Radio Broadcast (May 1929-Apr 1930)

receiver with variation of signal input in microvolts at the antenna terminal. This curve does not represent the absolute sensitivity of the receiver, but is used to show the action of the automatic volume control. Thus, it can be seen that after the signal input to the receiver passes 50 to 60 microvolts (at the knee of the curve), the output is practically constant up to 100,000 microvolts or one-tenth volt. Above this value of input, the first r.f. amplifier tends to overload and it is necessary to use the "sensitivity control" to reduce the input from the antenna to the r.f. amplifier. This control is discussed further in a following paragraph. The slight rise in the curve at 100,000 microvolts shows the beginning of this overloading effect. It should be remembered, however, that this value represents a very strong local signal. The proper operation of a receiver equipped with the automatic volume control necessitates some sort of resonance indicator; that is, some means which will indicate when the receiver is tuned exactly to the carrier of the signal being received, is necessary. The resonance indicator in this receiver is a "visual tuning meter" which is connected in series with the cathode circuit of the second r.f. amplifier and operates on the plate current of this tube. When the received signal increases in strength, as it does when it is "tuned in," the automatic control bias on this tube is increased and the plate current decreases. Thus, the station received should be tuned so that the minimum indication of current is obtained. The meter is mounted in an inverted position so that the receiver is tuned for the greatest deflection of the needle which moves from left to right (in the direction of the arrow on the meter dial). When operating this receiver, it will be found that some stations will come in with good volume without affecting the needle pointer. This is due to the high sensitivity of this receiver as shown by Fig. 6. The r.f. inputs in microvolts, shown on this graph, give a standard signal of 50 milliwatts output. Fig. 5 shows, as mentioned above, that the automatic volume control is designed so as to start to function at an r.f. input of from 50 to 60 microvolts, as this has been found to be the most satisfactory value in all respects. This indicates that quite a strong signal could be obtained from the loud speaker before the automatic volume control starts to function. Furthermore, such a signal would not give a volume in the loud speaker greater than that set by 750 1000 1250 FREQUENCY IN KILOCYCLES Fig. 6 — Sensitivity curve of No. 846 receiver. Note that the ordinate is plotted in microvolts input and not in microvolts per meter. FREQUENCY IN CYCLES' Fig. 4 — Overall fidelity characteristic of chassis of No. 846 receiver. the manual volume or level control. It should be borne in mind, however, that such signals are comparatively weak, and would not be recognized at all by many receivers. The Power Detector The linear power detector is of practically the same design as used in the Nos. 641 and 642 receivers, except that the grid leak and capacitor arrangement is omitted because the phonograph pick-up connec INPUI IN MICROVOLTS Fig. 5 -Graph illustrating the action of the automatic volume control. tion is not made to the detector grid in this receiver. The output of the detector feeds a very high impedance 1:1 ratio audio transformer. In order to obtain maximum possible plate voltage on this tube, it is necessary to carry the cathode (in series with which is the biasing resistor) and the grid return to the negative "B" line. The negative "B" line, in this case, is "below" ground potential on account of the voltages necessary for the automatic volume control circuit, where ground is the most positive potential used. This arrangement accounts for the rather unusual bypassing in the detector circuit. The Audio Amplifier The audio amplifier of the No. 846 receiver consists of two stages. The first stage uses a 227-type tube, and the second or output stage employs two 245-type tubes in push pull. The manual volume or level control, is a voltage divider across the secondary of the 1:1 input transformer, used to vary the signal voltage applied to the first audio grid. The moving contact, or lever, of this unit is connected to ground while the grid is connected to one side of the transformer (through the pick-up switch). This greatly simplifies the insulation problem and eliminates troubles due to hand capacity which would be present if the grid were connected to the moving element. The "Volume Control" in a set which provides a substantially uniform signal at the detector, must be at the input of the audio amplifier for satisfactory operation. The pick-up switch acts so as to connect the output of the pick-up outfit to the grid of the first audio tube in place of the secondary of the first audio transformer. This switch is actuated by turning the volumecontrol knob in a counter-clockwise direction as far as it will go. At this position a cam acts on the switch levers. When the pick-up switch js in position for phonograph operation, both terminals of the secondary of the audio transformer are grounded, greatly decreasing the possibility of capacity coupling of the signals from the r.f. to the audio amplifier. The push-pull input transformer is of large size, having a turns ratio of 1 :3.6 on each side in order to obtain enough amplification for operation of the phonograph pick-up unit. The Power Supply The power supply for this receiver consists of the usual power transformer with the necessary secondary windings. There are three lowvoltage windings; one for the heaters of the r.f. amplifier, first audio, and volume control tubes, one for the detector heater, and one for the power output tubes. The detector heater winding is provided with an adjustable hum balancer and the winding for the output tubes filaments has a fixed midtapped resistor. The remaining winding has a grounded mid-tap. The rectifier is of the conventional 280-type and supplies current to a special two-stage filter. Each stage of this filter is equipped with the tapped inductor, which, in combination with proper capacitors, gives much better filtering action with smaller coils and capacitors than does the conventional "bruteforce" type. The B supply for the output tubes is taken from the junction point of the two sections of filter. This allows the second inductor to be made considerably smaller. This connection also prevents " motorboating " due to coupling in the power supply between the output tubes and any of the preceding tubes as it introduces a whole stage of filter between the respective "B" supplies. The plate voltage to the detector is supplied through a 10,000-ohm resistor connected to the output of the second filter stage. The remaining voltages are supplied from the voltage-divider resistors and where necessary have filtering capacitors shunting the resistors. The built-in electrodynamic loudspeaker is very sensitive and, in order LOWER FREQUENCIES HIGHER FREQUENCIES KILOCYCLES OFF RESONANCE Fig. 7 — Selectivity curve of screen-grid receiver with automatic volume control taken at 1000 kilocycles. 356 • • OCTOBER 1 92 9 •