What does this thingy do?

[Axtman]

What does this vactrol do in the circuit? This is from an Ampeg BA115 combo amp schematic. If I were to venture a guess I would say it is a limiter of some sort. Usually I see vactrol as a photo resistor in a tremolo circuit.

[Enzo]

That is exactly what it is. Your snippet excludes the circuits that explain it. The line into the top is the amp output. ANything over about 35v there will cause the LED to light in the device. And that shunts across the unseen op amp below. Said op amp being the input to the power amp. Q6 is unrelated, and is the power on mute to prevent thump.

[Axtman]

Thanks Enzo. Is this circuit to prevent hard signal clipping or to prevent the signal from blowing out the output transistors or both?

[Enzo]

Mainly clipping I would think. There are zeners on the MOSFET gates to prevent over driving them with signal. At least that is how I look at it.