Improvising over DOMINANT 7 Chords: One chord swing backing track E7 - 120bpm

Improvising over DOMINANT 7 Chords
Key: A Major
Dominant 7 chord: E7
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Improvising over dominant 7 chords is a vital skill in jazz improv.
This is the first dominant chord of Sweet Georgia Brown in G.
Here are some options you can try:

Scale: E F# G# A B C# D
This is the Mixolydian mode (basically the A major scale starting on E - 3 sharps) or you can think of it as a E major scale with a flat 7 (D), which is the way I prefer to think of it, to avoid brain freeze due to too many scale names!

Arpeggios
The main chord tones are E G# B and D. (1, 3, 5, 7)
You should be aiming for these in most lines you play.

ADDED FLAVOUR
BLUES NOTES: A flat 3rd (G) or 5 (Bb) will give you that bluesy sound.

CHROMATIC RUNS: Fill in the gaps with chromatic runs,especially between the chord tones e.g. G#-A-Bb-B, D-Db-C-B, D-D#-E etc.

DIMINISHED ARPEGGIO: Based on the 3rd note of the scale.
Here it's G#-B-D-F in other words, the 3rd, 5th, 7th and flat 9th of the E major scale.

9b5 ARPEGGIO: E G# Bb D F#
A bit more "outside", nice to resolve on the third (G#) rather than the tonic.

THE HARMONIC MINOR SCALE
Another way to get that b9 spice is to use the relevant harmonic minor scale. Find this by going up a fourth (or down a fifth) from the dominant chord. It's also just the Harmonic minor of the key we are in - A.

For E7 that would be A Harmonic minor: A B C D E F G# A
In this scale you have the 1 the 3 the 5 the 7 and the flat 9.

LICKS: You can throw in any specific dominant E7 licks you have learned or made up. This will help to make the whole solo more "tuneful".

There are many more ways to approach the Dom 7th but these basic approaches should get you started. I prefer to come at it from the point of view of arpeggios and then "filling in the gaps" while heading for some chord tones, rather than learning loads of scale names, which can get really confusing really quickly. But, most important of all, use your ears to decide what sounds good. Happy noodling!