Radio Broadcast (May 1928-Apr 1929)

had been poled differently , the chances are that the entire amplifier would sing at some frequency with the removal of any part of the filtering. The only safe way to get a good characteristic out of good transformers is completely to isolate all a.c. circuits. The best place to do this, physically, is in the amplifier itself, for with this arrangement the amplifier circuit is entirely independent of the B supply. It will be noted that the series filter resistor must pass the plate current required for the tubes on the side furthest from the plate supply unit. Thus, in the circuit diagram, Fig. 3, the 50,000-ohm resistor must be able to carry the plate current of the detector or about 2 mA, and the 25,000-ohm resistor must pass not only the plate current of the first a.f. tube, or about 3 mA., but the current taken by the detector as well, or about 5 mA. in all. The resistors should be able to handle at least one watt. The series resistors in the grid circuit do not handle any steady current at all since there should be no current flowing in this part of the circuit. Mr. Crom, however, states that these resistors should be able to handle considerable current which is taken by the charging of the shunt condensers. It is his FRE QUENC AMEI Y CHARACTERISTICS OF tTRAN AMPLIFIER > D S J? AD f . *\ \ 1 — \ \ i / # / RAI HO BR( IADCAST ] LABORATORY • 300 500 1000 FREQUENCY IN CYCLES 3000 5000 10,000 Fig. 5 — These curves show the frequency response characteristics of the amplifier with various filter combinations experience that small grid leak type resistors frequently go bad in this kind of filter circuit within a few months after the apparatus is put into use, probably due to the momentary high currents passed through them. Noisy resistors, of course, should always be avoided as a never ending source of embarrassment and bother. The voltages which the condensers must handle are not very great, and ordinary 150 or 200-volt units of the paper-dielectric type will prove satisfactory. If the tubes are taken from their sockets while the power is turned on, however, the full voltage output of the plate supply unit will be impressed on the condensers, and if such conditions ever exist, it is well to use condensers that will stand up under the added strain. Although the filter condenser in the plate circuit of the first a.f. tube is required to stand up under a voltage of not much over 90 volts with the tube in the socket, if the tube does not draw plate current, or is taken from its socket, the voltage across the condenser may rise to the full 180 volts supplied to the power tube. List of Parts HP HE list of parts indicates what was actually used in the Laboratory. Equivalent apparatus may be used, of course. The complete list follows : Ci. C2, C3, C4 — Faradon condensers, 1-mfd., type WS3810-A; C6 Acme condenser, 4-mfd., Series A; Li Amerchoke, type 874; Ri Durham Powerohm resistor, 50.000-ohm; R2 Daven Hi-duty Glastor resistor, 0.5-megohms; R3 Aerovox Pyrohm resistor, 2000-ohm; type 992; R4 Durham Powerohm resistor, 25,000-ohm; R5 Daven Hi-duty Glastor resistor, 0.5-megohm; R6 Aerovox Pyrohm resistor, 2000-ohm; type 992; R? Electrad center-tapped resistor, 20-ohm; Ti, T2 Amertran DeLuxe Transformers; BOOK REVIEWS The B. B. C. Handbook, 1929. The British Broadcasting Corporation. Published in September, 1928. 480 pages, including advertising. The British Broadcasting Company's Handbook, issued for the second time, illustrates an underlying difference between the British and American forms of broadcasting. In the United States the large broadcasting chains content themselves with getting what newspaper publicity they can, and satisfying their Listeners and advertisers. In its Handbook, which of course is also a publicity medium, the B. B. C. assumes the role of a British institution, somewhat on the order of the House of Lords, the Boyal Institute, and the Church of England. In the advertisements which are included with the text we note that the British radio manufacturers, like their American prototypes, build perfect radio sets which will, however, be less than perfect when the next model comes out. The articles cover a wide range. There are recapitulations of important programs and a discussion of program plans for the future. Sports, the opera, educational material, and orchestral broadcasts receive considerable space. There are miscellaneous articles indirectly connected with broadcasting, such as the one on "Bands, Orchestras, and Instruments." Artists who have broadcast extensively give their views on the best technique. The fisteners are even told how to listen. The drama and religious broadcasting are not neglected. The press, poetry, and copyright limitations are other miscellaneous heads picked out at random from the text. Like the radio itself, the Handbook evidently aims to present a number of things of interest to its variegated group of patrons. The "Technical Section" contains a summary of progress in transmitter design, in which low-power transmitter modulation is favored, although not enthusiastically. The radiation pattern of the Daventry station is shown. There is quite an informative article on broadcast acoustics, including a disclosure of the "artificial echo" scheme. The output of the microphone amplifier is split, one channel going directly to the transmitter, while the other, through a re-inforcing amplifier, actuates a loud speaker in an echo room with bare walls and a consequently high period of reverberation. The sounds emitted by the loud speaker are picked up by a microphone in the room, and mixed with the straight studio output in any desired proportion. The method is an ingenious one and should give good results in selected cases where the distortion involved in repeating through a loud speaker is not objectionable. The article on microphones discloses the fact that in the British stations the movingcoil type of microphone which was standard at one time has been largely superseded by a carbon transmitter apparently of the Reisz type, in which the sound affects a layer of carbon granules through a rubber membrane, and by American condenser transmitters. Considerable material on receiver problems also appears in the technical section of the Handbook. The illustrations are interesting and some of them refute the idea which has proved so useful in American vaudeville shows, that the British generically lack a sense of humor. Carl Dreher Radio, By Elmer E. Burns, D. Van Nostrand Co., New York, 1928, 255 pages, price $2.00. The sub-title of Radio is "A Study of First Principles"; it is intended for "Schools, Evening Classes, and Home Study." The author is an instructor in physics in the Chicago high schools. The treatment, probably as a result, is more thorough in setting forth the theoretical basis of the art than most elementary books, and there is correspondingly less "How to Make" material. Graphs are used liberally but there is some application of simple mathematics, and here and there a vector diagram. At the same time Mr. Burns' text never becomes abstract or merely verbal. His first chapter, in fact, plunges abruptly into "Simple Receiving Circuits," without the preliminary wooing of the principles of electricity to which we have become accustomed. However, the author then retraces his steps, starting with electric batteries, and going through electric circuits and Ohm's Law, electron tubes, alternating currents, detectors and amplifiers, fundamentals of receiving circuits, oscillators and transmitting circuits, and radio measurements. In an appendix some of the common mathematical relations are brought together, and there are lists of graphic, mathematical, and code symbols . . An index is provided. Radio — "First Principles" provides an excellent course of study for students with high-school preparation in physics and mathematics. It might serve as an effective introduction to an advanced text like Morecroft's, for, in its restricted sphere, it shows some of the same care in preparation and choice of material. Carl Dreher. • february, 1929 page 252 O