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Variation in the gm or gain of a tube with changing frequency is a major effect observed when cathode interface impedance is present. In application, variations in the gain of wide band amplifiers and loss of power in pulse and trigger circuits are sometimes evidence of cathode interface impedance. In the course of laboratory investigation, it has been determined that the major interface impedance difficulties are caused by the formation of barium orthosilicate. Some other interface compounds also have low conductivity but give relatively little trouble. These include barium aluminate, barium tungstate and barium titanate. As the term "interface" implies, this compound develops between the cathode coating and cathode sleeve in the form of a resistive layer or film. In ohmic value, measurements have shown a range of from one ohm to several thousand ohms depending upon electrical operating conditions,
cathode temperature and the degree of development.
Several factors determine the rate of development, effect and measured value of interface impedance including the following:
1. High cathode temperatures produce a faster growth of the interface compounds; therefore, interface impedance difficulties are multiplied by long time operation at high heater voltages.
2. The effective interface impedance is temporarily reduced at higher cathode temperatures; therefore, high heater voltages will temporarily reduce interface resistance troubles after they occur. Conversely, after interface troubles occur, they will increase exponentially with a temporary reduction in heater voltage.
3. The operating conditions of the tube also determine the effective impedance of the interface compound since it varies with the level of current drawn through the interface.
4. The level of interface impedance can be changed for a short period of time by drawing
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heavy current through the interface. This tends to activate the interface, giving a higher conductivity for a short time. However, the interface returns again to its previous level in a short time.
The effect of interface impedance can be approximated by placing a parallel resistance-capacitance network in the cathode lead of a tube. In actual practice it will be found that the interface resistance is more complex than that approximated by a single resistive-capacitive network and, therefore, a closer approximation requires a series of these resistive-capacitive networks in the cathode lead, Figure 2.
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