Radio broadcast .. (1922-30)

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RADIO BROADCAST thrown. The automatic volume control in this receiver will automatically compen- sate all signal fading that could be com- pensated by continually adjusting a man- ual volume control. On the control grids of the r.f. tubes there is a steady d.c. bias of approximately 3 volts. The volume control circuit func- tions, of course, to increase these biases. The first two r.f. tubes receive the full grid-bias voltage available from the recti- fier. However, to prevent distortion in the third r.f. amplifier stage, the third r.f. tube receives only half this bias. The antenna circuit arrangement is in- teresting. In the first place the antenna primary circuit is resonant at a frequency less than 550 kc., which makes the an- tenna circuit tuning substantially inde- pendent of the size of the antenna—the circuit also gives a uniformly high voltage step-up over the entire band. These high inductance antenna circuits were first in- troduced into broadcast receivers by the Hazeltine Corporation about 1926. Since then their operation has been somewhat improved by the addition of some dead- end turns which increase the capacitive coupling between the antenna primary and the tuned secondary. The 5000 ohms which is in parallel with the antenna coil helps to reduce any periodicity of the antenna circuit and make it nearly aperio_dic. The local-distance switch connects into the circuit, for local reception, a 20-ohrn re- sistor in parallel with the 5000-ohm resistor. Coupled Tuned Circuits Ahead of the first screen-grid tube, two tuned circuits are used. These tuned cir- cuits are coupled together with both the mutual inductance, L, and the mutual capacity, C = 0.015 mfd, the former pre- dominating at the high frequencies and the latter at low frequencies. At 1500 kc. the inductive coupling is below the optimum value so that the circuits are coupled quite loosely at the high frequencies. This gives high selectivity at the high frequencies where it is so badly needed. At the low frequencies the capacity coupling is im- portant and the coupling reaches the optimum value around 550 kc. This effec- tively couples the two circuits quite closely, thereby decreasing the selectivity and preventing serious sideband suppres- sion. The combination of the two effects gives a selectivity characteristic for the antenna circuits which is quite uniform over the entire band. It will be appreciated that these two tuned circuits ahead of the first tube will effectively prevent cross talk. The r.f. amplifier as shown by the circuit diagram (Fig. 1) employs three screen-grid tubes. Between the first two tubes are tuned transformers which give an average gain of about 20 per stage. With the output capacity of the screen-grid tube the primaries of these transformers are re- sonant at a frequency slightly above 1500 kc. The gain at the high-frequency end would, with such a circuit, be expected to increase but this effect is prevented by the capacity coupling existing between the plate and grid ends of the primary and secondary coils. The capacitive coupling partly cancels the inductive coupling and prevents the gain characteristic from rising. This is not a "losser" method and therefore keeps the full selectivity. Untuned R. F. Transformer Because of the characteristics of the two-element detector it is preferable not to couple it to a tuned transformer. An untuned transformer is therefore used, the gain of this circuit averaging eight over the broadcast band. The 13,000-ohm re- sistor across the primary of this trans- former damps out any resonant peaks. There is one other point of interest re- garding the r.f. amplifier. As stated previously, the capacity coupling between the first two tuned circuits is due to the 0.015-mfd condenser which is effectively in series with the tuned circuits. So that all the tuned circuits will track properly similar 0.015-mfd condensers are placed in series with the following tuned circuits, and all these three condensers are used to bypass the bias voltages. Regarding the a.f. amplifier circuits, it will be noted that the output of the two- element rectifier is connected directly by resistance coupling to the grid of the follow- ing 227 a.f. amplifier tube. An ordinary detector functions in the dual role of am- plifier and detector, but we have in this set two tubes that in combination do what the detector does in an ordinary set. There- fore, the two-element detector and auto- matic volume control circuits associated with these two tubes (the two-element detector and following amplifier) has been termed by Philco a "multiplex detector circuit." The output of the second 227- type tube of the multiplex detector circuit feeds into a resistance-coupled stage con- sisting of a 500,000-ohm plate resistor, a 0.015-mfd. coupling condenser and, in the grid circuit of the next tube, a 500,000- ohm potentiometer, which is the volume level control. Volume Control Circuit After all these years that have seen a gradual trend towards the location of the volume control in the r.f. amplifier, it is interesting that it has once again returned to the a.f. amplifier. Probably it has re- turned to stay. Technically it is sound to control the sensitivity of the r.f. amplifier by means of automatic circuits designed so that there is applied to the rectifier a value of signal just sufficient to load up the power tubes when the a.f. volume control is set at maximum. The detector then works at a constant level, and the circuit can be designed so that at this particular level the detector circuit pro- duces minimum distortion. Such princi- ples will, we believe, be generally adopted within the next few seasons. The 227 detector-amplifier, V5, is sup- plied with a value of grid bias which is a function of d.c. rectified current of the two- element rectifier. The circuit constants are such that the grid bias voltage is always slightly greater than the peak value of the a.c. signal applied to the grid. In this manner it is impossible for the grid of the tube to swing positive on the audio peaks. The sensitivity of the entire receiver is essentially constant at about 5 microvolts per meter throughout the entire broadcast band—this sensitivity is obtained, of course, with the volume control at maxi- mum and the local-distance switch in the distance position. Summary In summary the features of this receiver are: 1. Two-element detector tube giving true linear detection and no overloading on signals up to 100 volts input—an input greatly in excess of that possible. 2. Automatic control of volume and fading, giving, in conjunction with the local-distance switch, satisfactory control of all field intensities in the ratio of 10,000 to 1. 3. A "multiplex detector circuit" being the name applied to the use of a two- element detector in combination with an ordinary three-element tube working as an a.f. amplifier direct connected to the output of the two-element detector. 4. A local-distance switch to prevent overloading on strong local stations. 5. Uniform-gain antenna and interstage coupling circuits to maintain a constant sensitivity of about 5 microvolts per meter throughout the broadcast band. 6. Simplicity of operation—the manual volume control need be touched but seldom. 7. Excellent fidelity due to the linear and non-overloading detector together with the resistance-coupled first a.f. stage and a push-pull power stage. ANTENNA CIRCUIT •*,- R F AMPLFtR .. MULTIPLEX DETECTOR ELECTRODYNAMIC SPEAKER 112* Fig. 1 — Complete schematic diagram of the Philco 95 receiver. • DECEMBER 1929 •