Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST. sistor piaced across the filament terminals or a center-tapped transformer should be used for plate and grid-circuit return. The grid swing (50 volts) required to work this tube at its maximum output is easily obtained with sets of present-day sensitivity although when used in "pushpull " circuits the signal input requirement is doubled and this arrangement will call for a set having high over-all gain. When increased power output is desired without increasing the plate voltage supply or grid swing capabilities of the receiver, two ux-245 tubes may be operated in parallel. The grid-bias voltage for the ux-245 should preferably be obtained by means of a voltage drop across a resistor of proper value placed in the platereturn lead. This is particularly true in resistance-coupled circuits where grid leaks are employed. This " self -biasing " arrangement automatically prevents overloading of the plate and takes care of normal line volt age variation and variations among tubes. When resistance coupling and grid leaks are used it is imperative that the self-biasing connection be used. The grid leak should be made not greater than one megohm so that the grid bias will not be materially reduced in the event that current should flow in the grid circuit. The plate voltage requirements of the ux-245 are such that the maximum potential (250 volts) may be obtained easily from the type ux-280 rectifier tube. However, the problem of plate voltage supply becomes more involved when two ux-245 tubes are to be supplied in addition to the excitation current for a dynamic loud speaker field and the plate power for the amplifier and detector tubes of the receiver. This condition of use is quite general in this year's receivers and the circuits shown in Fig. 1 will be of interest as they show several methods of obtaining all voltages and currents required without exceeding the rating of the ux-280. This is of particular interest because the use of the ux-280 to supply the plate voltage of a receiver means reduced cost of power pack and reduction in the cost of the receiver. The rating of ux-280 is as follows: A.C. volts per anode (r.m.s.) 350 (max.) D.C. output current 125 mA. (max.) This gives approximately 350 volts at 125 mA. (d.c.) Because of the many types of dynamic loud speakers in use no values have been assigned to the various components shown in Fig. 1 since the resistance of the loud 20 40 60 80 1C0 120 140 160 180 200220 240 260 280 300 320 340360 380 400 PLATE VOLTAGE Fig. 2 — The undistorted power output available from a UX-245 tube under various operating conditions may be determined easily from the above family of plate current-grid voltage curves. full voltage of the rectifier and a considerable voltage drop at a moderate value of rectified current is permissible in supplying the rest of the set. This would be in case the ux-245, either singly or in push pull, is used as the output tube and either ux-226 or uy-227 tubes are used in the rest of the set. Circuit c is of advantage where the output power tube requires approximately the full voltage output of the rectifier and only a low voltage drop at a low current is permissible in supplying the rest of the set. Circuit d„ shows a series reactor-type filter circuit for use under circuit conditions similar to those of circuit b. In this connection, the series reactor decreases the instantaneous peak current through the rectifier and thus permits that tube to work under more advantageous conditions. However, in order to get the full value of rectified current from this arrangement, the voltage on the tube must be increased beyond the present maximum value of 350 volts, and this is not recommended. The arrangement of loud speaker field as shown in circuit c may be used also in connection with circuit d. Conclusions I speaker winding and its power consumption determine their values. The following general discussion of the circuits will serve to illustrate the fundamental practice; the set designer can easily fit into his design the constants and select circuit best adapted to this problem. Four Power Supply Circuits Circuit a is of use where a considerable voltage drop at a fairly high value of current is permissible; for instance, where the ux-171a, either singly or in push pull, is used as the output tube and ux-226's or uy-227's are used in the rest of the set. Circuit b is of value where the output power tube requires approximately the N connection with a discussion of plate voltage supply for the ux-245 it might be well to point out that high plate voltage does not of itself produce appreciably higher volume. It does permit greater volume without distortion provided sufficient signal is available to swing the grid with maximum efficiency. In sets which do not have sufficient overall gain to swing the grid of the ux-245 to its full value (50 volts) it is good practice to reduce the plate voltage, as conservative operation of this tube, as well as other types, considerably prolongs its life. Under all recommended conditions of use a transformer or choke and output condenser should be employed to couple this tube to the loud speaker in order to prevent the plate current of the ux-245 from flowing through the windings of the loud speaker. BOOK REVIEWS The Radio Amateur's Handbook, The American Radio Relay League, Hartford. Conn., 4th Edition, 200 pages, paper covers. Price: $1.00. Like its predecessors, the fourth edition of The Radio Amateur's Handbook makes no attempt to cover the amateur radio field but confines itself rigidly to telegraphic amateur radio with much stress on message-handling. The compact first chapter, "What is an Amateur," has been replaced by a longer one on "Amateur Radio." The second chapter remains, as before, a very good introduction to code-learning. The quite exceptional Chapters 3 and 4 by F. E. Handy have been retained from the original text. They cover the troubled questions of "Fundamentals" and "How Radio Signals are Sent and Received" in a manner that is unusually clear and quite free from the usual semi-correct concessions to brevity. Chapter 5 on "The Receiver" has chosen to drop the design information of earlier editions and to substitute an array of "how-to-build-it" discussions. Although they are to be classified as amateur-band covering, only two of the designs include the 160-meter band. The three-tube receiver by Westman is unusually well designed and may well serve as a model. This reviewer must of necessity approve of Chapter 6 since it is written on the text of his own preachings to the amateur transmitter these past years. By this is meant the proper attention to adjustment of transmitters and the use of circuits with a high ratio of C/LR. The chapter presents an array of constructions which crowds out the very good coil-design information of the earlier editions. Chapter 7 on "Power Supply, Keying, and Interference Elimination" is sound and adequate. Transformer design, rectifier construction, and battery plate supply are discussed. The classic Dellenbaugh filter information has properly been retained. Chapter 8 does all for transmitting an tennas that one may reasonably expect in 10 pages. The 160-meter band again is somewhat lightly touched upon. Chapter 9 undertakes the impossible by attacking at once "The Frequency Meter" and "Radio Measurements." The discussion of frequency meters and monitoring oscillators is capable but the designs fail to reach the 160-meter band. "Radio Measurements" cannot be treated in 5 pages without references. Chapters 10 and 11 relate to the mechanism and manners of the message-handling game which is exhaustively discussed in the 26 pages covered by these chapters. In recognition of the manifest inadequacy of the space Chapter 12 on "The Experimenter" makes no effort at detail but contents itself with a one-page outline and a reference to some prospective activities. The appendix contains additional information on coded phrases, distance computations, a few formulas, and a good, though short, bibliography. — B. S. K. 168 • • JULY • 1 9 2 9 •