I've started improvising guitar solos over some backing tracks like this one, and the videos always start by stating the chord progression and then saying which scale you can solo in for it to sound good with the backing track. What I can't figure out is: what is the logic that lets you get from the chord progression of the backing track and to the scale of the solo, and vice versa? For example, if I have the chord progression Am-C-E-F, what pentatonic scales could I use to solo on top of it.
TL;DR: Find the home note and home chord.
Do not play random notes from a single scale
First of all, two bad ideas you should try to get rid of:
Bad idea #1: Select notes from a scale randomly.
Bad idea #2: Use one single scale for the whole song regardless of what's happening in the backing chords.
The scale you should think about often changes with the backing chord progression throughout the song. In your example progression Am-C-E-F, during the E chord you can play a G♯ note and it will create a certain effect, but over the F chord you'll most probably want to avoid G♯ except as a short passing note, because it will conflict with the harmony. Over E it will support the harmony, and over F it will fight against it! Know your chord tones.
Then to the actual question: how to get a scale. It's actually quite simple.
Find the default reference scale
Ask yourself these two questions:
- Question 1: What is the home note, i.e. tonic? What is the note that the song is centered on and feels like a home base, and a natural ending for the song. In your example, I think it's A. (could be C as well)
- Question 2: What is the home chord, is it a major or minor chord? In your example, I think it's A minor. (might be C major)
If you know these two, you get your basic, normal, regular, default scale.
If your home note is A and the home chord is a minor, i.e. A minor, then your basic reference scale is A natural minor. Notes: A, B, C, D, E, F, G
If your home note is C and the home chord is C major, then your basic reference scale is C major. Notes: C, D, E, F, G, A, B
It is immensely beneficial to see this as the normal default situation. The vast majority of Western music culture assumes a diatonic scale on the home note as the default reference. In music notation, the default scale is indicated by a key signature, and deviations from it are marked with accidentals.
Knowing the basic reference scale, you know the set of diatonic chords derived from that scale.
If your scale is A natural minor, the basic diatonic triad chords are: Am, Bdim, C, Dm, Em, F, G. If your scale is C major, the basic diatonic triad chords are: C, Dm, Em, F, G, Am, Bdim. (i.e. all the same chords, because A minor and C major have the same key signature!)
Find deviations from the default scale in the chords
Look at the chord progression and see if there are any chords outside the default set of diatonic chords. In your example there is the E major chord, which has the special G♯ note. Your diatonic notes are C, D, E, F, G, A, B. No accidentals i.e. sharps (♯) or flats (♭) in those note names, but G♯ has a sharp, so it's a special case you have to take into account.
Here I've written out the chords of your example progression. As you can see, on the E major chord there's a G sharp note that needs to be specially marked, because it's a deviation from the default scale.
Let's take another example. Suppose it's in Am i.e. A is the home note and Am is the home chord. Our example progression is: Am - C - D - F - Dm - Em - D - Am. You see D major chords ... a D major has an F♯ note, which is not diatonic in your home scale A natural minor! So, whenever the D major chord is playing, you'll know that you have to change your scale. You could change it in other places as well, but at least over the D major chords, an F natural (without ♯) note will probably fight against the backing chords. If you want, you can see the D major as fitting the A Dorian scale, but you can also see it as A natural minor but with the F temporarily sharpened.
What about the pentatonic scales?
Pentatonic scales are used for two reasons. (1) they can sound bluesy and jazzy, and (2) you can "cheat" a little by restricting your playing to harmonically more ambiguous notes, and thus avoid having to know what's happening in the backing chords. And if someone says you're cheating, you can always say you do it because of the bluesy sound, not because it's easier.
In your example progression, you can play notes from the A minor pentatonic scale (A, C, D, E, G) and quite well even a G natural on the E major, and it will sound bluesy (because then there are both a minor and major third for the E chord, which is one of the basic blues tricks). Furthermore, you avoid playing the F and B notes, which can be harmonically sensitive and clash with the backing chords. Just like the G♯ can clash.
Finding the appropriate cheat code scale is easy:
- if the key is X minor, then you use the X minor pentatonic scale
- if the key is X major, then you use the X major pentatonic scale
The minor and major pentatonic scales actually have the same notes. In your example, they would be A minor pentatonic, or C major pentatonic. If your goal in music is bad ideas #1 and #2, then either scale will get you going.
It's probably more useful to know why to use which, rather than which to use.
As a basic, with Am and C in the sequence, Am pent. and C maj. pent would be a good choice. As it happens, they're exactly the same notes! The pents work well, as they leave out the two notes which can be awkward to fit in: the 4 and 7 of maj. pent., or the 2 and 6 of min. pent.
The E and F are slightly more problematic, as they are one semitone apart, meaning there won't be many common notes. In fact, there's E, and A, and that's it !
So, one safe bet would be to use E maj. pent over E, and F maj. pent. over F. No great surprise there!
Now let's look at common notes between Am pent. and E maj. pent. A and E again. However, if a Blues tinge is needed, there's D (m7 of E) and G (m3 of E) that are available in Am pent. I'll let you work out what's common between Am pent. and F maj. pent!
We could go deeper, and say that just because a note isn't in an appropriate scale doesn't mean it can't be used. All twelve notes are available (and those 'in the cracks' for guitarists and sax players for a start), but you specifically ask about pents. And while some notes from one scale aren't oficially usable (!) they will add some colour to anyone's playing - used intelligently.
And - don't think of scales as much as a bank of notes from a scale. A lot of players fall into that trap, and their playing consequently sounds scalar.
A pentatonic scale is just a normal 7-note one with the two 'wrong' ones taken out. What we used to call the 'avoid notes'. But restricting yourself to those 5 notes just means you'll play something that doesn't sound wrong, not something interesting.
Look at all the available notes (and that's ALL of them!) and think what their place would be over a C major chord. Obviously C, E and G won't jar. D and A are fairly neutral, there's no place they particularly want to go. But F and B are far from neutral - F has a strong tendency to move to E, B to C. So, if you want to emphasise the C major chord, stick primarily to C, E and G, also allow D and A, (Hey! We've found that pentatonic scale!) avoid F and B. (Another way to look at it is to avoid notes a semitone away from a chord note. Same result.)
But don't just run up and down those 5 inoffensive notes. Hit one of the 'wrong' ones just for the pleasure of resolving it to a 'right' one. (Or don't - leave it hanging!)