Amplification from the Mains

Radio Record, Volume II, Issue 33, 1 March 1929, Page 30

Amplification from the Mains

Lecture to Wellington Amateurs

"THE speaker at the last meeting of the Amateur Radio Society was Mr. Preston Billing, who took as his subject "Power Amplifiers." In introducing the subject, Mr. Billing explained that in the future power amplifiers were going to be exceedingly popular. They could be operated directly from the light socket, and were capable of handling tremendous volume without in the least impairing the quality. At the same time, they were simple to construct, and gave very long service. ; Briefly touching on the theoretical side, such terms as "alternating current," "magnetic fields," and "magnetic flux," were explained, it being shown by diagrams the bearing of these in a magnetically-coupled transformer such as that used in the amplifier. When two coils were loosely coupled, that is, where coupled by their magetic fields only, and a current was passed through one of them, induction took place in the adjoining coil, and there set up an electric current. In order to intensify the field between these two coils, a core (usually of soft iron or stalloy) was employed. The quality of the iron had a good deal to do with the quality of the electric current produced, and secondly upon the resultant tone. This explains why cheap transformers give a harsh and unsatisfactory tone. It was explained that when the voltage was to be "stepped-up," the relation of this "step-up" was that of the number of turns on the primary compared to that on the secondary coil, that is, if 50 turns were put on to the primary and 150 on to the secondary, the output of the secondary would be three times the input of the primary, less the loss through the transformer. Commenting on the general design of transformers, Mr. Billing said that there were mainly two types of cores, the rectangular and the shell. The latter was the one generally employed because of its greater efficiency. The types of iron used were sylicon and stalloy. The former was the more satisfactory but very much more expensive. To constructors who were mak-

ing their own transformers, a word of warning was voiced. Enamelled wire was that usually used, but if the enamel happened to be chipped off and one wire on a layer fell through to the next layer, a short circuit would occur, and very rapidly burn out the transformer. If the transformer was to be subjected to much movement, then the wire should receive a covering of cotton over the enamel. The current draw was small, but care had to be taken that the resistance would not cause the voltage to drop too The principle of the gramophi pick-up was explained with diagrams, it being said that a good pick-up would give equal strength with a crystal set if telephones were used instead of an amplifier. Commenting on the amplifier, Mr. Billing suggested that a variable resistance should be placed across the primary of the first audio transformer. In effect, the gramophone pick-up was replacing the radio frequency and detector stages of the ordinary receiver, and should be considered as such. Rectification was accomplished by a valve, either the filament or the nonfilament type. Mr. Billing preferred the former, because they were capable of delivering a greater current at a higher voltage. The types were halfwave and full-wave. The full-wave was the more satisfactory because it» was able to deliver double the current of the single-wave type. In concluding, Mr. Billing mentioned two points on which the constructor should be careful :- 1. The transformer was delivering very high voltage, sometimes up to 1000 volts, and if great care were not exercised, the constructor might get a severe shock. 2. The chokes should be capable of handling 100 milliamperes and have a eapacity of 30 milliahenries. Mr. Billing concluded by remarking that not 5 per cent. of the sets in use could reproduce faithfully all the frequencies that were sent out by the transmitting stations, and in view of this, much of the talk about distortion and unfaithful reproduction emanated as a result of the incapacity of sets.