Watch the Mixoldian Flat Six online guitar lesson by Bruce Arnold from Total Modal
The Mixolydian b6 is commonly used over a dominant 7th chord but can also work over a dominant 7sus4. Be careful when using this scale over the chord symbol 7#5 because many times they are looking for a whole tone or Altered scale. Most of the time the chord will be written as 7b13 and this is when the Mixolydian b6 will work best. It is easier to memorize a Mixolydian b6 mode if you think of it as a Mixolydian mode that has a b6th. The Mixolydian b6 has one avoid note which is the 4th. Remember that with the modes that you don't know as well, improvising will really help you eventually understand the sound.
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Dominant 7th chords that are written with b13 i.e. C7b13 usually resolve to a minor chord up a fourth. So a C7b13 would resolve to F minor. There are two common scales used in this situation. The Mixolydian b6 or the Mixolydian b2, b6 which is build off the 5th degree of Harmonic Minor. Experimentation will help you decide which sounds best for any given situation. Keep in mind that depending on how you hear the progression you will choose scales accordingly. For example if you had a chord progression that was 2 bars of F minor to one bar of C7b13 most likely you will hear the entire progression in F minor therefore you would only use some sort of F minor mode and not change to a Mixolydian b6 mode on the C7.