Radio Broadcast (May 1928-Apr 1929)

OCTOBER, 1928 THE "HI-Q 29" 343 100,000-ohm potentiometer, Ri, connected across the 45-voIt B supply. The center tap of this potentiometer provides a variable voltage which is impressed on the screen grids of the two r.f. amplifier tubes. The amplification obtainable from these tubes varies within wide limits as the voltage on the screen grids is varied, being at maximum around 45 volts and dropping rapidly as the screen-grid potential is reduced. This provides a very smooth control of volume within wide limits without affecting quality or tuning in the slightest degree. While the screen-grid tubes have a very low value of capacity between plate and grid, thus almost entirely obviating the tendency to feedback through the tubes themselves, this advantage is nullified if feedback occurs in other parts of the receiver. Taking this into consideration, every effort has been made to isolate all circuits in which coupling might result in instability. The negative bias for the grids of the r.f. tubes is secured by the drop across individual 10-ohm resistors, R$ and R8 in series with the negative leg of each screen-grid tube filament. Since the screen grids of both these tubes are biased by the 100,000-ohm potentiometer, Ri, 5000-ohm isolating resistors, Rio and Rn, are inserted in the lead to each of the screen grids, which are in turn bypassed by means of separate 0.5-mfd. bypass condensers, Cs and Cio The plate circuits of these tubes are likewise isolated by individual filters consisting of separate radio-frequency choke coils, Li and L2, and bypass condensers, C9 and Cn. In addition to the above mentioned precautions the entire r. f. end of the receiver is thoroughly shielded. Each stage is entirely enclosed in a snug fitting aluminum box which is securely fastened to the metal chassis. The screen-grid tubes are so located that the leads to the control grids are as short as possible and well removed from the plate leads, which are also very short. By placing these tubes between the cans as shown in Fig. 4 the can sides are used also as tube shields, effectively preventing coupling between the tube elements and other parts of the circuit. This arrangement provides the minimum coupling between output and input circuits, which is extremely important. The audio amplifier is of the conventional type consisting of two stages transformer coupled. The a.f. transformers, Ti and T2, have a very flat frequency characteristic over theusual a. f. range. An r.f. choke coil is placed between the plate of the detector tube and the first a.f! transformer to prevent any stray r. f. voltages from getting into the a. f. amplifier. A 171 type tube is recommended for use in FIG. 4. HOW THE INSTRUMENTS ARE MOUNTED the last stage, although other types may be used if suitable \, B and C voltages are available. a. c. OPERATION ALTHOUGH the above description is based on the battery operated model, a complete all electric model has been developed. Arcturus a.c. screen-grid tubes are used and some slight changes in wiring are required. Otherwise the operating characteristics and set performance of the a.c. and d.c. models are identical. Constructional data on the a.c. model may be obtained from the kit manufacturers, or through Radio Broadcast. Fig. 4 is a top view of the a.c. set, which is similar in general appearance to the d.c. model. LIST OF PARTS THE parts used in the d.c. laboratory model of the Master Model "Hi-Q 29" receiver are listed below. The coil data is given in Fig. 3 in case it is desired to make these at home. All the other parts are of standard design, and substitution of other makes of equivalent parts may be made. Ai, A2, A3 — 1 Hammarlund coil set, No. HQ-29 O to C5 — 5 Hammarlund midline condensers, 0.00035 rnfd., No. ML-17 C6 — 1 Sangamo fixed mica condenser, 0.00025 rnfd. C7 — 1 Sangamo fixed mica condenser, 0.001 rnfd. Cs to Cn — 4 Parvolt bypass condensers, 0.5 rnfd., series 200 J — 1 Pair Yaxley insulated phone tip jacks, No. 422 Li, L2, L3 — 3 Hammarlund radio-frequencv chokes, No. RFC-85 Ri — 1 Carter "Hi-Pot" potentiometer with switch, 100,000 ohms, No. 11-S R2 — 1 Durham Metallized resistor, 2 megohms R3, R4, R5 — 3 Amperites, No. i-A Re, R7, R», R9 — 4 fixed filament resistors, loohms (included in foundation unit) Rio, Rn — 2 fixed resistors, 5000 ohms (included in foundation unit) Ti, T2 — 2 Thordarson audio transformers, No. R-300 Y — 1 Yaxley Cable Connector and Gable, No. 660 2 Hammarlund knob-control drum dials (walnut), No. SDW 5 Benjamin Cle-Ra-Tone sockets, No. 9040 2 Eby engraved binding posts 1 "Hi-Q 29 Master" foundation unit (panel, shields, chassis, shafts, binding post strips, clips, fixed resistance units R6> R7, R«, R9, Rio and Rn, resistor mounts, and all special hardware required to complete receiver.) To place the set in operation the following apparatus is necessary. 2 cx-322 tubes 2 CX-301A tubes 1 CX-371A tube Source of A, B, and C voltage (6 volts A, 45, 90, 135 volts B, 4j and 27 volts C) FIG. 5. THE CIRCUIT OF THE "HI-Q 29"