Watch the A7 Fill online guitar lesson by Joe Dalton from Acoustic Blues Handbook
An A7 fill added to these choruses will help smooth out the chorus. When we play the B7 chord we are implying a melody. When we go back to the pattern, we break up the melodic flow. The fill where that A7 comes in will help this melodic flow. The turnaround contains single notes, and the B7 implies a single note melody. The A7 fill bridges the gap. Played correctly, there will be no gap, just the melodic flow I was referring to.