The dependence of mutual conductance on frequency of aged oxide-cathode valves and its influence on their transient response

The mutual conductance, gm, of some aged thermionic valves having oxide-coated cathodes varies with frequency, being greater at higher than at lower frequencies; its variation has been ascribed to an impedance at the interface layer between the oxide coating and the nickel sleeve. A new null method of measuring gm enables the impedance to be assessed to within about 1% of 1/gm; results show that the previous representation of the impedance as a single parallel combination of resistance and capacitance is inadequate. The response of these valves to pulses of low level and short duration (about 1 microsee) involves overshoot, which has been measured by a method adapted from time-division-multiplex telephony; the decay of the overshoot is not exponential, as has previously been assumed. Although an interface layer whose impedance contained many components could explain the results found, explanations could also he based in part on effects not necessarily occurring at the interface and best not represented as impedance made up of conventional components.