DISTORTION IN RECEIVERS

Hawke's Bay Tribune, Volume XIX, Issue 300, 4 December 1929, Page 10

DISTORTION IN RECEIVERS

It should be the aim of every set owner to endeavour to make his receiver reproduce as faithfully as possible the original sounds made in the studio of the transmitting station (says an exchange). Many receivers, especially home-made ones, fall short in this respect, and in some cases the owners are not even aware that their receivers are not functioning correctly, because they have never tested them alongside a really good set. In a good many cases the trouble may be located outside the receiver, and is due to one or all of the following causes:—(l) Incorrect valves; (2) insufficient h.t. voltage; (3) incorrect grid bias voltage; (4) faulty or inferior speakers. With regard to item (1), u incorrect valves/’ in the earlier days of radio, it was the common practice to make one typo of valve serve the purpose of amplifier and detector. Later, however, valves possessing different characteristics were employed in the various circuits of a receiver. As an example, high MU valves were used for resistance capacity amplification; low impedance valves for audio ampification, and special power valves, or valves capable of handling relative ly large currents, wore employed in the final stage of a receiver. As the siginals progress from one stage to the next they increase in amplitude, and on reaching the last valve they are usually strong enough to work a loud speaker. We may, therefore, conclude that a valve which will handle very weak impulses satisfactorily will not necessarily cope with large powers and vice versa, a valve suitable for largo powers will not be satisfactory for very weak signals. This is of special importance in the case of the last radio valve, which should be of the power type if it is desired to get the best results. In the case of insufficient high tension voltage, failing to use the correct voltage on the audio valves (especially on the last audio) will produce distortion on the louder notes. This is caused through the grid fluctuations swinging the plate current past the straight portion of the characteristic curve of the valve. By using more high tension the usable portion of this curve is increased, with the result that the valve will handle louder signals. The instructions enlosed in the valve carton should be carefully followed, and the high tension batteries checked over from time to time with a voltmeter. When the high tension voltage is increased it becomes necessary to add more negative bias. Instructions regarding the Correct bias for various values of plale voltage are to be found inside the valve carton. It is general practice to write these instructions somewhat as follows:— H.T. volts 120 bias 9, h.t. volts 100 bias 6, etc. Therefore, if the high tension supply is 110 volts, n bias of 7or S volts should be applied. In the case of resistance amplification only a very low bias is required because. although 150 volts h.t. may he used, the actual voltage applied to the plate will not exceed 80 or 90 volts due to the R.T. drop across the coupling.