CARE OF VALVES

Radio Record, Volume I, Issue 27, 20 January 1928, Page 14

CARE OF VALVES

EXCESSIVE A BATTERY, Present-day valves, for the most part, have so-called thoriated tungsten filaments, the action of which, even at this late date, is not fully appreciated by the average listener. ‘he electronic emission of the thoriated tungsten filament depends upon the presence of a layer of thorium atouis on the outer surface of the filament. It will be noted that, unlike the oxide-coated filament found in some valves, the thoriated tungsten filament is not merely thorium-coated, but it is permeated throughout its entire mass with the rare element thorium. During in the normal operation of such a filament, the thorium on the outer surface ‘is gralually cvaporated, reducing the emission, rendering the valve shortlived. Wowever, while the heat of tlie filament 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. Valves Paralysed. When subjected to an oyer-voltage on the filament, however, the evaporation becomcs excessive, so that thie valves accordingly become more or less paralysed. Operating these valves at sub-normal is also liable to paralyse them slowly, as the filament temperature is so low that the process of boiling out the thorium from the interior of the filament becomes abnormally retarded. fence it is important that the thoriated tungsten filainent valves be operated strictly at their rated voltage, Ly means of hand rheostats with an accurate voltmeter, or, better still and simpler, by means of amperites, the self-adjusting rheostats.