What is a scg channel on a amplifier?

Sequential Cascading Gains - Fancy way of saying distortion. Usually, the distortion channels built in on amps sucks. It's better than nothing, but if you can, get a pedal that suits the sound you want. You can get a cheesy metallica sound using the SCG channel by turning the low & hi EQ knobs all the way up and the mid EQ knob all the way down.