6k7 Regenerative Receiver

IClose-wound.

2Spaced to make coil length l}4 inches. Ll and Lz all close-wound with No. 36 d.s.c., spaced H-inch from Lz or Li. Forms are National R-39, five-prong.

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111L iiu.ii-rntroiiM \\<:i: sitkkhet flc. t211 — (,i:.m.i{\l mew

IClose-wound.

2Spaced to make coil length l}4 inches. Ll and Lz all close-wound with No. 36 d.s.c., spaced H-inch from Lz or Li. Forms are National R-39, five-prong.

When everything is aligned the c.w. beat oscillator should be set so as to give about a 1000-cycle tone when heterodyning a signal tuned in "on the nose." Then the selectivity control should be brought up to just below oscillation, as indicated by the "ringing" sound. The signal will increase in intensity and, with tuning through zero beat, the audio image or "other side of zero beat" should be hardly audible. Careful manipulation of the alignment adjustments will bring out this desired single-signal feature to its fullest.

The value of the tickler L s has intentionally been left so that oscillation in the i.f. circuits can occur with the control resistor B9 almost, but not quite,

111L iiu.ii-rntroiiM \\<:i: sitkkhet at its point of highest resistance. The receiver never should be operated with the i.f. self-oscillating.

High-Performance Superhet With Regenerative First Detector

• The receiver illustrated in Fig. 722 is intended to give maximum performance for the number of tubes and circuits used, while combining good mechanical stability and adaptability to amateur construction. It is also designed to accommodate a noise-silencer and crystal filter unit, as described farther on, in case the builder wishes to include all the features available in the modern single-signal type receiver. The circuit line-up, as shown in the diagram of Fig. 723, consists of a regenerative first detector (or mixer) using a 6L7 tube, a separate high-frequency oscillator using either a 6D6 glass or 6K7 metal tube, an iron-core transformer coupled i.f. stage using a 6L7 with dual automatic gain control, a 6H6 duo-diode second detector and a.v.c. rectifier, a 6D6 or 6K7 i.f. beat oscillator, a 76 or 6C5 triode first audio stage and a 42 or 6F6 pentode output amplifier. There is also a 6E5 "Magic Eye" tuning indicator tube which, while not essential to operation of the receiver circuit proper, is an extremely useful adjunct. The receiver operates from a separate power supply, such as the heavy-duty type described in Chapter IS, which must be capable of at least 2.8 amperes at 6.3 volts for the filaments and 90 ma. at 250 volts d.c.

The oscillator-mixer circuit is similar to that shown in Fig. 616 of the previous chapter, the injector (No. 3) grid of the 6L7 being capaci-tively coupled to the cathode of the oscillator. This circuit shows negligible "pulling" effect as the result of mixer input tuning up through the 14-mc. band, and only slight effect at 28 mc., provided the coils are properly adjusted. The single-control tuning system employs the tapped-coil method of band-spreading and tracking with adjustable air condensers for

FIG. 724 — TOP VIEW OF THE HIGH-PERFORMANCE SUPERHET, SHOWING THE LAYOUT OF THE COMPONENTS

setting the range. Regeneration in the mixer input circuit is obtained by a cathode circuit feedback coil coupled to the grid coil, regeneration being controlled by a variable resistor acting as a r.f. shunt across this tickler winding. This method of control is the same as that used in the regenerative i.f. amplifier of the previous receiver. It maintains the electrode voltages constant, in contrast with screen voltage control as used in regenerative autodyne detectors, and has but slight effect on the mixer tuning, even at the higher frequencies.

Only two points need be mentioned in connection with the i.f. amplifier circuit. The No. 3 grid of the 6L7 is connected in parallel with No. 1 for d.c., but not for r.f., and a voltage divider instead of a simple series resistor is used for obtaining screen voltage. As shown in the diagram, the No. 3 grid is returned to the ground side of the i.f. transformer secondary, where it picks up the a.v.c. voltage along with the No. 1 grid. The rather heavy screen voltage divider maintains the screen at practically constant potential despite the bias applied to the grids, thus increasing the effectiveness of both the manual and automatic gain controls. The manual gain control is bled off the plate supply by the usual method.

mixer 6L7

00 to mixer 6L7

Schematic Tube 6k7 Preamplifier

FIG. 723 — THE HIGH-PERFORMANCE SUPERHET CIRCUIT DIAGRAM. SHELL PIN TERMINALS OF METAL TUBES ARE ALL

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