is caused directly by the sudden passage of a large current between heater and cathode. Examination of heater/cathode failures in valves tested under rigidly controlled conditions has indicated some of the laws governing the deterioration of the insulation. The effects of various parameters such as temperature of operation, thickness of insulation and applied potential have been studied, and comparisons have also been made for different heater and cathode materials. The experimental evidence is discussed in an attempt to explain the failure mechanism involved, and the conclusion that electrolysis is the principal factor is supported by the results of a number of experiments carried out with specially designed electrode structures.
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