Radio Broadcast (May 1928-Apr 1929)

178 RADIO BROADCAST JANUARY, 1929 No. 16. January, 1929. s service The Freshman Model 0 Receiver '"FHE model Q Freshman receiver is -1 unusual in that it employs a 222type screen-grid tube in the r.f. amplifier. The 222-type tube used is one designed for d.c. operation, but in this receiver it is operated on alternating current. The single screen-grid stage of radio-frequency amplication is followed by the usual 227 heatertype a.c. detector, the first audio stage with a 226-type tube, and a 17lA-type power tube. TECHNICAL DISCUSSION 1. Tuning system. This receiver contains only two tuned circuits, LiO and LiCj, and both of the tuning condensers are ganged to a single control. A small midget condenser, Ci, with a maximum capacity of 50 mmfd., is connected in series with the antenna. The two tuning controls, i.e., the main dial controlling Ci and C2, and the vernier adjuster controlling Ci, are interdependent and a slight change in one necessitates a change in the other. It should be noted that the plate of the detector tube connects through a fixed condenser, C4, to the lower end of the inductance, L?. The r.f. currents in the plate circuit must, therefore, pass through C4 to the lower end of L> and hence through Cs, with a capacity of 0.02 mfd., to ground. These currents in the plate circuit of the detector which flowing through the condenser Ci, connected in the grid circuit of the detector, thereby impress on the grid circuit of this tube a small voltage and causes regeneration which increases the gain of the receiver. 2. Detector and audio system. The grid-rectification detector in this receiver, using a grid leak, Ri, with a value of 2 or 3 megohms, and a grid condenser, Ci, with a value of 0.00025 mfd., is followed by a two-stage transformer-coupled amplifier with a 226-type tube in the first a.f. stage and a 17lA-type tube in the power stage. Cs with a capacity of 0.02 mfd. is part of the regenerative system mentioned in the preceding section. 3. Volume control. The volume control consists of a variable resistance, R2, connected between antenna and ground, and in this position it serves to regulate the amount of energy supplied to the first r.f. amplifier. MODEL Q 4. Filament circuits. Since four different types of tubes are used in this receiver it is necessary that the power-supply transformer contain four separate filament windings. A 3.1-volt winding, Si, supplies the 222-type r.f. amplifier, a 2.25-volt winding, S2, supplies the 227type detector tube, a 1.4-volt windings, S3, supplies the 226-type audio amplifier and a 4.8-volt winding. Si, supplies the 17lA-type power tube. It is interesting to note that all of these voltages are somewhat lower than the rated filament voltages of the tubes they supply. These low voltages are used because it has been found that the various tubes will give satisfactory emission with these potentials and that their life will be greatly increased. 5. Plate circuits. The plate of the 222-type tube is supplied with 170 volts, the detector with 50 volts, and the first audio and second audio tubes with 170 volts. The screen-grid of the r.f. tube is supplied with 50 volts. It will be noted that the 170 volts with which the platesof the r.f. tube and first audio tube are supplied is considerably above the maximum value specified by the standard tube manufacturers. The reason for the use of these high voltages is probably that the hum in the output of the receiver is less than with rated voltages, due to the fact that with high plate voltages high values of grid bias can be used and as a result the modulating effect on the grid circuits of any a.c. hum voltages is proportionately less. All the plate circuits are bypassed to ground with fixed condensers. 6. Grid circuits. Grid bias for the various tubes is obtained from resistors connected in series with the center taps of the various filament windings on the power transformer. The 3.1-volt winding supplying the 222-type tube has in series with its center tap a fixed resistance, R i, which supplies a negative potential of approximately 2.5. volts to the control grid of the r.f. tube. There is no bias on the grid of the detector tube except that due to grid current flowing through the grid leak, Ri. The 226-type audio amplifier tube obtains grid bias from the 1800-ohm resistor, R4; the voltage drop across this resistance places a negative bias of about 10 volts on the grid of this tube. Negative bias on the grid of the power tube is approximately 35 volts and is obtained from the voltage drop across the resistance, Rs. 7. Power supply. A 280-type full-wave rectifier is used in the power supply, this tube being supplied with 400 volts from the secondary winding, S«, and 4.8 volts on the filament from S6. The output of the tube feeds into the filter system consisting of a single choke coil, L3, and the two condensers, C« with a capacity of 2 mfd., and Cs with a capacity of 4 mfd. Cs has a rating of 2000 volts and Ce has a rating of 1000 volts. It should be noted that condensers of a far higher voltage rating than is actually applied to them are used in order to insure long life without danger of break down. The output of the filter circuit supplies the plates of all of the tubes except the detector, and by use of the voltage dividing resistors, Re and R7, the maximum output potential is reduced to 50 volts for the plate of the detector tube and the screen grid of the 222-type tube. R6 has a value of 12,500 ohms and R? is a 10,000-ohm re sistor. Note: As this issue goes to press we are informed that the Freshman Model Q receiver has been superseded by the model 3Q. The new receiver employs an additional tuned circuit — The EDITOR.