JCM 800 help!!! Please!!! expirienced guitarists???!!?

If you have the preamp cranked on this and you aren't getting distortion, then there is probably something wrong with the head. I would recommend taking it to a good technician so it can be repaired. It might be something as simple as replacing the tubes, but it could also be a lot more complicated. A pro will be able to figure it out for sure.