Self-heating thermionic tubes

The paper shows that an alternating current may be conducted by a single self-heating thermionic tube in which the conventional anode and cathode are replaced by two identical thermionic emitters, each acting alternately as anode and cathode and so designed that the heat generated in them by electron bombardment is just sufficient to maintain them at an appropriate working temperature. Because the full anode dissipation is made available for cathode heating, such a tube can be provided with cathodes of larger emitting area than is usual with conventional tubes of similar size, thus permitting satisfactory operation at lower supply voltages. Portions of the cathodes must be heated above a critical temperature by a starting heater or an auxiliary bombarding cathode before the self-heating action can commence. Three distinct types of self-heating tube are described. In the thermionic rheostat the equivalent resistance of the tube is controlled by varying the spacing between the cathodes. The ¿self-heating triode,¿ which has a grid midway between the cathodes, functions as a variable resistor or as a power oscillator. In the ¿thermionic voltage regulator¿ the heated cathode area increases with increasing current, the result being that the voltage between the cathodes is substantially independent of current. Self-heating tubes are potentially useful as voltage stabilizers, or to control small a.c. loads such as motors or solenoids. Also, the self-heating triode will generate high-frequency power for heating purposes. The operation of self-heating tubes is analysed and the methods used in the design, specification and application of conventional tubes are extended to cover self-heating tubes. Experimental self-heating tubes having oxide cathodes and operating direct from 240-volt a.c. mains are described.