Radio Broadcast (May 1928-Apr 1929)

z RADIO BROADCAST Chassis view of the Junior iiHi-Q 29" A. C. Receiver The Six-Tube, Screen-Grid, Junior Model "Hi-Q 29" THE "Hi-Q 29" Junior Model has been tested in the Laboratory and was found to be an efficient receiver; that is, it is sensitive, selective, and will bring in stations with good fidelity. In a single evening the receiver brought us a number of out-of-town stations in addition to the old stand-by's such as kdka and wgy. It has 20-kc. selectivity and this makes it possible here in New York to receive wlw when wor is on the air. Wor was audible in the background, but the frequency separation of these two stations is only 10 kc. It had more r. f. gain than could be used in this particular locality. The cost of parts, $54.60, brings it within the reach of nearly everyone. The circuit diagram of the d.c. -operated Junior model is shown in Fig. 1. Two screengrid tubes are used in the radio-frequency amplifier, and the first, which is untuned, is coupled to the antenna-ground input system through a 3000-ohm variable resistor which serves as the volume control. The receiver is a true single-controlled set, although it employs two sharply tuned circuits. The plate circuit of the first screen-grid tube looks into a specially designed transformer which has several primary taps, so the experimenter can choose the one that provides the best selectivity. The primaries are small and wound of many turns of fine wire. Following the first untuned antenna stage are two carefully tuned stages, the second r.f. amplifier and the detector. The voltage gain in the antenna stage is not high, but contrary to many such untuned stages using low-mu tubes (of the 201a type) it provides some voltage gain. The second stage, of course, has a high voltage gain and the detector input, being tuned, adds its bit to the overall radio-frequency amplification of the receiver. The coils are typical Hammarlund spaced winding solenoids of a good shape. C-BIAS DETECTOR The detector, which is a 20lA-type tube, is used in a C-battery circuit because of the greater input voltage handling ability and somewhat greater freedom from high audiofrequency loss. Following this tube is a threestage resistance-coupled audio amplifier, the first tube being a 240, or hi-mu type, the second a 201a and the final tube a 171a. The screen-grid tubes are placed outside the shielding boxes which enclose the tuning components. The control-grid leads are shielded, and thus there is little danger of unwanted feed back from one stage to another. The filtering in the r.f. amplifier consists of 5000-ohm resistors in the plate circuit of each tube and 0.5-mfd. by-pass condensers connected from the low-potential side of each primary to the filament of the tube in question. The advantage of such filtering has been pointed out many times in this magazine; its purpose is to keep the r.f. plate currents out of the B supply and to prevent them from becoming mixed with similar currents of another stage. Bias for the screen-grid tubes is obtained by connecting the low-potential end of the grid input circuit to the battery end of a filament resistor. The correct plate voltage for this value of C bias is obtained through the 5000-filter resistor — the voltage drop across this resistor is small since the normal plate current of the tube is only about one milliampere. Such a grid bias lengthens the life of the tubes appreciably. Bias for the resistance-coupled amplifier tubes is obtained externally as is that for the detector. A choke coil has been placed in the plate circuit of the detector to prevent r.f. voltages from overloading the resistance-coupled amplifier. In addition a condenser is placed across the input to the amplifier so that the detector will be provided with a lowimpedance output (to r.f. currents) with consequent better detection. All in all, the Junior "Hi-Q 29" is a sixtube receiver, using screen-grid tubes at their best, i.e., well shielded and well filtered, a C-battery detector, a resistance-coupled amplifier provided with values of resistance and capacity that will permit the amplification of all frequencies from below 100 to well over 5000 cycles, and a power tube designed to deliver at least 350 milliwatts of power to an average speaker. Because of the suggested plate voltage on the 171a, 135 volts, an output device is not necessary from the standpoint of protecting the loud speaker, although there is plenty of room for it on the baseboard of the receiver. LIST OF PARTS The picture gives a good idea of the internal appearance" of the receiver, and the list of parts below indicates the discrimination with which its designers picked out the components. There is an a.c. model of the same general circuit employing Arcturus tubes. The list price of the latter is $104.20. and the receiver as put together is a complet ely self-contained tuner, amplifier and power supply. The complete list of parts follows: Ci, C2 Two Hammarlund Midline condensers. 0.00035 mfd., type ML-17; C3 One Sangamo fixed condenser, 0.001-mfd; C4, C5, C6. C? Four Parvolt by-pass condensers, 0.5-mfd. ; Li One Hammarlund r.f. choke coil, type RFC-85; Ri, One Carter tapered volume control, 3000-ohm, type TP-3M; R2, R3 Two Durham Metalized resistors. 0.25-megohm; Ri One Durham Metalized resistor 0.1-megohm; Rs, R6 One Durham Powerohm, 100,000-ohm, 1-watt: R7 One Durham Powerohm, 50,000-ohm, 1-watt; Ti, T2 Two Hammarlund r.f. transformer, type SGT-17; Sw One Carter battery switch, type 2 ; One Hammarlund drum dial, knob-controlled; Six Benjamin sockets, type 9040; Two Yaxley phone-tip jacks, type 422; One Yaxley cable connector, type 660; One Hi-Q 29 Junior foundation unit containing panel, shields, chassis, coupling condensers, resistor units, resistor clips, binding posts, shafts, wire, screws, etc. • february, 1929 . , . page 269 •