Fig, 5—This four-component circuit switches time constants In the AVC line to make it suitable for either AM or SSB-CW reception. See tex+ -for discussion of Rl-Ct values and adjustment of attack and release times.

to pad the output back down to the nominal 1-volt level to eliminate readjustment of the audio gain control when switching from AM to SSB and back again,

Advantages of the sheet-beam detector include its high output, its resistance to overload, its circuit simplicity, and its low intermodulation distortion (where it's even better than the Crosby or the pentagrid, and is far superior to simple diode detectors).

Disadvantages are the requirement for push-pull circuitry, need for a separate BFCX and, most important, possible difficulties in obtaining the special tube used.

.Before leaving SSB detectors and moving into AVC circuitry for use with them, one more circuit deserves mention: Webbs "synchronous detector" adapter which allows reception of not only SSB and CW signals, but DS13 transmissions as well. While its ad vantages are numerous, so are its disadvantages: it is complex, expensive, bulky, and somewhat tricky in adjustment; Full details are found in the references; it's a full article in itself, All the SSB and CW detectors described here (with the exception of that shown in Fig. 1) share a common disadvantage—they make no provisions for AVC.

While AVC is no necessity for reception, no one will deny that it makes listening easier —especially in a round table sort of operation where some stations have strong signals and others are weaker. A good AVC which brings them all to common level eliminates blasting of the eardrums.

If your receiver is a Super-Pro or a similar design, using separate channels for signal and for AVC, there's little problem. In fact, such a receiver can be used with no change at all, especially it separate switches control the AVC and the BFO functions. However, the attack and release time constants best suited for AM use lead to a distinct "thump" on each syllable of a SSB signal, and put a chirp on everv CW station, w

The circuit of Fig. 5 puts an end to such problems. The diode {which must be a vacuum-tube type) presents very small resistance in one direction but almost infinite resistance in the other. Connected as shown, it provides an A\ C attack time measured lu microseconds, but release time stretches out to be almost in seconds. Thus, the AVC can cut back instantly when a strong signal arrives, but gain won't he restored between syllables (or between dits of CW). Between words, normally, the gain will return.

Roth the attack and the release times can be controlled independently by adjustment of values of CI and RL CI controls attack time; increasing its value makes the AVC take longer to respond to a signal. With CI set at the proper value, Rl determines release time. Increasing its value increases recovery time between signals. Values shown in the figure have been proved in practice, though they may seem unduly large.

Since SSB and CW AVC aren't suited to AM reception* the switch shown in the schematic is used to restore normal AM action. It is ganged to the FUNCTION selector on the front panel, or may be an added control.

This system of AVC won't work unless your set has separate if channels for signal and AVC lines. To install AVC for sideband or CW on a single-channel set, you have tluree choices: you can build up a complete separate if channel (which is bulky, expensive and

Fig. 6—This chanrieUsplitter (two cathode followers with paralleled inputs) allows the same if strip to be used independently for signal and for AVC. It is connected between the Ust if transformer and the detector Inputs. Interaction between the detectors is minimized by its isolating action.

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