VALVE PROGRESS

Evening Star, Issue 23699, 5 October 1940, Page 4

VALVE PROGRESS

COULD READ BY EARLY ONES USED POWER FOR FIFTY MODERNS The radio valve is one of the most intricate and marvellous devices in the radio world. It is because of the valve that broadcasting is on such a large scale, which, in turn, brings entertainment to millions of listeners throughout the world. There have been many changes in valves since their invention. One of the biggest is their reduction in size and the smaller amount of current they draw. Many modern valves are really several valves in the same tube. The early valves' were largo and clumsy and had not a very high efficiency rating. Some of them glowed so brightly that it was possible to read a book by their light. The discovery that a piece of wire, if made hot enough, would shoot out streams of electrons when placed in a vacuum, was the start of the valve. The piece of wire is called a filament. The filaments were made of tungsten, which needed a very high temperature before it would work. The simplest way to heat the filament, which was in a sealed glass bulb, was to connect it to a battery. Compared to the power the modern valve consumes, the older one used an enormous amount. It has been estimated that about 50 modern valves could ho worked on the power used by the old ones. Later scientists mixed different metals together so that the filament would give off electrons at.a much lower temperature. When the filament was lit the added metal (usually thorium boiled to the surface) formed the electron-emitter. This layer had a short life, and had constantly to be renewed, until the filament itself wore out.

The next important development in filament manufacture was the discovery that better filaments were made possible by spraying a coating of a suitable oxide on the filament proper. The majority of sets nowadays work from the power mains, which in most cases deliver power of alternating current. Except in the final or power stages of a set it is not possible to use filament-heated valves, as this would introduce a bad alternating current hum in the speaker. This hum is caused by the voltages applied to the filament appearing in the electron stream in the set. It was because of this hum that the alternating current valve was designed. It has a filament as in the battery valve, but the filament does not emit electrons. It is sprayed over with a coat of good insulating material, and over this is slipped a thin metal tube which is sprayed with the electron-emitting oxide. When the set is switched on the filament heats up, and its heat is transferred to the thin metal sleeve, or cathode, which also becomes hot and so emits electrons. In this way there is an electron-emitting element which has no contact with the alternating current supply. These valves glow at a dull rod heat.

There are still some output valves which are directly heated, hut this is (permissible because the small amount of hum can be minimised by balancing it out with a centre-tapped filament winding (or by using a centre-tapped resistance across the filaments) the ppri+re tan being regarded as an earth. Tbp fraction of a volt that is left could be very annoving in the earlv stages of the set. which would amplify it along with the signal, but jbe amplification in the final is low by comparison pud the signal ontnut volts are very high so that the total bum cannot be noticed.