Radio broadcast .. (1922-30)

728 Radio Broadcast chance of error, since the transformer and tubes, when in this position, almost connect themselves because of the proximity of binding posts to binding posts. The two input or primary terminals of the first transformer are connected to the two central prongs of the center jack in the set. The outside terminals of the secondary are then connected to the grids of the two tubes, each to the nearer grid. The central terminal is run directly to the negative post of the C battery, while the positive post of the C battery is run to the negative A battery binding post. The filament connections are made in the usual way, the two filaments being placed in parallel and being controlled by the one rheostat. The plate of one tube is connected to one of the two outside posts of the primary of the second transformer, the plate of the other tube to the other. The central binding post of the secondary is attached to the plus post of the high B battery voltage. The two secondary leads from the second or output transformer are wired to the push-pull jack. A .0005 mfd. fixed condenser C<j is connected from the upper grid side of the secondary of the input transformer to the lower side of the secondary of the output transformer as an additional stabilizing capacity. Various values of capacity may have to be substituted to obtain quiet operation. In some cases it may be necessary to include in the push-pull amplifier circuit the .0005 mfd. fixed condenser shown connected between the G terminal of the secondary of the input transformer and the F terminal of the secondary of the output transformer. Usually its addition to the circuit will eliminate any tendency of the tubes to overload and the shunting of the .006 mfd. condenser across the secondary terminals of the output transformer will clarify the tone output of the receiver. This latter addition largely depends upon the type of loud speaker used. PUSH-PULL AND THE LOUDSPEAKER JVAOST loud speakers have an impedance *•** out of all proportion to the impedance of the tube preceding. And yet theory has it that the best results are obtained when the impedance of the speaker or other "load" matches the tube impedance. The push-pull system corrects, or rather, overcomes this because the output transformer separates the loud speaker from the tube circuit. The ordinary straight audio system places the loudspeaker directly in the plate circuit of the last tube and consequently subjects it continually to the full plate voltage. This means that there is a constant drag on the diaphragm of the speaker which prevents that diaphragm from responding freely to weak or complicated tones. In the push-pull system, the full plate voltage gets no farther than the primary of the second transformer. It cannot be transferred to the secondary or loud speaker circuit because it is a direct current. Direct currents do not affect a transformer. Only when there is a current fluctuation does the loud speaker winding get an impulse, because only then is the current transferred from primary to secondary by means of induction. It will be seen at a glance that this is the ideal condition. Unless speech or music is coming through the amplifier, the loud speaker has absolutely no potential applied to it, yet when broadcasts are being amplified, the loud speaker winding gets a powerful impulse exactly in accordance with the matter being broadcast. SELECTIVITY ' I VHE circuit described is extremely selec* live in spite of the fact it has but one tuning dial. Frequently on demonstration, seven or more out of town stations have been brought in on the speaker through the powerful New York local stations. Referring to Fig. 2, it will be noted that there are three terminals on the primary side of the tuning unit, which are marked "SA", "LA", and "GND". The terminals "SA" and "GND" include the entire winding and should be used with a short antenna of from 40 to 100 feet. The terminals "LA" and "GND" include the larger part of the coil and should be used with a long antenna. If extreme selectivity is desired the antenna may be connected to "LA" and the ground to "SA." This uses only a small part of the coil and while the volume will be somewhat reduced, the tuning will be extremely sharp. OPERATION THIS receiver performs best with 0-301 -A ov-2 or uv-2oi-A tubes operating from a small six volt storage battery and 90 volts of B battery. At the same time good results may be obtained using a 201 -A tube in the first stage and a uv-igg type tube in the second stage, running the two tubes from six volts of dry battery. If this latter method is used, the 30 ohm rheostat controlling the uv199 type tube must be turned on only just