Metallon

Hmmm, not sure exactly how you wanna do it, and I don't know fully how it works with a twin channel JCM 800, as I hear this is supposed to work with a single-channel JCM 800 (although a hot rodded one). But yeah, lead channel, max out at least the pre-amp gain and I suppose the power amp gain as well, control the overall volume with the master volume, put the OD in front of your amp, crank the volume on the pedal and back off on the gain (but not all the way).