The receiver shown in Figs. 5-35, 5-36 and 5-38 is a two-band (80 and 40 meters) four-tube (2X4) receiver plus (+) provision for receiving WWV at 5 Mc. A dual-crystal filter provides good adjacent-channel selectivity, and two tuned circuits between the antenna and mixer give image rejection of greater than 70 db. on both bands. The receiver can be used on other bands by the addition of a crystal-controlled converter.
Referring to the circuit in Fig. 5-37, the pentode portion of a 6EA8 is used as a mixer, with the triode portion of the same tube serving as the oscillator. The I.F. is 1700 kc. and the oscillator tunes 5.2 to 5.7 Mc.; tuning the input circuit to the 80-meter band brings in 80-meter signals, and all that is required to hear 40-meter signals is to swing the input tuning, C1, to the low-capacitance end of its range. Although, e.g., a 7.0-Mc. (5.3 + 1.7) and a 3.6-Mc. (5.3 - 1.7) signal will appear at the same setting of the tuning dial, the two signals cannot be received simultaneously because the double-tuned circuit between antenna and mixer grid provides the neces-sary rejection. Coupling between the two circuits is provided by a small common inductor, L4. A series inductor in the antenna lead makes the coupling more nearly constant on the two bands (reduced antenna coupling at 7 Mc. compensates for increased Q of L5). For the reception of WWV at 5 Mc., a small relay, K1, cuts in additional capacitance across the oscillator and brings its frequency to 3.3 Mc.
The mixer is followed by the dual crystal filter at 1700 kc. and a stage of amplification. I.F. gain is manually controlled by a variable bias control in the cathode circuit of the 6BA6 I.F. amplifier stage. One end of the gain control is brought to a jack, J2, for possible use of the break-in or muting circuit of Fig. 8-9; if the circuit is not utilized the end of the gain control is grounded by using a shorting plug in J2.
One triode of a 6CG7, V-3A, serves as a grid-leak detector and the other is used as the B.F.O. A two-stage audio amplifier follows, providing low-impedance output for low-impedance phones or speaker when connected as shown; if high-impedance phones are used they should be coupled to the plate of V-4B through a 0.02 mf. capacitor, as shown by the dashed lines. The audio power is sufficient to give more than enough headphone volume and quite adequate loudspeaker volume in a quiet room.
The power supply uses silicon diodes in a bridge rectifier circuit, and a 0B2 provides stabilized +105 volts for the high-frequency and beat oscillators. Capacitor C-5A appears to be in an unusual place in the circuit, but it is required to eliminate a slight hum that is present without it. No A.C. line switch is included because it is assumed that the receiver will be turned on by a wall or other switch. The receiver is fused in the line plug, P1.
Fig. 5-35-The 2X4+ receiver uses four tubes and tunes 3.5 to 4.0 Mc. and 6.9 to 7.4 Mc.; the range is selected by the setting of a two-section input tuning capacitor housed in a Minibox (extreme right). The toggle switches under the input tuning knob turn on (top) a 5-Mc. range for reception of WWV and (bottom) the B.F.O. Knobs either side of the central main tuning knob control (left) audio gain and (right) I.F. gain. B.F.O. frequency is set by the knob at the left.
Power-supply components, including on unshielded VR tube, are mounted along the rear of the chassis. The mixer/oscillator 6EAB tube is at the right, near the input tuning housing, and the audio amplifier 6CG7 is at the left, nearer the front of the chassis than the 6CG7 detector/b.f.o.