Radio Broadcast (May 1928-Apr 1929)

70 RADIO BROADCAST OCTOBER, 1928 No, 10, Radio Broadcast's Service Data Sheets on Manufactured Receivers THIS sheet is devoted to a discussion of a six-tube, completely self-contained electric receiver. It employs a.c. tubes, is single controlled and consists of three stages of r.f., a detector and two stages of transformer-coupled audio amplification. It is designed for operation from a 110-volt, 60cycle light socket. Type 226 tubes are used in the r.f. amplifier and in the first a.f . stage. The detector is a type 227 tube and the output tube is a type 171A. The antenna circuit is arranged with several taps to adapt the operation of the set to short, medium and long antennas. TECHNICAL DISCUSSION 1. Tuning System. The tuning system comprises four r.f. transformers and tuning condensers consisting of Li and Ci, L3 and Ci, L5 and O, and L; and Ci. A single-gang assembly contains all four tuning condensers, each section of which has a capacity of 0.00035 mfds. The antenna is connected to Ai, Aa, or As, depending upon whether it is short, or medium length, or long. The small variable inductor forming part of Li is in the circuit so that the effect of the antenna on the first tuned circuit may be compensated and this circuit brought into exact resonance at all wavelengths. This receiver is not neutralized but is stabilized by grid resistors, Ri, R2, and Ri, each with a value of 600 ohms. This method of oscillation control is used in many receivers and is quite effective. 2. Detector and Audio System The grid-leak-condenser type detector employed in this receiver uses a 0.00025-mfd. grid condenser, C3, in conjunction with a 2-megohm grid leak, R5. The detector is a type 227 tube and its output is fed into a two-stage transformer-coupled audio amplifier. The output of the detector circuit is bypassed by Ci with a capacity of 0.0001 mfd. The first stage audio transformer, Ti, has connected across its secondary a small fixed condenser, Cs, of 0.00025 mfd., probably to prevent singing in the a.f. amplifier at high frequencies. The output of Ti supplies signal voltage to the grid of V5, a 226 type a.c. tube. The loud speaker is isolated from the plate circuit of the power tube by a choke-condenser combination consisting of choke coil X2 and condenser Cn; this condenser has a capacity of 1.0 mfd. A NEW SPLITDORF MODEL This beautifully housed receiver is the new Splildorf Abbey Senior. It incorporates many new features not found in the older Splitdorf circuit described in this Sheet. It uses six tubes, one of them a 250 type, and is equipped with a phorwgraph jack and a novel " sensitivity-selectivity" switch. 3. Volume Control. A 500,000-ohm variable resistance, Ri, is connected across the input to the detector in this receiver and functions as the volume control. The input circuit of a grid leak and condenser type detector is generally quite low and as a result the tuned circuit preceding it has poor selectivity. It is therefore a good idea to place the volume control at this point, for at this point in the circuit it cannot affect the selectivity of the receiver to any marked degree. Since the control is ahead of the detector it is possible to regulate the volume to prevent overloading of the detector tube. 4. Filament Circuits. Filament current for the various tubes in the receiver is supplied by the power transformer, T3. Secondary winding Si supplies 1.5 volts to the 226 type tubes, secondary S2 supplies 2.5 volts for the heater of the detector tube and secondary S3 supplies 5.0 volts for the power tube. S3 also supplies current for the dial light. All the filament circuit leads are twisted to prevent hum, and 30-ohm potentiometers, R6 and R7, are connected across the secondaries, Si and S2, to make it possible to obtain a very accurate hum balance. 5. Plate Circuits. The output tube, V6, of the audio amplifier is supplied with the maximum voltage from the power supply unit — 180 volts. All the r.f. tubes and the first a.f. tube receive 1 10 volts and the detector is supplied with 45 volts The plate circuits of the r.f. tubes are bypassed in the set with condenser C2, whose capacity is 0.5 mfd. The detector plate supply bypass condenser is Cs, located in the power unit. Its value is 1.0 mfd. 6. Grid Circuits. The output tube of the set has a bias on the grid of 40 volts, supplied by sections of the resistor, Rs, in the power unit. The bypass across this resistor, C10, has a value of 1.0 mfd. Section d of the resistor supplies bias for the first a.f. and all the r.f. tubes. This bias voltage is 7.5 volts and the bias resistor is bypassed by the 2.0-mfd. condenser, C9. The tone quality would be very poor, due to loss of the low frequencies, if these bypass condensers, Cs and C10, were not included in the circuit across the C bias resistances. 7. The Power Supply. The A-B-C supply for this receiver consists of the power transformer, T3, the rectifier, V;, which is a type 280 tube, the filter system and the voltage dividing resistor, Rs. This resistor has a total value of 13,200 ohms divided as follows: section a, 2500 ohms; section b, 5400 ohms; section c, 4500 ohms; section d, 150 ohms; section e, 650 ohms. The filter circuit consists of the filter choke coil, Xi, and the two filter condensers, C6, with a value of 4.0 mfd., and C7 with a value of 6.0 mfd. The primary of the transformer is fused at F and the entire receiver is turned on and off by the switch, Sw. RECEIVER AND POWER SUPPLY CIRCUIT