Radio Broadcast (May 1927-Apr 1928)

JANUARY, 1928 "STRAYS" FROM THE LABORATORY 229 Differentiating this expression with respect to N and solving for a maximum, Rp And u.tfpL ^RpRs whence power in Rs = If Rs = ^Eg2 8 Rp If the speaker Rs is in the plate circuit of the power tube without the output transformer, and if the impedance Rs equals Rp the power will be as indicated above, showing that the transformer is indeed an impedance adjusting device and that it is possible to secure maximum power in a load impedance which is not equal to the tube impedance provided the output transformer has the proper turn ratio. A little consideration will show that if the primary impedance of this transformer is equal to the tube impedance and if the secondary impedance is equal to — "matches," is the usual word — the load impedance one half the voltage existing in the plate circuit will be expended in heating the plate. It is obvious, then, that the primary impedance again should be several times that of the tube impedance, but since the final tube has a low impedance, the ux-171 (cx371) type for example has an impedance of 2000 ohms, a manufacturer of transformers needs to put nothing like the impedance in his output transformer primary that he puts into an input transformer. Likewise the secondary impedance must be high compared with the load, or speaker. As long as the turns ratio is proper, energy will be transferred from the tube to the speaker with a minimum loss, and for all practical purposes the output transformer may be neglected in calculations. Some of Radio Broadcast's High-Towered readers are technically in'Bunk clined, and must enjoy the monthly battle of wits among radio advertising writers whose "eulogies" appear in many radio publications. We saw recently a new loud speaker advertised which consisted of a "tone column of new design; by means of a scientifically designed tone distributing chamber a forced crossing of sound waves is accomplished, and a divisional tone chamber of unique design segregates high and low tones, reproducing both with equal facility." Bell Laboratories engineers should take notice of this new scheme for attaining perfect fidelity, and should forget all about loud speakers which will look like a pure resistance to a power tube at all frequencies. All that is necessary is to segregate the low voice from the high one and then re-combine them. Presto! There is also a new antenna that promises much, for, according to a widely circulated advertisement, it meets the need of radio users, whether for small or large sets. It has been thoroughly tested for over two years under all kinds of conditions and on many kinds of sets. In every set, it has proved its many advantages: 1. Easy to put up or take down. 2. Picks up waves in any direction because it is round. 3. Proven more selective. 4. Greater volume. 5. Greater distance. 6. Helps eliminate static. 7. Small and compact. 8. Neat in appearance. 9. Low in cost — only $7.50. What more could anyone want? This kind of bunk is not only confined to the advertising pages of the media in question, sadly to recount. The following passages are quoted from a recent article describing a new and revolutionary receiver that anyone can build for about twice as much as he would have to pay for a well known and thoroughly engineered set. "Its volume is due to the high amplification in the audio end of the circuit and to the fact that two 71 tubes in push-pull are used in the last stage." Off hand this seems reasonable until one considers that there are three audio tubes — which furnish most of the amplification — and that there are eight tubes which precede them! It reminds us of the early days of radio when the Laboratory had its hands full weeding out the good apparatus from the poor. A receiver came to Garden City equipped with thirteen tubes, guaranteed to pick up any signal in any part of the world at any time. The total plate current of this super-receiver, including a power tube, was nine milliamperes. It seemed incredible — and was, until we discovered that eight of the thirteen tubes had no plate voltage on them. But let us continue with this totally new receiver. "With all that amplification and power handling capacity there will be undistorted volume enough to make the welkin ring. But all of this would be merely potential volume were it not for the almost incomprehensible amplification in the intermediate amplifier. It is here where the weak signals from the remote stations are pulled from infinitesmal levels and placed on the plane of the signals from local stations. Both the amplification and selectivity could be expressed in numbers but they would be so large as to be meaningless to the human intelligence." There you are, and five more columns of it for good measure! THE NEW SCREENED GRID TUBE The bulb of a "dead" ux-222 tube was broken so that this photograph of the interior construction might be taken. The Radio Broadcast Laboratory is expending considerable time experimenting with these tubes so that the information contained in articles in these pages might be backed up with actual experience. We can promise readers some particularly fine articles along such lines in the very near future During recent months, the <j^ew Laboratory has received for cApparatus test the following apparatus: Power units from Kellogg, Wise McCIung, Valley, Universal, Briggs and Stratton, Boutin Electric Co., Sterling Mfg. Co., and Grigsby Grunow; tubes from the following tube plants, Arcturus, R. C. A., Cunningham, Supertron, CeCo, Manhattan Electric Supply Co., Connewey Laboratories, Zetka, Cable Supply Co., Televocal, Van Home, DeForest, and Supercraft; audio transformers from Modern, Silver-Marshall, Samson, Tyrman, Sangamo, Amertran, G. W. Walker; the new Eby sockets, a Muter double-impedance amplifier, a Pacent Phonovox electrical pick-up unit, the excellent looking Abbey receiver manufactured by Splitdorf and already illustrated in Radio Broadcast, a useful floor cord made by Belden which enables one to place wires under a rug without danger of tripping or of impairing the appearance of the room — the wires may carry house lighting current for a lamp, or loud speaker wires; a complete assortment of Acme Wire Co. Parvolt condensers; rheostats, etc. from Carter; a complete push-pull amplifier and B supply units from Samson, Thordarson, and General Radio; resistances for stabilizing grid circuits, for plate supply circuits, for center taps on a.c. tube circuits, etc., from General Radio, Electrad, Frost, Daven, Amsco, Gardner and Hepburn, Aerovox; and coils from Precision Coil Co. Many other interesting pieces of apparatus have been received, previous mention of which has been precluded pending test and on account of space limitations. Among the larger items are Silver-Marshall's interesting Time Receiver, a huge and much involved horn loud speaker from Newcomb-Hawley, using a Baldwin Unit, and known as a Console Grand Reproducer, a Peerless loud speaker from the United Radio Corporation of Rochester, and a Holmes Piano loud speaker from the International Radio Corporation, of Los Angeles. The Newcomb-Hawley loud speaker has a very long air column secured by giving the neck several convolutions about the wide opening. The Peerless is an attractive loud speaker with an excellent element. Both loud speakers cover wide frequency ranges and go down particularly well. The Laboratory was especially interested to ' receive one of the "tuned" impedance amplifiers about which so much is heard, this one delivered by Mr. Kenneth Harkness in person, giving us the opportunity of meeting him for the first time. Frequency curves are being prepared on this amplifier, and will be ready soon. Transformers for the new a.c. tubes were received from Amertran, General Radio, Mayolian, and Northern Manufacturing Company. New rheostats came in from Frost, Centralab, and Carter. Some large heavy-duty resistors from the Electro-Motive Engineering Corporation also made their appearance. Among the other items are new Samson apparatus, loud speaker units from the Balsa Wood Corporation, Engineers Service Company, Baldwin, Vitalitone, and Magnaphon Company, a series of new and probably very efficient inductances made by the Precision Coil Company, a "Subantenna" (is there any way of testing this antenna without digging up the garden?), two sheets of beautifully burnished copper for shielding, from C. G. Hussey & Company, of Pittsburgh, condensers from X-L Radio Laboratories, Electrad apparatus and a fine looking White socket power unit from Sioux City, Iowa.