Renewing Valve Life

Radio Record, Volume III, Issue 17, 8 November 1929, Page 29

Renewing Valve Life

A Method for Restoring Emission SOME types of valves, notably the older dull-emittters, employ metal filaments coated with thorium. If this‘ class of valve is overheated part of the filament surface coating. evaporates, thus considerably reducing emission. Distributed throughout the core of the filament is additional thorium, and if this can be brought to the surface the life of the valve is renewed. . The loss of emission may be due to two causes, Firstly, the B battery supply may have been momentarily flashed across the filament by accident... Secondly, the A battery. voltage supplied to the valve may be higher than the manufacturer’s rating. Restoration of filament efficiency can be accomplished with valves in which the filament is of the thorinm-coated variety. Oxide-coated filaments, which usually operate at a duli red glow, do not appear to benefit from this treatment when once their emission has been lost. The actual method of applying the restoring process is to connect the filament in geries with some constant source of supply. The filament is then maintained .at its correct temperature

of operation for two hours or more, There are two simple methods of carry« ing out the above "cooking" process. (a) An accumulator of suitable volt. age is connected directly across the fila« ment pins and left in circuit for about two hours. The yalve is then tried in a set, and if results are not up to stat~ dard, the "cooking" process should be tried for another hour or so. (b) The second method of applying the necessary constant voltage is by the use of a step-down transformer which delivers across the secondary terminals an approximately eorrect voltage for the valve to be treated. It is an advantage to secure a transformer. arranged with taps on the secondary, so that it may be used for 2, 4, or 6volt valves. A rheostat is arranged in series with the secondary winding ta give an accurate adjustment of the voltage applied. The actual treatment of the filament is identical with that employed when an accumulator is used.