Radio broadcast .. (1922-30)

New Apparatus and Its Applications Transformers Now Available for Linking Dynamic Loud Speakers With Push-Pull Amplifiers this month a change is made in the method of presenting new apparatus in these pages. Parts submitted by different manufacturers usually are treated as separate items, but in this issue several dynamic- speaker output transformers of the push-pull type are described in one article. In this way it is possible to give a more lengthy dis- cussion of the applications of these devices, as space is not wasted in duplicating descrip- tions. A second article under this heading describes a dual push-pull public-address amplifier. A complete list of parts is specified in the text, and much valuable data is given on the design of amplifiers of this type. THE story is told of a man and woman who, while riding through California, expressed great curiosity regard- ing some immense fields of French artichokes, with which they were un- familiar. It was explained to them what these vegetables were, what a tooth- some dish they provided, and the sug- gestion made that they try them. To which the gentleman replied with ut- most finality, "Oh, no! We never eat strange foods." An antipathy towards "strange foods" is fortunately not a characteristic of radio enthusiasts to whom new things are a staff of life. The newest "strange food" to which many of us have probably devoted considerable thought during the past few months is concerned with moving- coil loud speakers—how good they are, how to operate them, and so forth. This article is devoted to one particular angle of the subject, the reason for the use of a coupling transformer between the moving coil of the loud speaker and the plate circuit of the power tube. The coupling transformer, T, in a moving-coil loud speaker is connected in the circuit as indicated in Fig. i, and it has one major purpose in life—to "match" the impedance of the mov- ing coil to the plate resistance of the power tube. It is true that the trans- former will also serve to keep the d.c. plate current of the tube out of the moving coil, but this purpose is secondary in comparison with that previously mentioned. Now, since the pur- pose of the transformer is to match the tube to the moving coil, it would seem to follow that manufacturers would have to put into the mov- ing-coil loud speaker a coupling transformer that can only be used with a tube of a definite plate resistance—unless it is possible to design the transformer so that it will be satisfactory for use with all types of tubes. Let us see if we cannot answer this question very briefly. It is not our purpose in this article to enter into a discussion of transformer characteristics. From such a discussion we would finally reach the following conclusions: (a) that a properly constructed coupling transformer designed to work out of an impedance of 5000 ohms, cor- responding to a 1I2A- or 2io-type tube, will also work satisfactorily out of 2000 ohms, cor- responding to a 171 A- or 2>o-type tube, and Center-tapped Choke FIG. 4 FIG.3 FIG. (b) that a transformer designed to work out of an impedance of 2000 ohms, corresponding to a 171 A- or 250-type tube, will not operate satisfactorily out of a H2A- or 2io-type tube, because with such a transformer the two lat- ter types of tubes would operate with a load impedance in their plate circuit of less than twice the plate resistance of the tube and, for this reason, the plate current-grid voltage char- acteristic of the tube will be curved and distortion will result. For these reasons we find that the coupling transformers which always are built into a moving-coil loud speaker are designed according to (a), and the trans- former will, therefore, work satisfactorily out of a H2A-, I7IA-, 210-, or ajo-type tube. In some cases, however, we may want to operate a moving-coil loud speaker out of a push-pull stage. When this is to be done, two possibilities are open to us—we 194 FIG. 5 can either remove the transformer supplied with the loud speaker and substitute one designed for use between the moving coil of such a loud speaker and a push-pull stage, or we can just connect fthe output terminals of the present push-pull transformer in the set to the input terminals of the coupling transformer incorporated in the loud speaker. If we use the former arrangement the result- ant circuit will look like Fig. 4, and if we use the latter arrangement the circuit will look like Fig. 5. Whether the arrangement of Fig 4 or Fig. 5 is used depends upon various circumstances which are listed below: (a) If the present output push-pull transformer in the receiver is a good one designed for use with ordinary cones with a nominal impedance of about 2000 or 4000 ohms, then the circuit of Fig. 5 may be used with satisfactory re- sults. The power loss due to the use of two transformers will only be about 2 TU —a negligible loss. (b) If the present push-pull output transformer in the set is thought to have a poor frequency characteristic it will be best to remove it and use the circuit arrangement Fig. 4. (c) If a choke output circuit, Fig. 3, is used in the output of the push-pull tube then the circuit of Fig. 5 may be used (i.e. terminals I and 2 of Fig. 3 may be connected to the leads from coupling transformer supplied with the loud speaker) if 171 A tubes are used in the push-pull amplifier, provided that each of the choke coils have an in- ductance of not less than 30 henries. (d) If a choke output circuit is usedwith 210- type tubes then the chokes and the coupling transformer in the loud speaker should be re- moved, and a special transformer substituted and arranged in the circuit as per Fig. 4. There are listed in the following table a number of special transformers, designed to replace the coupling transformer of a dynamic speaker: