[MrSandMan]

You mean the circuits inside the amp or the button you push?

Some amps have gain stages, others (like vintage Marshalls) you just drive them by cranking up the volume. Actually, driving the amp is a really cool method to get a Brown Sound distortion (aka overdrive). Peavey's 5150 is really good at that, hence why it is a Van Halen Sig amp. But they also have gain stages to produce some massive distortion.

Not sure if this answers your question, if not, sorry about the ramble :winkx: