from some random interview on the net:
GW: One music-oriented question before we move on to "Houses of the Holy": Tell
me how you got that sound on Black Dog.
Page: We put my Les Paul through a direct box, and from there into a mic
channel. We used the mic amp of the mixing board to get distortion. Then
we ran it through two Urie 1176 Universal compressors in series. Then
each line was triple-tracked. Curiously, I was listening to that track
when we were reviewing the tapes and the guitars almost sound like an
analog synthesizer.
http://www.iem.ac.ru/zeppelin/docs/interviews/page_93.gw