Its right that the old echoplex was the original, after that he used and tried a load of different stuff.
The "more involved" setup Henri mentioned was a system of gates and delays which were volume activated. This was also the reason why there were a bunch of AC30's at the live gigs.
Basically this system tried to copy the system devised in the studio to make the sound get bigger and bigger when the volume went up. This system was typical for the wide dynamic rance guitar sound
The way this was done is by placing one AC30 in a long corridor. close milked. Then a series of mics at set intervals, each subsequent one gated a bit more, so the final one was gated most and only opened up when Brian turned his guitar way up.
Play low volume and you get just the close milked one. Turn it up a bit and the gate of the 2nd mic, some 8 ft away, opens. The distance crates natural delay and the sound get bigger. Louder and the next one kicked in etc.