Radio Broadcast (May 1928-Apr 1929)

A Six-Tiibe Screen-Grid Receiver TT'HE receiver described here is one of the ■*■ new models in the line of kits to he narketed this season by Silver-Marshall. It is in excellent and attractive receiver and the essentials are priced to make the eyes of the borne constructor and professional set builder ipen wide. The complete receiver was tested n the Laboratory and it performed very well ndeed "in all departments," as the sporting vriters say. The "gain" of this set is high, vhich is advantageous when the user wants disance. It should be remembered, however, that 10 high-gain receiver can successfully be operited where there is much man-made or other nterference. It is simple enough in conditions ike that to cut down the gain and to tune to tations where signal level is above that of the wise. Additional constructional data is availible from the manufacturers or from Radio Broadcast. — The Editor. THE new Screen-Grid Six receiver is a highly perfected development of the receiver known to readers of Radio Broad:ast for three years as the Silver Six, Shielded >ix, and Shielded-Grid Six. The new Screen-Grid jix takes full advantage of all meritorious feaures of its predecessors, and, like the latest of hem, it employs three stages of high-gain r.f. Lmplification with screen-grid tubes. In performance, the receiver will give 10 to 15 ic. selectivity, in almost any location, and vill bring in from forty to one hundred stations n a single average evening's tuning. Such are the esults that have been had with numerous models )uilt to the new design operated in many differ:nt locations in and about Chicago during the :arly months of 1928. And yet it can be built it home for less than seventy-three dollars! As an example, the log on this page was obained in two hours' time in a typical residential ocation within a few miles of Chicago's twenty)dd local stations. All the stations listed were eceived on the loud speaker with practically 'local" volume, using only a 35' antenna. Since the Screen-Grid Six design would ;eem to represent not only a new high level )f radio receiver performance, but also a new low evel in cost, almost regardless of performance, t is felt that a description of the engineering eatures of the design, together with other points )f interest, will not be unwelcome to Radio Broadcast readers. From an examination of the photographs and -I L J 1 J i i 1 1 — si WO £00 700 800 900 1000 1100 1200 1300 1400 1500 FREQUENCY-KILOCYCLES FIG. I By McMURDO SILVER Silver-Marshall, Inc. drawings it is seen that the Screen-Grid Six is a six tube t.r.f. receiver consisting of a three-stage radio-frequency amplifier using screen-grid tubes, followed by a detector and two-stage audio amplifier. In this respect it is not at all unusual (though at this writing there are known no ready-made sets incorporating the full advantages of screen-grid r.f. amplification.) If there is anything at all unusual about the set, it is the fact that the three r.f. tubes have been made to average about two hundred and fifty times the amplification obtained from this same number of 201 a tubes in some receivers using an r.f. choke in the input. In addition, the audio gain is greater than that of the ordinary audio system of the same number of stages, due to the employment of the Clough audio system. How this is done is easily seen by taking as a typical example a representative ready-made six-tube set costing about one hundred and fifty dollars. In this common type of set the first r.f. tube has its grid circuit connected across a small r.f. choke directly in the antenna circuit, across which is developed the signal voltage. This type of coupling gives no voltage step-up between antenna and first r.f. tube, and does not contribute in any way to selectivity. The first tube with the t.r.f. transformer that follows it shows an average amplification of less than 10 times between 200 and 550 meters (this figure is quite generous). The two additional r.f. tubes, with two more t.r.f. transformers, each give a gain of 10 per stage. Thus, 10x10x10= 1000 shows the amplification between antenna and detector grid circuit. LOG OF SCREEN-GRID SIX Station Location Right Dial' Kc. WNBA Forest Park, III. . 