On my first ever amp, which was a very tiny 15w amp, the distortion worked by cleverly using the Volume and Master Volume knobs. Granted it doesn't sound anything like the effects I have had in the past but it did give me a taste for distortion that I didn't have before that.

The way it worked was that if you move the Volume knob up to the highest setting, which is 10 on most amps :D, and then used the Master Volume to set the actual volume, you get a over-driven sort of sound.. And if you have it other way around (ie. Master Volume set at max, with Volume controlling the real volume), then you end up with the standard clean sound.