CAUSES OF “BURNED OUT” VALVES.

Sun (Auckland), Volume 1, Issue 16, 9 April 1927, Page 20

CAUSES OF “BURNED OUT” VALVES.

The chief cause of bmaed out tubes is a weak lilamcnt unless the tube has been ebused. Seldom, however, does such a tube last any length of time at all. The weakness has been with it from the beginning. More frequently they burn out after an accidental passing of the B battery through the filament. This is possible from crossing grid and plate wires, if minus B connects to positive A and there is no condenser in the grid circuit. The only other possibility is crossing the plate wires with the filament wires. It is also entirely possible to burn out n filament properly connected by putting the plate voltage too high. 1 his is not apt to happen with the standard tungsten filament tubes, but may with coated filament tubes, that is. those tubes having barium or strontium oxides on their filament to reduce the temperature of filament necessary for liigh emission or electrons. Particuit sucl) tubes tQiae degree

FILAMENTS AND THEIR PECULIARITIES.

of gas in them, the phenomenon known as ionization takes place. In all tubes there is some gas and there is gas in the metal of the parts within the evacuated space. Too higii a plate voltage causes the gas to ionize. The positive parts of the ionized gas travel toward the tilamvnt, the negative parts go toward the plate. The bombardment of the tllament heats it, usual! v in a weak spot, and the cumulative effect of heating and bombardment results in a break. It may even result in a broken tube. Luckily this does not happen often. It is more frequent in experimental work where several hundred volts are available than in most sets where not over 90 to 120 volts are needed. ’ Fortunately, too. most of the storage battery tubes' available are of a type which do not ionize at any reasonable voltage. The one exception is the soft detector tube still used for its sensitivity, but this is always used at a very low voltage and ionization results in a very apparent blue glow and complete cessation of the tube operation. After a short rest such a tube will operate *4 well aa »v<J«