Cathode Interface Impedance Desimplified

Cathode interface impedance has usually been treated as if it could be represented at any given time by a parallel R-C combination. In actuality, however, the impedance can be represented accurately only by an R-C network containing four elements. Moreover, the interface undergoes a reversible change of state with a relaxation time near one second as the cathode current is changed. These characteristics merit important consideration when specifications concerning cathode interface impedance are written. Modern methods of preventative maintenance frequently allow replacement of those tubes with cathode interface impedance before they can cause the failure of large electronic systems such as digital computers. However, serious cases of cathode interface impedance may cause the tube population of such a system to have a short average life, perhaps less than 10,000 hours. Under such conditions, catastrophic failures - primarily an affliction of young tubes - very likely will be more prevalent than would be the case if the average life were greater. Any increased level of catastrophic failures will cause a reduction system reliability which may be attributed indirectly to cathode interface impedance.