POWER VALVES

Shannon News, 14 June 1927, Page 4

POWER VALVES

The first.- essential of a powci valve is, of course, the ability to handle, effectively the power necessary to operate a loud-speaker to give loud signals. Almost any valve can be made to give loud signals, but only the special valves, the so-called “power” valves, can ac complish this without giving distort ‘ ed reproduction of music and speech. Fortunately recent developments in tho design of receiving-valves have enabled both British and American valve manufacturers to produce valves which can handle large voltage swings bn their grids without 1 difficulty. The valves have a com paratively low amplification factor and will therefore not give louder signals, as is frequently expected, than ordinary receiving valves. However, they will give louder undistorted signals and this is the essential requirement. The valves have a low importance, and as a result a better production of bass notes is obtained.

To obtain the best results with the power valve it is necessary to follow the maker’s instructions carefully and in doing so it will be found that high plate voltage, with correspondingly high valve of the voltage of the grid biassing battery i s demanded. No success will attend the use of the'power valve unless these instructions are obeyed. With the high plate voltage and the low impedance of the valve the plate current becomes much greater than that taken by the ordinary valve. Where batteries are used their life will be correspondingly reduced, but to meet this difficulty the battery manufacturers have developed over-size types and it should be unnecessary to add that where a dry- cell B-battery is in use in conjunction with a power valve the large size must be used. 'ln alternating current areas where battery eliminators are used, the increased current taken by the power valve is of no importance. . The time is approaching when a power valve will always be installed in the last stage of an audio-fre-quency amplifier.