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Peak-level control

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Dave Mann, ex-W3MBY and now W6HLY of Woodland Hills, has a worth-while method for insuring that his s.s.b. rig isn't hit hard enough to be driven beyond the linearity limits. He finds this very useful with visitors, and particularly so with any phone-patch work, where the level may be down 20 dB or so from the station microphone. The pertinent parts are shown in Fig. 1, and it is no more than an output (or high-level) rectifier that is used to control a low-level r.f. amplifier. In Dave's case, some of the r.f. developed at the 807 is rectified and fed hack (through level-setter R3) to the grid of a variable-μ r.f. amplifier following his sideband filter. The gain of the amplifier is set by R1, but the fed-back control voltage is set by R3.

C110 nF
C2,C62 nF
C315 pF mica
C4100 pF mica
C5100 nF
R110 kΩ potentiometer
R2220 kΩ, 1 watt
R31 MΩ volume control
R41 MΩ
R512 kΩ

Fig 1
Fig. 1. This peak-level control circuit, used at W6HLY, will readily handle variations in audio input over a 20 dB range.

The crystal rectifier is biased (through divider R4R5), so it doesn't rectify immediately but only when this bias is exceeded. Thus the effect is more like limiter action than straight compression, but without introducing appreciable distortion. The capacity divider, C3C4, makes the r.f. voltage applied to the rectifier substantially independent of frequency.

The control circuit cannot be installed in every existing exciter without modification, because it requires that the existing exciter have 20 or 30 dB of gain to spare. However, installing the control tube in an existing design will insure that this requirement is met. In the interests of good linearity, the controlled tube should be used in the circuit at a point where the signal level is low (less than 1 volt). To avoid feed-back troubles, the controlled stage should be on a frequency different than that from which the control voltage is derived.