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Show Hlililiiir! I Copyruhl, RADIO BROADCAST 1936 Circuit Specially Developed for Use With a Torold Coil ws Radio Frequency Fre-quency Coupling Device. We have seen and heard a great deal about the torold coll during the lust few months, and much space has been devoted to the publication of data for the Inclusion of these colls In already existing circuits. Few writers have gone to the trouble of describing circuits which have been especially designed to make use of the torold Inductance, however. The Radio Broadcast Magazine's "Lab." circuit Is one of the few examples where special attention has been paid to this matter. A series of articles on the "Lab." circuit appears In the Radio Broadcast Magazine, from the pen of Keith Henney, the director of the laboratory of that publication. "The great advantage of the toroid coil," he says, "lies In Its restricted field. Tills means that energy from the coil is not likely to expand about the coll itself and Interlink with other fields, and by the same token it Is difficult dif-ficult to get energy Into the coil. For this reason a circuit in the neighborhood neighbor-hood of powerful broadcasters is not so likely to be disturbed by local signals sig-nals except those arriving at the receiver re-ceiver through the proper means the antenna. The fact that the field is restricted re-stricted Indicates that the coils may be placed closer to metal end plates of condensers, closer to wires carrying carry-ing radio frequency currents of wrong phases, and even closer to each other without the many troubles incident to Interstage coupling. The Advantages. "The use of toroids makes possible the conservation of space, the maintenance main-tenance of the resistance of circuits at their lowest point, the reduction of 'pickup' from external sources, the prevention of interstage coupling by stray electromagnetic fields, and naturally nat-urally makes neutralization of high-gain high-gain amplifiers a much simpler problem. prob-lem. All of these things are possible, of course, provided that proper precautions pre-cautions are taken, and provided that the experimenter does not play 'with an inherently unstable or unbalanced circuit." In the circuit diagram which is reproduced re-produced here the toroid coil is employed em-ployed as an interstage coupling device, de-vice, and is shown as P and S. The ordinary primary of the toroid is left disconnected, the P and S as marked on the diagram consist of the secondary second-ary winding of the toroid divided. "Determining "De-termining the proper place to tap the coil is a question of utmost Impor- tance, and the correct ratio o the P and S windings Is found by the following fol-lowing formula," writes Mr. Henney: NSq. Root of 55,000 Kp In the formula N is the turn ratio, while Rp is the plate impedance. The plate Impedances of various tubes, with 90 volts on the plate and a nega- ' tive 4.5-volt bias on the grid, are as follows: Type Turn Tube Impedance Ratio 12 1S.000 1.75 199 22,400 1.57 20J -A 13,000 2.00 112 0,000 3.00 High-mu 30.000 1.35 (00 volts B, 1 volt C) Not a Difficult Operation. Neutralization In the Radio Broadcast Broad-cast "Lab." circuit Is. accomplished by the Rice method, and this is not a difficult operation if due care Is exercised. exer-cised. It is best for the fan who intends in-tends seriously experimenting with the "Lab." circuit to start with a type 199 tube, for this latter will be found easier to neutralize than the 201-A type. Those fans who have tried a 201-A in the Browning-Drake will readily agree on this point. By reference to the circuit diagram It will be noted that the secondary coil of the antenna tuning circuit has a center tap. It is important that the tap be taken more or less exactly at the center turn, otherwise trouble will be encountered In the neutralization neutraliza-tion process. The size of the various tuning condensers con-densers employed depends on the colls used. The standard coils usually take a 0.0005-mfd. tuning condenser. The condenser connected across the primary pri-mary and secondary of the toroid coil has a maximum capacity of 0.00025 mfd. The one connected from the plate of the detector tube to the primary pri-mary of the toroid coil Is the regeneration regen-eration control condenser, for the capacity ca-pacity feedback system Is employed. It is a 15-plate midget condenser. The ordinary three-plate neutralizing condenser con-denser will be found satisfactory for N. Any satisfactory tuner may be employed em-ployed in the antenna circuit. A Samson Sam-son choke coil was found satisfactory In the plate circuit of the detector tube, but this may be wound at home If necessary. The output, which may be connected to any good audio amplifier, am-plifier, is indicated by an arrow on the diagram. |