When playing a tune that has both, minor and major chords, what scale would be safe to use for soloing? It seems that a Minor Pentatonic would work for the minor chords, but not that well for the major ones
If I understood you correctly, you're trying to play notes from a scale that's always rooted on the root note of whatever chord there is at that moment. And that's why for example if your chords are C - Dm - G - Am, a minor pentatonic scale seems to work nicely on the Am and Dm, but feels a bit funky on the C major and G major chords. If this is what you're doing, there's something quite important you're missing, and that's the concept of key.
Songs are usually in a key, for example C major. Not the C major chord, but the key of C major. (Unfortunately music is full of words loaded with multiple different meanings depending on the context.) Being in the key of C major means two things:
- (1) C is a very special note in this song. It's the so-called tonic, which is a home note, a place of rest, the central perspective where you stand and look at everything. All the notes and chords happening in the song live in a C centric universe.
- (2) Other notes in the song come mostly from the C major scale: C, D, E, F, G, A, B. (Many songs do have notes that aren't from the key's base scale, and those notes slightly change the harmonic feeling.)
If your song is in C (which is another way of saying "in the key of C major"), it might have a chord progression like: C - Am - Dm - F - G - Dm - G - C. During the whole chord progression, your ear only expects to hear notes from the C major scale, and the most expected ending note for a melody would be the C note. All of the notes of all of the chords are from the same scale:
- C : notes C, E, G
- Am : notes A, C, E
- Dm : notes D, F, A
- F : notes F, A, C
- G : notes G, B, D
For soloing on a song like that, you select the C major scale for "safe" notes: C, D, E, F, G, A, B. If you don't know what you're doing and want to be even more safe, leave out the F and B notes (which I highlighted in the chord listing above), which might disagree with the backing chord changes here and there. This leaves you with C, D, E, G, A. These should be quite "safe" to play even completely chaotically.
So, find out what key your song is. Where is the tonic, the home note? When you know that, then you can assess your scale choices, and begin to reason about why and when something works or doesn't work. If you don't know the key, i.e. which note is the tonic, then you're floating around in space eyes closed and don't know which way is up, down, left, right - you're disoriented. You need a reference point to reason about the notes, and the musical key is your reference point.
(If I misunderstood what you're doing, then this whole answer is pointless.)
It would help if you provided an example. In any Key there are a combination of major and minor chords. Specifically the folling list of 7th chords are in each Major key,
IMaj7, ii-7, iii-7, IVMaj7, V7, vi-7, vii-7(b5)
So, in principle if you see anything from this combination would could just stay on the I major scale and be fine. For example, in the changes I, vi, IV, V which is in many Rock ballads and similar to Rhythm changes, you could stay on the I major scale all the way. There is no need to modulate and use the minor scale over the vi chord, etc. If, on the other hand you see I, vii-7(b5), III7, vi-7 then because of the insertion of the Major chord on the iii you really have modulated to the vi and you will hear it that way.
There is a chord stream for the harmonic and melodic minor scales similar to what I posted above for the major scale. I wouldn't pick a scale for each chord but rather use the patterns to determine the key changes if any and stick to the key.
Here is an online tool where you can enter a sequence of chords, and see what scales and modes are a best fit for those chords:
Below the form for entering the chords you'll also find an explanation of how are the scales and modes chosen to fit the different chords.
For disclosure, I programmed that online tool a few years ago, so if you find any mistakes, I'm the one to blame... :)
The minor pent. has been used for decades by guitarists to solo over both major and minor chords.
It's the mainstay for three chord wonders, 12 bars et al.
Let's take a key. A major, when there are simply three chords - A D and E, then A pent. works over most of the song. In fact, a lot of guitarists would make it work over all the song.
One could also use A maj. pent. again for most or all (being judicious) of the song.
The question asks 'what scale?' So in key A major, which may well have the chords A, Bm C♯m D E F♯m, then it makes sense to use the major scale diatonic to those - A.
Or have I missed the point of the question?