1 1 140 KSTP St. Paul, Minn. . . . 4 1360 WJKS Gary. Ind 6 1290 WGES 8 1240 WOK 12 1190 WJAZ Chicago 19 1140 WTAS Elgin, 111 18.5 1090 WENR Chicago 23 1040 WTMJ Milwaukee .... 24 1020 KMOX St. Louis, Mo. 26 10IH) WHT Chicago 28.5 980 KDKA Pittsburgh 31.5 950 KFAB Lincoln, Nebr. 32.5 940 KOA Denver, Colo. 34.5 920 KFQB Ft. Worth, Texas . 36 900 WSM Nashville, Tenn. . 37 890 WLS Chicago 42 870 WEBH Chicago 47 820 WDAF Kansas City .... 47.5 810 woe Davenport . . . 50 800 WGY Schenectady .... 51 790 WBBM Chicago 54 770 KWKH Shreveport, La. 55 760 WTAM Cleveland 57 750 wcco Minneapolis .... 59.5 740 WGN 62 720 WOR Newark, N. J. ... 63.5 710 WLW Cincinnati .... 65 700 WQJ Chicago 71 670 WJZ New York .... 73 660 KRLD Dallas, Texas . . . . 75 650 IKFI Los Angeles .... 77 640 1WSB 79 630 WCFL 82 620 WEAF New York . . 84.5 610 KTHS Hot Springs .... 86.5 600 WOW Omaha, Nebr 89 590 WFLA Clearwater, Fla. 92 580 KYW Chicago 95 570 WHO Des Moines .... 98 560 KSD St. Louis, Mo. 100 550 * Reading of left dial is not given, since it varies a few degrees for different antenna hngths. Both dials "track" very closely. 28l For the Screen-Grid Six a tuned antenna input circuit was designed having the very best possible characteristics which could be attained in practice. This circuit consisted in its final form of a very low resistance coil consisting of 89 turns of No. 20 enameled wire upon a threaded bakelite form 3j" long and 2§" in diameter. A tap on this coil about 40 turns from the filament end is used when the set is to operate with a small antenna; when a long antenna is used or greater selectivity is required the antenna is connected to this tap through a 75-mmfd. midget condenser. Any student familiar with average coil resistance will realize that the values of 3.3 ohms at 550 meters and 11.5 ohms at 200 meters obtained with the coil, tuned by a 0.00035-mfd. condenser, represent an unusually good circuit (the coil itself has a "figure of merit" practically double that of the best coils on the open market). This input coil, Li in Fig. 7, is tuned by a single condenser, Ci, actuated by the left-hand drum, Di. In a test made to determine the characteristics of the antenna circuit, a representative antenna of 400 mmfd. capacity, 25 ohms resistance and 28 microhenries inductance was coupled to the input coil first through a small primary coil of 20 turns, and then through a large primary of 35 turns in series with a 75-mmfd. midget condenser, which was employed to regulate selectivity. Curve A of Fig. 1 shows that the voltage step-up provided by this tuned antenna input circuit varies from 64 at 200 meters (1500 kc.) to 28 at the middle of the broadcast band and rises to 100 at 550 meters (545 kc.)! And 545 kc. is the point at which the greatest step-up is always needed, for the amplification of any practical r.f. amplifier always falls off at long waves, as will be shown. The shape of this curve is not wholly ideal, but there is certainly no comparison between the voltage amplification that can be had from this circuit, with its one additional tuning dial, as compared to curve B of Fig 1. (Curve B represents the voltage amplification of a good untuned antenna coupling choke, and is hardly distinguishable from the base line of the curve of Fig. 1.) The dip in the center of curve A shifts with different size antennas and may further be shifted by adjustments of the midget antenna coupling condenser, C5, so that it is seldom necessary to operate the receiver with as low a voltage step-up in the antenna coupler as is shown by the lower bend of curve A. THE R.F. AMPLIFIER TP HE antenna input circuit is followed by * three identical tuned circuits, each housed in individual copper shielding cans, SHi, SH2, SH3. These circuits employ small plug-in inductances, L2, L3, L4, the secondaries of which consist of 985 turns of No. 29 enameled wire wound upon a threaded moulded bakelite form i\" in diameter FREQUEHCTKILOCVCLES FIG. 2