Ahhh......a semi-theory thread. My absolute favorite; you see theory is my thing. What Jehle and Snork say is almost the same thing. Jehle's "Magic Scale" is a great idea, used not just by Morse, but by Eric Johnson too (check out the "Total Electric Guitar" video - an all time classic). Now we can also say SRV uses this idea too since Dorian = minor pentatonic + two tones. Here's the breakdown:
Minor Pentatonic: 1, b3, 4, 5, b7
Major Pentatonic: 1, 2, 3, 5, 6
Dorian: 1, 2, b3, 4, 5, 6, b7 - All we have added from MP is the 2, 6! Now, here's something you probably didn't know.....
If you play the first "shuffle" progression that like Johnny B. Goode and whatever else uses....you know (without having me tab it out) the one where you play the root fifth interval (two note "power chord" 6th string, 5th fret and 5th string 7th fret) and you slap your pinky on the 5th string, 9th fret to create the shuffle, and then you do the same thing for the D(5th string, 5th fret and 4th string, 7th fret) and then the E (you get the idea) here's what you get in the progression:
The A: 1, 5 and then 1,6 so, A and E then A and F#
The D: 1, 5 and 1,6 is D and A then D and B
The E: 1, 5 and 1,6 E, B and E, C#
Now if you play a minor pent solo over this....doh! Do you see what is implied? Through the A and D chords you imply Dorian. Why? the F# in the A chord dealy is the 6th degree of A, and the B of the D chord dealy is the 2nd degree of A + minor pent and you have Dorian. What about the E chord? C# is the 3rd degree, not the b3. This would imply a major tonality. This the the doh! This is not a problem, play the minor pent.....the major/minor tonal ambiguity is a trademark of the blues. If you just played the root-fifths and didn't do the pinky thing, then switched back and forth between Major and Minor pentatonic you would get the same effect. I suppose this is what Snork means by Clapton = major pentatonic. I know for a fact he plays plenty of minor pentatonic along with that major.
Hope its not too confusing. There's more to the minor pent + two tones deal. You can get Aeolian (natural minor) and Phrygian the same way. I won't torture you with any more for now. Have fun!