Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST none of the r.f. currents in the plate circuit of the detector tube are permitted to pass into the audio amplifier. For this reason there is included in the plate circuit of the detector the r.f. choke coil, L6, and the small 0.0002mfd. by-pass condenser, C5. These two units comprise a filter system which causes the r.f. currents to pass directly back to the filament of the detector tube but which does not prevent any of the audio-frequency currents from passing into the audio amplifier. The receiver contains two tuned circuits, L2Ci and L4C2. Were it not for the fact that regeneration was incorporated in the detector these two tuned circuits woidd not give sufficient selectivity. By means of the regeneration control, however, it is possible, when necessary, to bring up the selectivity to a point where satisfactory discrimination between different stations is obtained readily. The various taps are placed on the antenna coil, Li, so that when the set is first placed in operation reception may be checked with the antenna connected to the different taps and the lead can be soldered finally to that tap giving the most satisfactory combination of sensitivity and selectivity. The volume control consists of a variable resistor, R6, connected in the filament circuit of the r.f. tube. Output Arrangement HPHIS receiver has been operated in the JL Laboratory with a il2A-type output tube feeding into a good moving-coil loud speaker, and excellent quality was obtained at moderate volume. If more volume is desired a 171atype tube may be used with 135 volts on the plate and a 27-volt C bias. Under these conditions the latter tube will deliver about 330 milliwatts of undistorted power as compared with about 120 milliwatts which is obtained from a 112a tube with 135 volts on the plate and a C bias of minus 9 volts. The only disadvantage of using a 17lA-type tube rather than a 112a is the increased plate current drain which will raise the total load on the B batteries from 10 mA. with a 112A-type tube to 19 mA. with a 17lA-type tube. With the latter tube the batteries will, therefore, have a life of about 250 hours. We feel that in most cases the 112A-type tube will prove satisfactory although, if sufficient volume is to be supplied for dancing, for example, then it will probably be necessary to make use of a 17lA-type tube. In operating this set in the Laboratory it was interesting to note how clearly defined is the overloading point of the resistance-coupled amplifier. When using transformer-coupled amplifiers a certain small amount of overloading may exist on peaks without experiencing serious distortion, but with a resistancecoupled amplifier even slight overloading tends to make the tubes block so that reception is practically ruined. All this simply L250 Turns No.24 L340 Turns No.32 L ) 15 Turns No.28 tapped tapped at center every 5 turns L450 Turns No.24 L5 50 Turns No.32 Fig. 2 — Coil Specifications for battery set means that in operating the set the volume control must be kept at a point low enough to prevent overloading. Incidentally, the plate and grid voltages supplied to the first and second-audio tubes are such that these tubes will not overload if called upon to supply a peak potential of about 50 volts to the grid of the power tube. It follows, therefore, that the amplifier has more capacity than necessary to supply either a 112a or a 17lA-type tube, since the latter tube (with a C bias of 27 volts) doesn't require a peak potential of more than 27 volts on its grid. The model of this receiver which was constructed is illustrated in the various pictures. The baseboard is 10 inches deep and 20 inches long and the panel is 7 inches high and 21 inches long so the set will fit in any of the standard cabinets which are generally 21 inches long and about 12 inches deep. The various parts which make up the set are lettered on the circuit diagram, Fig. 1, to correspond with the lettering in the pictures and in the list of parts. With these data it will not be difficult to locate the various units on the baseboard and to lay out the panel. The drilling templates supplied with the Render drum dials are used in locating the various holes on the panel. The volume-control rheostat, R5, the on-and-off switch, and the regeneration control may be placed as indicated in the pictures. In starting the construction of the receiver, the coils and condensers should be placed on the baseboard first, their corresponding position relative to the panel determined, and the latter drilled as indicated above. The various sockets, resistor mounts, and condensers are then mounted on the baseboard. The cable connector is located along the rear edge of the baseboard. The tuning of this receiver should not be difficult. To tune-in a station regeneration shoidd be increased until the detector circuit oscillates, the carrier wave should be tuned-in by locating a heterodyne squeal, and then the first condenser may be tuned to resonance. Maximum selectivity will be obtained when considerable regeneration is used in the detector circuit; therefore, in those locations where great selectivity is required it is advisable to operate the set with some regeneration, reducing the volume if necessary by means of the volume control. Parts Required THE parts used in the model illustrated in this article are named below. Other parts electrically equivalent may, of course, be substituted if desired. The complete list follows: Ci, C2 Two Render condensers, 0.0005-mfd.; C3 One Polymet grid condenser, 0.00015-mfd;. C4 One Hammarlund neutralizing condenser; Cb One Frost fixed condenser, 0.0002-mfd. ; C6 Three Frost fixed condensers, 0.005-mfd.; C7, C8, C9 Three Polymet by-pass condensers, 1-mfd.; Li, L2 One Hammarlund antenna coil.; La, L4, L5 One Hammarlund coil, type TCT-23; L6 One Hammarlund r.f. choke, type R.F.C. 250; Ri Three Daven Glastors, 0.25-megohm; R2 Three Daven Glastors, 2-megohm; R3 Three Daven Glastors, 0.1-megohm; R4 One Frost fixed resistance, 0.8-ohm; Rs One Daven Glastor, 2-megohm; R6 One Yaxley filament rheostat, 15-ohm; S One Frost Filament switch; Five Frost sockets; One Frost cable, type 780; One Frost cable connector, type 781; One coil Frost hook-up wire; One Micarta panel, 7 x 21-inch; One Baseboard, 10 x 20-inch; Two Render drum dials, type 40; Four Fahnestock clips. The total cost of the parts listed above is not more than $40.00. The following accessories are required: Two 201\-type tubes; Two 240-type tubes; One 112A-type tube; Three Eveready heavy-duty B batteries; Two Eveready C batteries, 4i-volt; One Storage battery, 6-volt; One Loud speaker. The set described in the preceding paragraphs was constructed as a result of requests from many readers for data on a receiver that might be operated economically from B batteries. Those who desire such a receiver but who, perhaps, do not want to go to the trouble of constructing one will be interested in the Eveready receiver. This set contains six tubes, is single controlled, and uses 240-type high-mu tubes throughout, except in the power stage where a 112a or a 17lA-type tube is recommended. This Eveready receiver contains three stages of r.f. amplification, each stage of which is neutralized and shielded. The detector is followed by a resistancecoupled amplifier. The set fists at $85, without tubes or batteries.