Receiving Tubes Employing Secondary Electron Emitting Surfaces Exposed to the Evaporation from Oxide Cathodes

Previous use of secondary-emission multiplication in receiving tubes has been accompanied by difficulties with life and has been accomplished in oxide cathode tubes by shielding the secondary emitter from cathode evaporation. This shielding required a complicated tube structure and additional electrodes and leads. This paper describes methods of controlling cathode evaporation to obtain satisfactory tube life and even to enhance secondary emission gain in simple tube constructions in which the secondary emission surface is directly exposed to the oxide cathode evaporation. A 1.4-volt filament tube is described which may be operated to give a transconductance of three times the normal input transconductance, or may be operated to give the normal input transconductance with a reduction to 40 per cent of the original total battery power. The construction and characteristic curves of an indirectly heated oxide cathode tube with the dynode directly exposed to the cathode are given. A transconductance of 24 ma/volt and a wide-band figure of merit of two to three times that of conventional tubes is obtained. A high cathode temperature reduces dynode life, but a variation of ±10 per cent in heater voltage seems satisfactory.