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AUGUST, 1928
A DUAL CONTROL A. C. RECEIVER
193
Cable
FIG. I. THE PICTURE WIRING DIAGRAM OF THE RECEIVER
fier is that the various circuits have been stabilized by means of grid resistors. In the final stage a neutralizing condenser is also used. Without the neutralizing condenser in the circuit or with the condenser improperly adjusted the amplifier will oscillate over the entire band. With the neutralizing condenser correctly adjusted, however, the circuit is entirely stable.
The volume control is located in the r.f. amplifier and consists of a variable resistance across the primary of the third r.f. transformer. Care is necessary in the selection of the method of volume control. A filament rheostat on the r.f. tubes would not be very satisfactory because the output voltage of an A-power unit increases when the load is decreased, and therefore, if the volume is lowered by decreasing the filament voltage on the r.f. tubes the filament voltage across the
other tubes in the set would rise above normal and shorten their life considerably. A resistance in series with the B-plus lead to the r.f. transformer is also unsatisfactory because it will cause wide variations in the voltages applied to the plates of the detector and first audio tubes. A variable resistance across the primary of an r.f. transformer has none of these disadvantages. It operates very smoothly and does not cause any changes in the plate or filament voltages applied to any of the tubes.
An r.f. choke is placed in the plate lead of the detector to keep r.f. currents out of the audio amplifier, and a small 0.002-mfd. fixed condenser
1-2.1-4,1.6,1.8 all similar coils, consisting of 85 turns of No. 24 d. s c. wire on 2 inch tube
Li . L 3 , L 5 similar coils, consisting of 10 turns of No. 28 d.s.c.wire
I.7 30 turns No.28 d.s.c.wire, with tap at center
FIG. 2. THE COIL DATA
bypasses the radio-frequency currents from the plate to the filament of the detector tube.
As was mentioned previously, an extra socket is placed in the receiver so that a phonograph pick-up may be used. The plate terminal of this
SOCKET
FIG. 3. THE RECEIVER FROM ABOVE