Radio Broadcast (May 1928-Apr 1929)

Armchair . Chats on Short" Wave Subjects By ROBERT S. KRUSE THE writer now understands perfectly the feelings of an actor who is pushed suddenly on the stage and told to "fill" until some delay back stage can be "unscrambled." Luck is certainly with him if he has practiced something that is not included in the regular performance of the show. This is a complicated way of explaining that the article describing a short-wave receiver, which had been scheduled tentatively for this issue, has been held over because of some changes on the part of a manufacturer who suddenly made it impossible to obtain certain apparatus which had been selected for use in the final version of the set. Of course, it would have been possible to substitute other parts but the time available prior to publication was insufficient to permit checking thoroughly the sensitivity and selectivity of the revised receiver. Therefore, as a matter of policy the receiver will not be presented, as it is considered unwise to describe a set before it has been tested, even if one feels sure of the results. As a result it is a case of " Better late than — early," and the receiver will retire for the present in favor of the assorted comments on short-wave subjects which form the basis of this month's article. Push-Pull Vs. Back-to-Back SEVERAL correspondents simultaneously have requested a brief and simple explanation of the differences between " backto-back" and "push-pull" circuits. Unfortunately this is not a subject which lends itself to brief treatment, but in the following paragraphs an endeavor will be made to cover the more important aspects of the two transmitting systems. Although the term usually is applied to audiofrequency amplifiers, the "push-pull" system is also used in many radio-frequency amplifiers and oscillators. On the other hand, because it is employed only in oscillators, the "back-to-back" system belongs exclusively to the transmitting fraternity. Both circuits always require the use of at least two tubes, but there are several important differences in the ways the tubes are connected. In order to appreciate fully the features of the two systems it is necessary to study carefully the circuit diagram of each. For the purpose of this comparison four versions of the old standby oscillator circuit devised by R. V. L. Hartley have been selected (see Fig. i). In diagram a of Fig. I we have a standard shunt-feed one-tube Hartley oscillator with a dotted line separating the tuned circuit, L1C1, from the tube and its plate supply, stopping condenser, and grid leak. The terminology employed is standard, R. F. C. being the r.f. choke coil, Cp the stopping condenser, and Cg and Rg the grid condenser and leak, respectively. Of course, it is understood that Cp and Cg both have capacities larger than the capacities of the tube; therefore, they do not affect the tuning, but serve only to s This month's subjects , 0 include "Push-Pull Vs. j> r Back-to-Back Circuits" 5 and "Short-Wave Reception Troubles" keep the d.c. plate supply out of the r.f. tuning system to the left of the dotted line. In diagram b of Fig. i is the schematic circuit of a shunt-feed Hartley oscillator with two tubes connected in parallel. It should be noted that in this diagram there is no fundamental difference in the circuit; in fact, the apparatus to the left of the dotted line has not been changed, nor have any additional feed chokes been added. It would even be possible to connect the grids of the two tubes directly together and use a single grid leak and condenser. However, smoother operation usually is obtained with the arrangement shown. THE BACK-TO-BACK SYSTEM THE diagram of a Hartley oscillator with two tubes connected "back-to-back" is given in diagram c of Fig. I. This circuit appears very similar to diagram b but its operation differs in an important manner, even though the tuning system to the left of the dotted line has not been changed. Whereas in diagram b the two tubes operate simultaneously, an analysis shows that in diagram c they operate alternately, due to the fact that each tube operates only when its plate is positive. A comparison of the two circuits will quickly show the difference between them; in circuit b the plates of both tubes are supplied with current which flows through the choke coil, R. F. C, but in circuit c the currents for tubes Nos. i and 2 pass through separate choke coils, R. F. C.i and R. F. C.2, which are connected to the two high-voltage terminals, M and N, of the power transformer, Tr, the rcenter-tap terminal of which is connected to the filaments of the two tubes. Therefore, when M is positive tube No. 1 operates, but at this instant N is negative and tube No. 2 is inoperative. THE PUSH-PULL ARRANGEMENT THE circuit arrangement in diagram d of Fig. 1 shows an oscillator with two tubes connected in "push-pull". This arrangement has features which are similar to both b and c, but the operation of the system is quite different from either of the former circuits. Neglecting the dotted lines of the diagram and tracing the connections, it will be noted that the plate power, which is supplied to the center-tap connection of the coil Li, flows in both directions through the coil to the plates of the two tubes. Therefore, the two plates are at the same potential as far as the power supply is concerned, but the r.f. voltage between them is obviously the entire voltage across the tuned circuit LiQ. Accordingly, we may say that in diagram c (back-to-back)the plates are at opposite sides of the plate-supply cycle, while in diagram d (push-pull) the plates are on opposite sides of the radio-frequency cycle. Conversely, the plates of the tubes in the back-to-back system are connected together through two large condensers and, therefore, have no r.f. voltage between then, while the plates of the tubes in the push-pull system are connected together by wire and have no lowfrequency voltage between them. In the above paragraph, we believe, the important differences between "back-to-back" and "push-pull" oscillator circuits have been clearly explained. However, some readers may be interested in the relative merits of the two systems. We shall, therefore, devote some space to this subject. An important. feature of the back-to-back system is that by careful adjustment of the various condensers, choke coils, etc., it is possible to obtain a r.f. output with a low modulation, i.e., a good tone. From the viewpoint of the reader this may prove a very unexpected conclusion, but let us study the action of the tubes. Inasmuch as the tubes are supplied with pure a.c. one would [expect that tube No. 1 would oscillate briefly for 60 periods each second — if 60-cycle current is used — and that tube No. 2 would go through the same performance while No. 1 is resting, i.e., each tube would operate for riv of a second, or during one half of each cycle (see diagram a of Fig. 2). If this were true the r.f. output of the transmitter would be similar to the curve in diagram b of Fig. 2. In practice, however, this is not exactly what takes place in the tuned circuit, L1C1, of Fig. ic. On the other hand, by careful adjustment it is possible to make each tube oscillate for more than one-half cycle, and, since this is true, it is evident that the r.f. output may be much smoother than indicated in diagram b of Fig. 2, as the train of oscillations of one tube The Japanese Schoolboy's "Q_T Signals A T THE late International Radio Conference at Washington, D. C, ■*• *■ many curious things were said and done, hut a special height of foolish sublimity was attained in the new list of "Q" signals and their definitions. It is not known just why anyone felt that it was necessary to tinker with the old list, hut one can say with some certainty just who was retained to write the definitions of the abbreviations. It was positively Hashimura Togo, Wallace Irwin's Japanese schoolboy. The inspired ability to choose the wrong word, the faultless gift for reversed word order, the special genius for associating ideas that have no connection — all are present. Hashimura has exceeded his Saturday Evening Post performances in some ways, perhaps because the opportunity was so good. The "Q" Signals are so called because they all begin with that uncommon letter, and since the meaning of each is purely arbitrary Hashimura had free rein. Thus, he was able to use the abbreviation "QSF?" to mean "Is my automatic transmission good?" and to make the corresponding statement read "QSF" " Your automatic transmission fades out," Of course, only God and Hashimura know what conceivable connection there may be between fading and quality of keying. Again he was able to go through the list of questions, carefully removing the word "Shall" from each and replacing it by "Must" to secure the desired Asiatic flavor. Again he was able — but why go further? Who let this silly set of definitions escape without proof reading of a capable sort? — R. S. K. 189