WHY VALVES BECOME PARALYSED

Radio Record, Volume I, Issue 41, 27 April 1928, Page 12

WHY VALVES BECOME PARALYSED

IREUSUNT-DAY dull emitter valves, for the most part, nave so‘alled thoriated-tungsten filaments, the action of which, even at this late date, is not fully appreciated by the average listener. The electronic emission of the thoriated- tungsten filament, according to the chief engineer of the Radiall Company (of America), depends upon the presence of a layer of thorium atoms on the outer surface of the filament. Thoriated-tungsten. It will be noted that, unlike the ox-ide-coated filament found in some valves, the thoriated-iungsten filament is uot merely thorium-coated, but it is permeated throughout its entire mass with the rare element thorium. During the normal operation of such « filament the thorium on the outer surface is gradually evaporated, reduceC UOTE LEC EP eE EE Pee: Ue

ing the emission current, and if permitled to continue, rendering the valve short-lived. EIowever, while the heat of the filauieut serves to evaporate the thorium particles on the surface, it is also boiling fresh thorium particles out of the mass and up to the surface. Thus the surface is being continually _ replenished. Just so long as the filament voltage is not increased beyond 10 per cent. above the rated value, this evaporation and replenishing process continues at an equilibrium rate, so that a constant layer of thorium is maintained on the surface. Wlien subjected to an ovet-voltage on the filament, however, the evaporation becomes excessive, 60 that the valve accordingly becomes more of

less paralysed. Operating these valves at sub-normal voltages is also liable to paralyse them slowly, as the filament temperature is then so low that the process of boiling out the thorium from the interior of the filament becomes abnormally retarded. . Need for Filament Control. Hence it is important that the thori-aled-tungsten filament valves be operated strictly at their rated voltage by means of hand rheostats with an ac-

curate voltmeter, or, better still and Kimpler, perhaps by means cf self-ad-justing rheostats, such as Amperites.