http://www.ericjohnson.com/flash/amps.html

Try the above link, it shows his effects setup and layout. You'll also find a bunch of other stuff on that page about him and his stuff. I've never been too good at replicating sounds.. at least on purpose.. sometimes I just accidentally get the sound..

My guess is the Tube Driver plays a big part in the scream.. but I am not that good at effects..