Soloing over modulation: a specific case

Question

Consider the following chord progression:

G F#m G F#m G F# ...going on with G again...

1 measure each chord

The melody is in key of D (at least until F#)

Soloing over this progression requires me to choose notes from a different scale over the last chord F# to avoid unpleasant tones. I tried this in 3 different ways. I considered the F# section as:

1. F# major, using all the relative tones

2. B major, centering the phrase on F#

3. C# major, centering the phrase on F#

However I noticed that only the second option really sounds good although in theory even options 1 and 3 are possible. What could be the reason?

I think the fact that F# chord act as sort of secondary dominant (V/vi resolving on G that is IV) could impose the use of F# Mixolydian (i.e. option 2), but I leave the word to you.

Answer 1

I see elements of two different questions. One is "what should I play during the F# major chord," and the other is "How do I analyze this looping progression?" The second question can affect the first.

The simplest answer is "Well, if the chord is F#, then play in F#." As you noted, this "sounds" most easily satisfying. The suggestion of a B scale would bring in a D# that would be at odds with the C# in the F# chord, as well as awkward for returning to G. The C# major would have even more issues, with its E# and G#.

The question of why there's an F# chord and where it's going can help. As long as the loop keeps repeating, it's hard to think of this as being "in the key of D," even if the overall piece is. One approach would be to think of the section as temporarily tonicizing G (IV in the key of D). In that case, the F#m lower neighbors create a mixolydian relationship, and the F#M certainly seems, as you suggest, like a pivot chord that one would expect to be a secondary dominant leading to B. (Perhaps when this riff is done looping, it does?)

Another approach would be to think of this loop as tonicizing the F#m (iii in the key of D) by giving it a Spanish-style lowered-II upper neighbor of G. If we take that thought, the F#M is just a change of mode to major.

Answer 2

Since the rest of the song is in D major, then for the F#M chord, I would use the fifth mode of the B harmonic minor scale: F# G A# B C# D E F#. This scale incorporates all of the notes in the chord while staying as close as possible to the key of D major — differing by only one note: the A#.

Answer 3

As Andy Bonner wrote, there are many ways to interpret this progression.

|: G F#m G F#m G F# :|

A possibility is to treat F# as a secondary dominant in Bm resolving deceptively to G, which for that short moment is VI in Bm. Then you can choose from a variety of dominant scales to play over F# depending on the flavor you like. What supports this interpretation is that Bm is vi in the key of the song, D, thus very closely related.