As far as I know, Angus was using his stock SG thru a Marshall for "Let There Be Rock". I have heard something about his Marshall Head catching on fire while recording that track. That may explain the "burning" sound he is getting on that track.
Ya, he pretty muich goes stock, and his ampp did catch fire, I actually don't realy like his tone on that whole record!! Although its OK...but he has had WAY better tones than that... Angus is awsome, and so is Malcolm.