Watch the Scale Applications 3 online guitar lesson by Robben Ford from Blues Revolution
Moving on from C#7, Robben considers the tritone substitution of C#9, which has the same notes as G7 b9b13 with C# in the bass. The scale for this chord is the "half-diminished, half-whole step" scale that Robben showed earlier: G-Ab-Bb-B-C#-D#-F-G.
He varies the riff he played over the C#7 to fit over the C#9 here like this: G-Ab-Bb-B-D#-G-F-Bb-Ab-G-F and so on. (Notice that the pattern "down a whole step, up a perfect 4th" does not work over this scale.)
Robben's discussion of scales and triads as musical devices is another added bonus in this lesson.