07-09-2025, 05:05 PM
X1 is a dampening diode. It increases the range of gain reduction the AGC can achieve before reaching the point where the transistor is no longer working in class A. I mentioned them earlier (post #12).
As the AGC drives TR2 towards cutting off, the voltage across R9 falls. At some point, this allows X1 to conduct, reducing the voltage from TR1. There's often a low-ish value resistor (hundreds of Ohms) in series with the diode.
I see it "stage 2" in the evolution of AGC in transistor sets (after what was seen in sets like the BC506). Stage 3 was - to me, at least - when the AGC voltage was applied to 2 IF stages - at this point the dampening diode was no longer used. After that, ICs took over.
Of course, that's a simplification, and there is much variation in practice, but a lot of manufacturers did follow this pattern, which of course is straight from the Mullard books of the time. I did have a flick through a couple of mine yesterday, and it's striking just how similar many of these sets were to the examples given in the books - right down to resistor values in many cases. Fair enough, of course...
This is one of my favourites: https://www.worldradiohistory.com/UK/Mis...s-1960.pdf
Page 199 (72 of the PDF) talks about AGC with and without the dampening diode (see figure 5).
As the AGC drives TR2 towards cutting off, the voltage across R9 falls. At some point, this allows X1 to conduct, reducing the voltage from TR1. There's often a low-ish value resistor (hundreds of Ohms) in series with the diode.
I see it "stage 2" in the evolution of AGC in transistor sets (after what was seen in sets like the BC506). Stage 3 was - to me, at least - when the AGC voltage was applied to 2 IF stages - at this point the dampening diode was no longer used. After that, ICs took over.
Of course, that's a simplification, and there is much variation in practice, but a lot of manufacturers did follow this pattern, which of course is straight from the Mullard books of the time. I did have a flick through a couple of mine yesterday, and it's striking just how similar many of these sets were to the examples given in the books - right down to resistor values in many cases. Fair enough, of course...
This is one of my favourites: https://www.worldradiohistory.com/UK/Mis...s-1960.pdf
Page 199 (72 of the PDF) talks about AGC with and without the dampening diode (see figure 5).
