Radio Broadcast (May 1928-Apr 1929)

Data on an Improved Circuit AN EFFICIENT PUSH-PULL A. F. SYSTEM THE common concept of the advantage of push-pull amplification seems to be the possibility of providing an undistorted output greater than that which would be available from a single tube in the power stage. As a matter of fact, the available output from the push-pull stage is somewhat greater than is available from the same two tubes used in simple parallel relationship. The manner in which the signal impressed on the grid circuit of the push-pull arrangement is amplified in the plate circuit, while the harmonics generated (distortion) within the pushpull circuit cancel out in the plate circuit, has been so well discussed in texts on vacuum tubes (Thermonic Vacuum Tubes by Van der Bijl, page 261) that it would be superfluous to go over the matter here. The writer has completed recently several designs of push-pull amplifiers using tuned transformers and it is felt that some expression of the performance of the combination of these two principles will be of interest to the readers of these columns. Analysis of Circuit TURNING to Fig. 3 we have two power tubes connected in push-pull to a divided choke coil. This coil may be the primary of either a transformer or an auto-transformer. In the use of the latter, however, resides several advantages of the push-pull system from a design standpoint. It is apparent that, when two well-matched tubes are used (passing the same plate current), the ends of the winding A and A1, are at substantially the same d.c. potential. For this reason the speaker can be connected across these points without any danger of damaging current flowing through the speaker winding. This ehminates the expense of the blocking condenser which is ordinarily used with plate chokes. In the same way the two points, B and B1, are at the same potential when placed equally distant from the center and may be used for speaker connection when a step-down ratio is desired for operation of low-impedance speakers. In addition, the simultaneous use of all or a portion of the winding as both primary and secondary results in a closer magnetic coupling between the tube and speaker circuits, with consequent improvement in efficiency and frequency By KENDALL CLOUGH Engineering Dept., Silver-Marshall, Inc. characteristic. A further result is the economy of window area for the copper in the iron core. This results in either more liberal size of copper wire in a given lamination or a greater reduction in the overall size of the device than would result with the use of two separate windings. We are grateful to Kendall Clough for explaining, in an engineering manner, the advantages of using more than one stage of push-pull in audio amplifiers. Several engineers have advocated such an unorthodox amplifier, but few seemed to have any good reason for it. In this article Mr. Clough states that there are distinct advantages and explains what they are. The amplifier he sent to prove his contentions made the entire Laboratory Staff discontented with their own personal equipment— which was not double push-pull! — The Editor. In considering the more intimate details of the design, theory indicates that the inductance of the choke should be large compared with the impedance of the tubes out of which it operates. Of the four tubes available for power use, the 112, 210, 171 and 250, the first two are of about 5000 ohms impedance and the latter two of 2000 ohms. The 112-type tube has never enjoyed much favor because of its limited output, while the 210 is not as commonly used at present, probably because it requires as high a plate voltage for its operation as the larger and more capable 250-type tube. For these reasons only the 171 and the 250 will be considered. Fortunately, these are the two low-impedance tubes of the group which simplifies the problem of attaining a sufficiently high impedance for good operation at the low frequencies. Calculation and measurements indicated that a total inductance of 32 henries would be sufficient with a 4000-ohm output circuit (the plate impedances of the two tubes are in series) to give very good response at the low frequencies with several speakers that were at hand. Another advantage inherent in the pushpull circuit made this value of inductance rather simple of attainment. In the ordinary single choke or transformer the iron is subjected to a continuous magnetizing force due to the direct current flowing through the winding to the plate. In order to prevent this force from magnetizing the core to or near the point of saturation, it is necessary to place a good size air gap in the magnetic circuit. This, in turn, increases the reluctance of the magnetic circuit making it necessary to use a much larger core (in order to secure the necessary inductance) than would be necessary if this magnetizing force did not exist. Even when the adequate inductance is attained in this manner, the core is subjected to a considerable magnetizing force, causing the signal to operate on an asymetrical magnetization curve which is as distinctly undesirable in an output transformer as in an interstage device. This type of distortion was pointed out in the author's previous article (July, 1928, Radio Broadcast) on the subject of audio amplifiers. Advantage of Circuit IT WILL be seen in Fig. 3 that, although the two halves of the choke are wound in the same direction, the plate currents flow in opposite directions to the plates; hence the magnetizing forces in the core, due to the two halves of the winding, cancel, and the resulting force in zero. [It will be noted that the author has indicated the direction of current as passing from plate to filament, thereby conforming to the convention that currents always travel from the point of positive potential to the point of negative potential. The actual electronic flow is of course in the opposite direction. Editor.] It should not be inferred, as many designers seem to have done, that this permits dispensing with the air gap altogether, although it does permit a smaller air gap, resulting in more economical use of the iron in securing the requisite inductance. A small air gap must be included in order that the original value of inductance, measured at small values of audio-frequency current, may be maintained at high signal levels. Before proceeding with the design features of the input transformer that is to supply the two voltages, Cg, these voltages SM-227 UX171-A SM-248 Fig. 2 february, 1929 . . . page 241 •