Radio Broadcast (May 1928-Apr 1929)

JANUARY, 1929 RADIO BROADCAST No. 15. January, 1929. Radio Broadcast's Service Data Sheets The Br erne r-T idly 8-20 Radio Receiver THE Bremer-Tully model 8-20 receiver consists of four stages of radio-frequency amplification followed by a detector and a two-stage audio amplifier. TECHNICAL DISCUSSION 1. Tuning System There are four main tuned circuits in the receiver, consisting of L1C1, L2C2, L3C3 and LiCi. The fifth tuned circuit, L5C5, is in a sense a wave trap; it is not connected between the grid and filament of the first r. f. tube as the first tuned circuit generally is, but instead is coupled to the small primary coil L6 which is connected between the grid and filament of the first r.f. tube. It should be noted that the leads from the neutralizing condensers to the grid circuits are not connected directly to the grid of the tube, but in-tead are connected to a tap on the secondary coils at a point a few turns from the grid end of the coil, the designers of the receiver having determined that with such a connection the receiver is neutralized more easily over the entire broadcast band. Type 226 tubes are used in all the r. f. stages, with about 100 volts of plate potential. 2. Detector and audio system. A leak-condenser-type detector is used, the grid leak resistor, Ri, having a resistance of 3 megohms and the grid condenser, C11, having a capacity of 0.00025 mfd. The detector tube is a 227-type with about 45 volts on the plate, and its output contains a 0.006-mfd. condenser, C12, to bypass the r. f. currents directly to the cathode. The r. f. choke, L?, also helps to keep the r. f. currents out of the a. f. amplifier where they might cause distortion. The choke coil, L7, connects, through the phonograph jack, J, to the first audio transformer, Ti. 3. Volume control. The resistor R4 is the volume control, and it will be noted that it is actually connected across the grid-filament circuit of the first r. f. tube and also in series with the plate circuit of the second r.f. tube. When the movable contact on the volume control is at the position of maximum volume (2) all the resis tance is connected between the grid and filament of the first tube and there is zero resistance in the plate circuit. As the arm is moved toward the other end (1), the resistance connected between the grid and filament of the first tube is gradually reduced and at the same time the resistance in the plate circuit is proportionally increased. 4. Filament circuits. Filament current for the various tubes in the receiver is supplied by several filament windings on the power transformer located in the powersupply device, the 226s being supplied with 1.5 volts, the 227s with 2.5 volts and the power tubes with 7.5 volts. Across secondaries Si and Sjare placed center-tapped 8-ohm resistors, it being necessary to connect all the grid and plate returns to a center point of the filamentcircuit to preventhum. Similarly a 15-ohm resistance is connected across the 2.5-volt filament winding. Si, supplying the 227-type tubes and a 40-ohm resistance across the 7.5-volt filament winding, Ss, which supplies the 210-type tubes. 5. Plate circuits. The plate circuit of each r. f. tube contains an r. f. choke, marked R.F.C. on the diagram, to prevent any of the r. f. currents passing into the platesupply device. The by-pass condensers, Cu, are connected from each r. f. choke to ground. All of the r. f. tubes are supplied with the same plate potential, and the other plate leads connected between the receiver and the power supply furnish voltage to the detector and audio amplifier tubes. 6". Grid Circuits. All tubes in the receiver are biased by connecting resistors of the correct value between the center points of the filament circuits and negative B. Grid bias for the first two r. f. tubes is obtained by the voltage drop across a 770-ohm resistor, R5. The drop across this resistance is about 6 volts. A similar resistance, Rc, supplies grid bias to the third and fourth r. f. tubes. Grid bias for the first audio tube is furnished by the 1125-ohm resistance, R7, and grid bias to the power tube is obtained from the resistance, Ra, which has a value of 770 ohms. 7. Power supply. The power-supply transformer, Ti, contains a primary winding, P, tapped for line voltages of 110 and 125, and six secondary windings. A 281-type half-wave rectifier is used. The filter system consists of the two choke coils, L» and L9, and the 2-mfd. filter condensers, Cm, C15, and Cic. Cu has a voltage rating of 800 volts, and C15, and Cm each have a rating of 600 volts. The filter circuit is ordinarily closed by terminals A and B in the power supply. If, however, a dynamic loud speaker is to be/used, and the field of this speaker is to act as one of the filter choke coils, then the connecting link between A and B is removed and the terminals from the field winding of the loud speaker are connected to terminals B and C. The output of the filter system is fed directly to the plate of the 210-type tube. After reduction by the resistor, Ra, the voltage is correct for application to the plates of the r. f. detector and first audio tubes. One-mfd., 200-volt, by-pass condensers are connected as indicated on the diagram between negative B wire and various points in the circuit. They prevent coupling which might cause distortion. The power to the receiver is completely controlled by the switch connected in the primary of the power transformer. Dynamic Speaker CIRCUIT DIAGRAM OF RECEIVER AND POWER UNIT