Radio Broadcast (May 1928-Apr 1929)

A CABINET MODEL OF THE SHORT-WAVE RECEIVER The short-wave set shown above uses the same parts and employs the same circuit as the receiver described in this article. It differs only in the aluminum panel and shielding used; these are obtainable through Silver-Marshall, Inc., or may be home constructed A Screen-Grid Short -Wave Receiver THE tremendous interest in short-wave reception which has been sweeping the country of late has brought about a great :hange in the requirements of a good short-wave receiver. The old-style short-wave receiver of the "ham" days, while still as sensitive as ever for c.w. work, has proved inadequate for receiving the modulated signals of telephonic broadcasts with the smoothness and quality of the reception obtainable in the higher broadcast bands. The short-wave receiver described in this article provides this higher degree of performance, plus freedom from radiation, for the oscillating detector is isolated from the antenna by a screen-grid r.f. amplifier tube. This tube does not add a tuning control, its input circuit being untuned, yet it improves the reception of telephone signals, and entirely eliminates "dead spots" at which the set will not oscillate, since it effectively isolates the antenna from the sensitive detector circuit. An unusual degree of smoothness of regeneration control, freedom from "putting" and "fringe effect" noises as the set goes into oscillation is effected by careful circuit and coil design, notably by using a small coil, which on the lower waves, particularly around 20 meters and below, provides smoother and sweeter control than the two three and even four-inch short-wave coils generally used. The coils are actually a refinement of the popular "tube base" or "Scottish" idea which has been found to give such excellent and economical results. These forms are slightly larger and longer than the average tube base, making it possible to design more efficient coils than are possible on the or By Howard Barclay dinary tube base (often not available except at the expense of breaking good tubes). A winding space 15" long and 15" in diameter is available, with a tickler slot \" deep and tV wide at the filament end. On the bottom of the moulded THE short-wave receiver described in this article employs a screen-grid tube as an r. f. amplifier and incorporates an audio system that amplifies high quality signals without distortion. It was designed in the Research Laboratories of Chicago for Silver-Marshall, Inc., and uses a new and ingenious type of coil wound on a form that fits into a five-prong or Y type of tube socket. In the Laboratory the receiver seemed remarkably free from the noises which often ruin short-wave reception, and on the sometimes swinging, sometimes steady signals of 5SW in England we received good dance programs from the Savoy Hotel in London. — The Editor. form are five hollow lead pins, properly positioned to fit any five-prong a. c. tube socket. These coil forms are so cheap that any number of experimental coils for different wave bands can be wound at little cost, to be tuned by any size of condenser that may suit the builder's fancy. In this matter of "builder's fancy," however, it is well to remark incidentally that while a code receiver can be thrown together almost any old way and still work, physical placement of parts and wiring details must be most rigidly watched in order to get a good modulated signal receiver. In the set described, the tickler con denser hardly reacts at all on tuning, over 20 degrees at 40 meters being needed to tune a c. w. code signal out of "readable" audibility. CONSTRUCTION OF THE RECEIVER THE set illustrated is mounted on an 8 x 18 x \" seasoned wood baseboard, with all parts placed in a simple straight line as shown in Fig. 1, instead of being tied up in a knot (as in broadcast band receivers) difficult of assembly and "trouble shooting." In Fig. 1, at the left is the antenna chpke coil Li, next the screen grid r.f. tube socket, then the five-prong coil socket up on \" studs, next the grid condenser, Ci, and grid leak mounting, detector tube socket, plate r.f. choke, L3, and the two audio tube sockets, with a pair of flat characteristic 3:1 transformers, T, behind the a.f. tubes. At the rear are the Fahnestock connection clips, and on the front panel are the .00014-mfd. tuning condenser, C4, the 20-ohm detector filament rheostat, R2, and the .00035 tickler condenser, C5. The circuit diagram, giving the proper connections, is shown in Fig. 2. The matter of a good short-wave variable condenser is an interesting one, for few good broadcast condensers, even of properly reduced capacity, are good at 20 meters and below, where bearing noises develop to an annoying degree. A noisy broadcast type of condenser can often be quieted for short-wave work by insulating its bearings, at increased cost and labor. However, the type of compression bearing found in the General Radio and Silver-Marshal condensers is quiet at 20 meters, and offers all the advantages of a good mechanical bearing of 203