Watch the Altered Scales online guitar lesson by Robben Ford from Blues Revolution
In this section of the course, Robben devotes a lot of time discussing and presenting scales and their applications. Thus far, Robben has presented many altered chords, which are the chords where the 5th, 9th or 13th has been raised or lowered -- now he drills down on the scales to play over those altered chords, and this helps to unravel the signature Robben Ford sound.
For diminished chords or chords that contain the notes of the diminished 7th chord (such as the dominant 7 b5), the diminished scale works perfectly.
The diminished scale is simply a series of whole step - half step pairs. For example, over the G13b9 chord it is Ab - Bb - B - C# - D - E - F - G - Ab.
Any altered chord wants to go somewhere. They need to be resolved. For instance, G13b9 wants to resolve to an unaltered C chord such as C9 or C13.
A G b13 resolves to a C7 or C9, and it takes the other common altered scale. This scale has two names, the more descriptive being "half-diminished, half-whole tone" meaning that the first half comes from the diminished scale (half step - whole step - half step) while the second half is all whole tones. So over G b13b9 the scale is G - Ab - Bb - B - C# - D# - F - G.
Another name for this scale is the ascending melodic minor scale starting a half-step up (that is, over Ab for a G chord). So, the ascending melodic minor scale of Ab is Ab - Bb - Cb (or B) - Db (C#) - Eb (D#) - F - G - Ab. Either way you think of it, it's best to start playing it on G to keep G as the tonal center.
Notice too that G b13b9 has the same top notes as C#9. Since a common blues move is to move a C9 up a fret (to C#9) and back, we can play the same altered scale over that C#9 as we would over G b13b9. And again, just changing the bass gives us a whole new chord that we already know how to play. Robben ends with a beautiful arpeggio that fits nicely over this altered chord.