R.L. Drake

Drake Mods - Receiver Gain and AGC

Authored by VE3EFJ


    Periodically you will read concern regarding 'gain balance' in a receiver and how mucking with the blanker gain, for example, can upset the gain balance in the receiver. It is a concern. One should not confuse gain with sensitivity. Although both are related, a sensitive receiver is noted for high gain and low internally generated noise. Once the receiver is sensitive enough to increase its white noise on preselector peaking more gain just makes the noise louder, but the ratio of noise increase will remain much the same. The various RF stages that comprise a receiver (RF amp, mixers and IF) act as a unit. Each stage acts as a signal conditioner as the desired and undesired signals are amplified and filtered prior to detection. With a properly designed receiver, increasing the gain in one unique area through modification ultimately affects the AGC'd stages since they are part of this entire loop. One typically mistakes a higher S meter reading after modification to mean 'more sensitivity'. This quite often is the receiver attempting to compensate through the AGC. With the AGC now more active on weaker signals and with the different AGC characteristics of the vaccuum tubes now receiving AGC sooner than designed, the receiver is actually now partially shut down.

    AGC is very important in a receiver. Factors that affect AGC performance are loop gain, hysteresis, decay time and filtering. Close inspection of the R4 series receivers will reveal use of vaccuum tubes with different Gm curves (sharp and remote cut off) and different AGC filter time constants to each section. This accounts for the excellent AGC characteristics of the receivers.

    If one dives into this equipment making mods 'for more RF gain' or 'reduce the AGC pumping with sharp filters', these AGC relationships in the receiver as a whole get skewered and your Drake will not be any better. Neither will it sound like a Drake anymore. While almost any AGC is better than no AGC, excellent AGC requires attention to detail. The results are worth the design and R&D effort.

    For fun, place an R4B next to any mid priced foreign transceiver and try an A/B comparison. Now, an R4B cannot compete in the selectivity sweepstakes against a radio 20 odd years younger, but have a listen to what the B sounds like and watch/hear the AGC do its thing. If you listen carefully you can hear the AGC recover and the receiver open up and recover from a strong SSB or CW signal. It actually 'breathes'. This is good, well engineered AGC.

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