As I recall, it's just a I-IV-V w/ power chords. Here it is in E.
(I don't remember if that's the original key or not, if not, try C or D)
It starts with a pick slide then:
E E E E B B B B A A A A E E E E in rhythm.
The "interlude" part is just those same chords arpeggiated, and the
"reggae" part with the humming is the same exact chords with an echo/delay
and a backbeat feel.
What the hell was I thinking. . . . What I posted before was in A'.
Here are the proper chords for P.D in E:
Chords Texts STRADLIN IZZY Pressure Drop. Chordsound to play your music, study scales, positions for guitar, search, manage, request and send chords, lyrics and sheet music