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Vintage Radio

The development of AGC (automatic gain control) circuits in the late 1920s and early 1930s was an important milestone in domestic radio receiver design. It allowed stations to be received at the same volume when tuning across the band, regardless of signal strength.

By Rodney Champness

When radio (or “wireless”) first made its appearance, it was necessary for receivers to use every bit of RF (radio frequency) and audio frequency gain available. However, as receivers became more sensitive and gains improved, this situation quickly changed. Receivers became much more capable of pulling in weak signals but there was one drawback. If a receiver was tuned to a weak signal, the gain then had to be manually reduced when tuning to a strong station in order to produce the same audio output level or volume.

On a set with a loudspeaker, there would be a burst of quite loud and probably distorted audio until the volume control was hastily wound back. That was bad enough but if the listener was using headphones, their ears would be ringing for quite some time afterwards.

Once caught, most listeners would turn the volume control down while they tuned across the band. This meant that strong stations were easily heard but it had the disadvantage that weak stations might be inaudible. So it was a compromise as to just how far the volume control was wound back while tuning.

DXing

In the early days of radio, many listeners became interested in the hobby of “DXing” which involved receiving and identifying distant stations. Of course, these stations were much weaker than any local stations and it was all too easy to get a sudden loud burst of sound from the loudspeaker as one of the local stations was tuned.

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The Mullard Meteor 600 4-valve receiver (circa 1947) was another economy receiver with no AGC. The volume was controlled by varying the back-bias to the first two valves (both variable-mu types) in the line-up (ie, to the ECH35 converter & EBF35 IF amplifier stages).

Coupled with the inevitable static crashes, this ran the distinct risk of not only damaging the loudspeaker but also frightening hell out of the listener (and any innocent bystanders). Permanent damage to the listener’s hearing was also possible if headphones were being used.

To minimise this problem, it was therefore necessary to keep one hand on the volume control as the set was tuned. That made tuning receivers with no AGC (automatic gain control) a rather tedious and awkward job.

In the simpler receivers, the regeneration control acted as the volume control but more complex receivers did have a separate audio-stage volume control. In fact, many of the more complex receivers were likely to have both audio and RF stage gain controls.

Another problem with early receivers was the variations in audio level due to signal fading. At night, the signal strength from a distant station often varied continuously, from almost non-existent at times to quite high at other times. This signal “fading” was inevitable and forced the listener to continuously vary the volume control to keep the audio at an almost constant level.

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