Radio Broadcast (May 1928-Apr 1929)

SIMPLICITY IS THE KEYNOTE OF THIS SUPER-HETERODYNE A "One-Spot5 By' W. H. HOLLISTER >uper FOR many years past the writer has been an ardent radio fan industriously building every new circuit which has been devised in the search for better and still better radio reception. Literally, every type of manufactured kit and every new circuit w,hich has appeared from time to time has been built and tested, but no standard circuit ever quite came up to expectations of what radio reception should be. The author's expectations were certainly not unusual and are probably shared by the majority of radio fans, who have believed that a good radio set should give absolutely faithful reproduction of all musical frequencies, great enough amplification to bring in any station louder than local noise and play it on the loud speaker, and sufficient selectivity to cut consistently within 10 kilocycles of all normally interfering local stations. A qualifying requirement would be that the receiver satisfying these expectations must be easy to build, dependable in its performance, simple to tune, and low in initial cost and upkeep. Early in the search for such a set, attention was attracted to the superheterodyne system, which theoretically seemed to provide opportunity for high amplification and extreme selectivity without seriously impairing tone quality, and, at the same time, to provide a receiver simple of construction. Super-heterodynes without number were built and tried with many circuit variations tested in each receiver. One quite satisfactory set was evolved which was used by the writer in his home for several years. It employed a 50-kiIocycle intermediate amplifier and with it Los Angeles and other Pacific Coast stations could be tuned-in almost any evening in the week with clocklike regularity and with volume equal to local stations. As time went on and more broadcasting stations came on the air, the repeat points upon the oscillator dial of this receiver made its selectivity less and less satisfactory, and the writer was forced to resume the search for a better receiver. At just about this time, the screen-grid tubes were made available, and birth was given to the hope of even greater amplification than was had with the old standby set using 201A tubes. Because many of the writer's friends have been so favourably impressed with the performance of the final receiver incorporating these new tubes, he has been led to prepare this article describing the processes by which it was developed, and telling how an exactly similar set may be constructed by the fan at home. As previously stated, the goal of this receiver was to be 10-kilocycle selectivity against any local station (and within a few miles of the writer's home in Chicago there are some twenty powerful broadcasting stations operating every ggfr THE present article by Mr. Hollister describes the first really "one-spot" super-heterodyne which has appeared in Radio Broadcast. Heretofore, the "bug" limiting the usefulness of most "supers" has been the upper and lower tuning points on the oscillator. Often, when receiving a station with the oscillator tuned to one of these points, the user would find interference from another station which would beat with the oscillator to produce perhaps a frequency similar to the intermediate frequency. The set described here is really "one-spot." It employs eight tubes, three of which are screen-grid tubes in the intermediate amplifier, which is designed to work at about 350 kc. (S55 meters). Volume is satisfactorily controlled by a potentiometer which varies the voltage on the screens of the 222 type tubes. The high intermediate frequency is chosen to prevent double tuning points. The i.f. transformers are equipped with small midget condensers so that each may be adjusted by the user to the same frequency. In testing this set in the Laboratory it was found easy to do this. With all the transformers accurately tuned, the amplifier oscillates, which can be regulated by the volume control. In the Laboratory test, fidelity seemed to be improved when the i.f. transformers were slightly detuned, with, of course, a certain decrease in sensitivity. The cost of parts is about $go. Constructional details in leaflet form are available from the manufacturer of the intermediate transformers and requests should be addressed to Radio Broadcast. — The Editor. evening), the finest possible tone quality which could be provided consistent with 10-kc. selectivity, sufficient sensitivity to give a wide range of program choice, both local and distant; and all this to be had with absolute dependability, simplicity of control, and low initial and upkeep cost. The answer to all these requirements finally took shape in the form of an eight-tube super-heterodyne receiver having a screen-grid first detector, a conventional oscillator, three individually tuned 400-kilocycle "one-spot" intermediate amplifier stages, a second detector, and two audio stages using the new Clough system of audio amplification. The new superheterodyne is pictured in the accompanying photographs and diagrams. DESIGN FEATURES A T THE outset the writer endeavored to use standard parts available on the open market for the construction of his receiver, but it soon became apparent that the requirements laid down for its performance could not be met with standard parts then available and it became necessary, to enlist commercial aid in the development of special transformers for the receiver. The balance of the parts in the author's set are standard and may be easily procured upon the open market. As work upon the set progressed, it was given tests night after night and the results obtained averaged over a space of months. In final form, as pictured herewith, the eight-tube receiver operated satisfactorily in the steel frame office building where it was built and would regularly bring in stations within a thousand to fifteen hundred mile radius on the loud speaker, using a 15-foot wire strung up in the author's office, for an outside antenna could not be had. Stations such as wsn, only 10 kilocycles away from local wls; wjz, 10 kilocycles away from local wmaq, only two miles away; kmox and wdaf, 10 kilocycles away from local webh; weaf. 10 kilocycles away from local wcfl; and kwkh, 10 kilocycles away from wbb.m, 249