I was asked recently, in a YouTube comment, why soloing with the Major scale over power chords does not sound good, but a minor pentatonic does sound good. A C Major scale and C minor pentatonic were being used by the person with the question.
Without knowing the exact chords being used to solo over, I can only make a guess. It sounds to me that the chords being used are not in the key of C. If the chords were in the key of C, playing a C Major scale on top of those chords would fit perfectly. Because the C Major scale does not fit over the chords being used, and the C minor pentatonic does, I would guess that the chords are closer to being in the key of C minor.
The power chords that can be used when soloing with the C Major scale are C5, D5, E5, F5, G5, B5, and A5.
You can mix and match any of those chords and you will be playing in the key of C. This allows you to solo over the chords using the C Major scale. The C minor pentatonic would not work over some of these chords, but some of them would be fine.
The C minor pentatonic uses the notes C, Eb, F, G, and Bb. You can use a power chord with any of those 5 notes as root notes, but that doesn't mean those are the only chords that can be used.
A minor pentatonic can fit over a lot of different things. Because a pentatonic scale only has 5 notes, you can use the scale over a lot of things where a Major scale would not fit. Remember, a Major scale has 7 notes.
The minor pentatonic can fit over so much in fact, you can find 3 different minor pentatonic scales within any Major/minor key.
In the key of C you will find an A minor pentatonic, D minor pentatonic, and an E minor pentatonic.
Going over several possibilities of where the minor pentatonic can be used would be a big, long lesson. If you are interested in seeing this lesson get put together, please let me know!
In conclusion, if you are soloing over a group of power chords and the Major scale sounds bad, but the minor pentatonic scale sounds good, you are probably not playing in key with the chords when using the Major scale. Also, maybe you just don't like the sound of the Major scale!
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John Taylor - guitar and bass instructor for Mile High Shred.