Watch the Eric Clapton online guitar lesson by Jeff McErlain from Kings of Tone
Clapton's basic pedal setup is very simple for the Cream era. It is basically a Wah pedal. Yup that's it. I know people often say he used a Rangemaster to boost the front end of his amplifiers but that hasn't been confirmed as far as I can tell. I can tell you that when you crank a Marshall of the vintage he is using, it does get to those levels of distortion by themselves, especially if you are using humbucking pickups. So I tend to believe that apart from the wah he was plugging straight into the amp.