Acoustic 220 PA questions

[gbono]

I finally have this PA working again but have a question about the AGC circuit. Are Q1/2 some sort of detector used in the AGC loop? How does the AGC work in this design?

This amplifier just about puts out 18Vrms (@2.7 ohms) before the positive part of the input signal starts to degrade. I'm also only getting 60 V out of the secondary at 120 V on the mains. I'm going to assume this was typical ACC production control in 1977.

[J M Fahey]

Are Q1/2 some sort of detector used in the AGC loop? How does the AGC work in this design?

No, the detector is Q11; Q1/Q2 are the Fets that attenuate the input signal.
I'm also only getting 60 V out of the secondary at 120 V on the mains. I'm going to assume this was typical ACC production control in 1977.

Then Acoustic had excellent production control, that's the correct secondary voltage.

This amplifier just about puts out 18Vrms (@2.7 ohms) before the positive part of the input signal starts to degrade.

Increase the drive slightly until the other peak just starts to clip and re-measure.
Remember this a MI amplifier and max. power was specified at 10% distortion.
Anyway you measured 120W RMS into 2.7 ohms , probably will get around 160W into 2 ohms, not bad at all.
And when you measure as specified by Acoustic, you might get closer to 200W.