EXPERIMENTAL PUSH-PULL AMPLIFICATION

Radio Record, Volume I, Issue 48, 15 June 1928, Page 13

EXPERIMENTAL PUSH--PULL AMPLIFICATION

XPERIMENTERS. have been inquiring for a method of testing push-pull amplification without employing special — transformers. The following method was pubfished recently in a London radio journal, and is reproduced here for the benefit of those who are interested and care to try it out. Although this arrangement has not all the advantages derived from the use of a pair of pushpull transformers, the use of the two valves in one stage will certainly allow eonsiderably move volume to be handled. The inevease in actual amplification is very small when a push-pull stage is substituted for the single valve arrangement, but increased vyolume is gained because it is handled without distortion, and this gives the same effect as greater amplification. It is also necessary for the speaker to be capable of carrying the increased volume without overloading. THE method employed is to use au ordinary audio transformer ~ as the input transformer of the push-pull stage, and as this is not provided with 2 centre-tapped secondary winding, the

equivalent is provided by shunting the secondary with two high value resistances and making the grid-bias eon--nection at the junction of the two. T is not possible, however, to perform this operation on the primary of an output transformer owing to the large D.C. voltage drop which would be caused across them, with the result that the voltage on the plates of the valves would be absurdly low. . AS is well known, the windings of a 2000 ohm Joudspeaker, for instance, are wound on two bobbins, each bobbin having a D.C. resistance of 1000 ohms. These bobbins are connected in series, and it is obyious that the plates of the two output valyes could be connected to the ordinary terminals of the loudspeaker, the ¥L.1, plus connecting via a third ferminal (which could be fitted in a convenient position) to the junction petween the two bobbin windings where they are connected in series; care must be taken in the soldering operation. This can be done even in the case of a: super-power valye, for no risk of magnetic saturation of the loudspeaker core will take place, as the magnetising current in each bob- pin will cancel out just as is the case with the two halves of the primary of an output push-pull transformer.