Radio Broadcast (May 1928-Apr 1929)

A Loop-Operated Browning-Drake By J. H. GOCKEL FRONT VIEW OF THE AUTHOR'S RECEIVER ANY set-builders are anxious to construct a receiver which is not bound by antenna and ground wires to one certain place in the home or in a room thereof. The place where such wires are available is often the wrong place for the receiver as a piece of furniture. Or the home or apartment may not provide a suitable location for an outside antenna. These fans require a loop-operated super-heterodyne, but the people we have in mind are not prepared for the outlay in funds which such a receiver requires. They are interested chiefly in good tone quality, and would rather spend what money they have on the audio transformers, the power pack, and the reproducer. Still, they have an occasional desire to satisfy their wanderlust by listening to stations several hundred miles away. Consequently, the receiver they need must be selective enough to cut through locals, and sensitive enough to reach out a thousand miles or more under favorable conditions. To sum up: the receiver required must be loop-operated, yet not an expensive multi-tube set; at the same time it must be selective, sensitive, and deliver good tone quality. The writer believes that the receiver illustrated in the accompanying pictures and diagram fills this peculiar and seemingly self-contradictory order. An examination of the pictures reveals but a single coil, and even this one coil is minus its primary winding. Yet this receiver, located within five miles of kdka with its 50,000-watt signals, is capable of bringing in wht, separated by only 30 kc. from kdka, with only a whisper of kdka's signals audible. Or it can receive the "Slumber Hour" from wjz, 10 kc. from a 500watt local station, without a trace of the latter's signals. Broadcasting stations in Texas, Florida, Nebraska and Canada were brought in with good volume during the early part of May. The quality of its detector output is such as to deserve the best of audio amplifiers. In short, it compares quite favorably with an eight-tube super-heterodyne which was used in making comparative tests. It did not possess the same degree of sensitivity, of course, but it also lacked two other things quite noticeably; namely, the noise that the eight tubes of a super-heterodyne can make on their own account, and the "repeat points" which kdka scatters so generously over the dials of a super-heterodyne. NOT A FREAK CIRCUIT THIS receiver is not a freak. The idea for it was obtained from Mr. Browning's article in May, 1928, Radio Broadcast, under the title: "A Two-Tube Screen-Grid Tuner." After applying his suggestions to an old Browning-Drake receiver which had done remarkably well in Arkansas, but which kdka had forced into retirement in Pittsburgh, the thought came, "Since the screen-grid tube is such an efficient amplifier, why not operate the receiver with a loop?" To make this experiment an old-fashioned "barrel loop" was obtained at a mere fraction of its former list price, and rebuilt into more modern form by using two curved pieces and two straight pieces of its frame, plus four triangular pieces of hard rubber notched to receive the wire. The loop having been reconstructed, the antenna and ground wires were disconnected, and the antenna coupler removed from the receiver. The loop connections were then made as shown in the diagram. Kdka was tuned-in, and then tuned-out almost completely by carefully turning the loop to a ninety-degree angle to the direction of the station. Three points down the dials wht was found, then kmox, wsmb, and others which had been drowned out before. It HERE is a story by a reader who applied a screen-grid r.f. stage to a BrowningDrake circuit which he was constructing for his own use. The result was a set which could deliver satisfactory signals with a loop antenna. The many readers who are using Browning-Drake sets of early vintage may find in this experience article just the information they want to help them in converting their set to one with greater gain and somewhat increased selectivity. — The Editor. is the loop, of course, which accounts for the selectivity of the receiver, and the use of the screen-grid tube explains its sensitivity. When the receiver is operated near a powerful broadcasting station, such as kdka, it is advisable to enclose the entire set in a sheet copper box, made by a tin-smith. The shield used by the writer was made just large enough to fit snugly around the base, so that the base panel would lend rigidity to the copper shield. Further shielding was given the shield-grid tube by using a Tyrman socket, which is supplied with a shield made especially for this tube. When a shield is placed about the entire receiver, the wiring is arranged as shown in the diagram. The usual method of mounting the grid leak, R4, directly across the grid condenser, C4, by means of clips cannot be employed, as this connection would not permit the tuning condenser, C2, to be mounted directly on the metal shield, as the rotor of this condenser, as well as the grid return of L2, would have to connect with the Fplus wire, unless a 200A-type tube were used. In this connection it might also be mentioned that neither shielding nor the turning of the loop will avail much in keeping out unwanted signals from a powerful local station if a ground wire is attached to any portion of the receiver. Fortunately, at least in our case, the removal of the ground wire from the power pack caused no increase in hum from the loud speaker. OBTAINING BIAS AT FIRST a 15-ohm fixed resistance with a 10ohm tap was used in the negative filament lead of the screen-grid tube, not only to reduce the filament voltage to the required amount, but also to supply the necessary grid bias. But 15 ohms seemed to give too much bias, and 10 ohms did not provide quite enough. Therefore, a 15ohm rheostat, Ri, was converted into a potentiometer, and the grid return of the loop antenna was connected to the contact arm. After the proper adjustment of the potentiometer has once been determined by experimenting, the knob hardly need be touched again, and so this potentiometer may be placed on the base panel. However, in the set illustrated, Ri is mounted on the front panel and is used as a volume control on strong local signals, rather than burn the filament of the screen-grid tube too low by reducing R-i. Either our ear or our imagination told us that the tone quality was better with this form of volume control for local stations. Furthermore, we have learned that burning the filament of a tube too far below its rated voltage does not 171