Deep Purple In Rock: guitar sound?

[nessbass]

Hi folks!

Does anyone here have any information about the guitar sound on Deep Purple's In Rock album? I just recently dug around in old cassette tapes and found a copy of it. I was blown away by that sound! It is so highly resonant and vibrating, just as if the speaker was about to explode! So is there a source of information about the equipment and techniques used, and where can I find it? I'm especially talking about the first track 'Speed King', BTW.

David