G Mixolydian and Triads by Guy Fenocchi is a free guitar lesson that will teach you the G Mixolydian Scale and three major triads that come directly out of it. Guy will then show you some soloing ideas over a G7 chord. To learn more about these concepts check out Improvising with Simple Blues in G.
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Here is the tab for the G Mixolydian and scale and the triads

Improvising with Simple Blues in G by Guy Fenocchi is a downloadable Quicktime video lesson that will teach you some blues soloing concepts. We'll use track 5 from the Let's Jam! More Blues & Rock as our jam track. Working through each chord of the progression, Guy will show you appropriate scales and arpeggios that can be used. The download includes five .mp3 jam tracks that allow you to practice over each section of the song. A .pdf featuring tablature and chord charts is also included with lesson. $4.99
Watch a preview: Quicktime or Flash