Radio Broadcast (Nov 1926-Apr 1927)

36 RADIO BROADCAST NOVEMBER, 1926 1 1 KING CHART LAB. RECEIVE N Tt R.B. •s. — FIG. I Both dials of the "Lab" Receiver read alike; should they both read approximately 50 for a certain station, you may be sure that the station is transmitting on about 1000 kc. (300 meters) Choke coils are advantageously employed in both tuned circuits. In the secondary circuit of the antenna stage, an 85 millihenry Samson choke is inserted in series with the lead connected to the center tap leading to the C battery. It is used to prevent oscillation of the circuit at the extremely high frequencies around 3750 kc. (about 80 meters). In the detector stage, a similar choke is employed to prevent the radio frequency currents from passing on through the primary of the first audio frequency transformer and through the B battery to the ground. These currents are more useful when passing through the small Precise variable condeners to cause regeneration. Another feature — purely a mechanical one — adding to the factory-like appearance of the finished receiver, is in the use of the new Marco illuminated dials. On the front of the panel a small pie-like segment of a bezel is mounted forming the window through which may be seen the white celluloid dial-piece. Behind the dial is located a small lamp which, when lighted by the thumb switch — a part of the dial proper and located above the bezel — illuminates the figures and scale markings so that one may tune the circuit very accurately. The full volume output of the audio amplifier may be diminished by merely turning the knob on a variable resistance unit which shunts the secondary of the first audio transformer. OUTPUT DEVICE EMPLOYED AS IS the custom with the many modern types of receivers, a choke coil and condenser are arranged in the output circuit so that the windings of the loud speaker may be operated free from the excessive drag exerted by the d. c. component of the high B potential on the plate of the last tube. Also, better quality of tone is obtained with this system due to the fact that a Samson tapped output coil is employed, making possible the adjusting of the impedance of the coil to approximately match the particularcharacteristics of the loud speaker used. The condenser employed in this combination must possess especial qualifications, vil-, it must be fairly large in capacity — 4 mfds. — but not large physically. Furthermore, it must be capable of withstanding approximately 350 volts. The Tobe-Deutschmann 4mfd. bypass condenser is suitable for this purpose, and is employed in the receiver described. Wiring throughout the receiver is accomplished with the use of Belden hook-up wire. This wire, consisting of a number of tinned twisted strands of fine copper wire, is well insulated with a rubber covering. It is obtainable in the following colors: Red, blue, green, and gray or natural. The use of this wire greatly simplifies the connecting of the various units of the receiver, and permits the battery leads to be twisted and denser of 50 mmfd. The Precise is very satisfactory for this work and is used here. T! A, B, and C cabled. So that a power tube may be used in the last audio stage, the filament wiring to this socket includes a separate ^-ampere filament ballast, "pVER since Keith Henney's article in Radio Broadcast for June ' describing the fundamentals of the R. B. "Lab" Circuit appeared, a considerable stream of correspondence has come into the office asking when a constructional article would be printed on the circuit. The "Lab" Circuit is not a new circuit in the sense that it is revolutionary; such circuits do not exist. The circuit is not easy to build; it cannot be "thrown" together, but this complete article by John B. Brennan tells how to build a model embodying many features of decided interest to the home constructor who desires to put together a set which uses many circuit refinements brought forward by the manufacturers for the 1927 season. The constructor who builds this set will have a receiver of neat appearance, great sensitivity and selectivity, and one which delivers a signal of high quality — all with four tubes. Development work on the circuit was done over a period of several years, by Keith Henney, director of the Radio Broadcast Laboratory. We prophesy that this circuit will enjoy a wide popularity.— The Editor. LAYOUT IMPORTANT O CONSTRUCT a receiver similar to the one described, detailed layouts, wiring diagrams, and explanatory sketches are furnished so that there need be no great difficulty in this respect. Only when actual duplication of the layout and wiring of the receiver as herein described is attempted, can successful operation be assured.' This point cannot be stressed too greatly. Mr. Henney, in his previous articles, dealt at length with the advisability of placing parts and wiring correctly to prevent objectionable feedback effects. The use of Airgap sockets, which introduce a minimum of grid-plate capacity, aids in reducing this feedback. The receiver described in this article is the result of much experiment, and it is doubtful whether with the apparatus employed a more successful arrangement can be obtained. It is for this reason that constructors are urged to follow closely the design as given. To begin actual construction of the receiver, the following parts are required: such as a Brachstat. The other three tubes are connected with their filaments in parallel and controlled by a single f-ampere filament ballast. Regeneration in the detector circuit is accomplished by means of a small variable con 3* > No. 28 Holes v B No. 18 ^Hole 1 & Y Drilled & tapped for 6-32 Screw 2 Cardwell 0.0003 5-mfd. Variable Condensers,type 169E. 2 "Lab" Circuit Tuning Coils, General Winding. 2 Marco Illuminated Dials. 4 Sockets, Airgap. 2 Amertran Audio Frequency Transformers, 1st and 2nd stages. 1 XL Neutralizing Condenser, type N. 2 Samson Choke Coils — 85 millihenries. 1 Samson Output Impedance, type O. 1 Tobe-Deutschmann Bypass Condenser, l-mfd. 1 Tobe-Deutschmann Output Condenser, 4-mfd. 1 Electrad Royalty Variable Resistance, 500,000 ohms. 1 Electrad Filament Switch. 1 Electrad Grid Condenser, 0.00025-mfd. 1 Electrad Fused Metallic Grid Leak, 4 megohms. 2 Brachstats, 5-ampere and £-ampere. 2 Frost Pin Jacks. 1 Precise Microdenser, type 940, 50mmf. 9 X L Binding Posts. 1 Radion Binding Post Strip. 1 Panel 7 x 21 x T3S inches, Formica. 1 5-wire fused Belden battery cable cord. 1 Fritz Cabinet. -8 turns No. 22 D. S. C. I D c D U 1%"- ■< 1%" > 1 77 turns, 77 turns j No. 22 D. S C. III No. 22 D. S. C. F-1 KH rr=n 2% All No. 18 Holes COIL ASSEMBLY FIG. 2 Data for the construction of the coils and supports are given here. Reference to the text on page 37 and Fig. 3, will simplify the making of the coil units Having obtained the necessary parts, we now lay out and drill the panel in accordance with the panel sketch, Fig. 4. Each Marco dial is furnished with a steel template which enables the builder to drill the holes in the panel necessary to mount the window. Now cut and prepare a base board, details of which are shown in Fig. 11. Brackets, to fasten the panel to the base board and the C batteries to the rear wall of the cabinet, are shown in Fig. 10. Brass strip, i-inch wide and tV-inch thick is used for this purpose. Also in this diagram are shown the details necessary to prepare the binding post terminal strip. The next thing to do is to fasten