NEW TYPES OF VALUES

Manawatu Times, Volume LIV, Issue 6843, 22 February 1929, Page 9

NEW TYPES OF VALUES

QUESTION OF PERFORMANCE,

Several new types of valves wore introduced several months ago, and it w now possible, says an expert writing in the London Morning Post, to scribe their behavious in various receivers. . . , - The first striking point arising trom practical tests is the great efficiency ot the new types in comparison witn those marketed last season. In certain instances the improvement amounts to as much as 100 per cent., or expressed in more practical fashion, with the newer valves twice as much magnification may be obtained for tho same quality of reproduction. A receiver fitted with new types will therefore provide much greater amplification, for it must be remembered that if the three stages of a threevalve receiver, for example, each give twice the amount of magnification when fitted with this season’s valves as compared with last season’s, the total magnification is eight times as. much. This great improvement in the magnification may appear of first importance, but in practice it generally leads to difficulties. Self-oscillation, for example, which produces in its worst form howling and squealing and in any form bad quality of reproduction, is much more liable to occur even when the re-cently-popularised methods of avoiding certain forms of low frequency interaction are employed. This is because the valves and their couplings are so effective that they tond to oscillate in a similar manner to high-frequency circuits. The danger of self-oscillation is not as great when resistance couplings are used, as this apparatus does not possess a natural or resonant frequency. But when choking coils or transformers are employed, as they usually are in popular receivers, there is always the danger of high-pitched whistles and other distortions, and it is difficult to eliminate them without employing means which reduce amplification.

It is better therefore to choose valves that will not give as great magnification, for then the quality of the reproduction will be better. The new power valves should certainly enable better quality to be obtained, although usually more high tension current is required. This is a disadvantage in many instances. When a -power unit is available the amount of high tension passed by the power valve is of no particular importance provided it does not overload the ,unit ,but when a dry battery supply is used it is necessary to restrict the current or the battery will quickly dis 1 charge. Portable set users should make careful inquiries before replacing valves, as in many instances receivers of this type will not function correctly when fitted with the newer and more effective valves.

The makers should be able to specify modern valves which will improve the performance, and no doubt there are dealers who understand the receivers, and are competent to advise. The really modern receivers of the three-valve type, having a shielded grid high frequency amplifying stage, a detector ,and a pentodo or five electrode valve out-put stage, appear to be giving satisfactory service when a main’s unit is employed for providing the high tension current. These sets have not always proved reliable when supplied in the portable or suit-case form, because of tho heavy drain upon the dry cell battery. The manufacturers of these - receivers are, however, usually able to choose valves which do not unduly strain the batteries, and their receivers may therefore be relied upon. In the shielded grid series of valves the manufacturers appear to have concentrated upon the production of types having two-volt filaments, as it is rather difficult to obtain specimens fitted with six-volt filaments. Tho majority of users employ two-volt valves, and it is only natural that this type should be put into production before the six-volt class, but it seems that owing to the special construction the manufacturers are quite unable to produce the necessary quantities.