What does the detector tube do? What’s automatic volume control? How does AVC work? Antique radio restoration.
This section of our schematic shows the detector and AVC circuits. Let’s start with the detector. The detector circuit, detects the audio signal on the IF, so it can be amplified and sent to the speaker.
The audio signal exists in the positive part of the IF’s signal and in the negative. The positive and negative audio signals are exactly the same, but opposite in polarity.
The detector must remove one polarity of waves or they will cancel each other out, and no sound will be heard. As you’ve learned, removing one polarity of alternating waves is called rectification.
The detector tube has a rectifier section to handle this. It also has an amplifier section to boost the audio signal. That’s why the detector is also sometimes called the first audio amplifier tube.
Here’s how the circuit works.
The signals from the IF transformers are connected to the two plates in the detector tube. As current flows to the plates from the cathode, the negative waves of the IF are blocked, and only the positive portions can pass. For more details on how this works, please see my earlier segments on rectification and the rectifier tube.
To isolate the audio from the IF, the signal passes through capacitor C18 which filters the 456 kilohertz carrier frequency away. The remaining audio signal is sent to the volume control R5 which adjusts its strength, and passes it to the grid of the detector tube, where it is amplified and sent to the next tube.
Now let’s look at the AVC section of the circuit which stands for automatic volume control. As we learned, early radios required lots of adjustment to tune, but the volume control also required constant adjustment to compensate for constantly changing radio signal strengths.
AVC is an ingenious design that does this volume compensation for you. It’s so useful that every radio you’ve ever used probably had a similar circuit. Without it, a radio’s volume will never stay stable and the the volume differences between weak and strong stations would be tremendous.
Here’s how it works. As we’ve learned, the audio signal comes from this point in the volume control. I’ll discuss the volume control in more detail in the next section. When the signal at this point is positive, the signal at this point is negative. Both signals are exactly the same, but opposite in polarity.
The AVC circuit takes this opposite, negative signal, and sends it to Resistor R4 and capacitor C16, which adjust the voltage and smooth the signal. From there, the negative signal is fed back to the secondary coils of the antenna and first IF transformers. This reduces the signals going to the converter and IF tubes, which reduces their output and volume level.
So, when the volume gets too high, AVC voltage increases, causing tube current, and volume to go back down. When volume gets too low, AVC voltage is reduced, current flow increases, and volume goes back up.
This automatic volume control happens instantaneously and in the background, keeping volume output at a steady level regardless of signal strength.
Of course, you can still control overall volume manually using the volume control. Let’s give it a closer look in the next section.