Barry Harris in his workshop videotape has an extensive section on the
application of chromatic tones in creating bebop lines which is hard to
summarize in a single post, but I will try to simplify it. The tape and
booklet are very good study aids and should probably clear up a number of
your questions. I got the order phone number from an ad in the back of Jazz
Times magazine.
He bases his chromatic notes on where they rhythmically place the chord tones
in a melodic line. If you transcribe Charlie Parker or Dizzy Gillespie solos
and melodies they tend to add chromatic notes in key places in their scales
so that the R,3,5, and 7th fall on the strong part of the beats: 1,2,3, or 4
and the chromatic notes fall on the weak parts of the beat: on the "ands" of
the beats.
The simplest way to do this is to add the major 7th to the mixolydian scale,
David Baker coined this the "Bebop Scale." This results in a scale which
when played in eighth notes descending from its root starting on the one will
end with its root on the one of the next measure. Ending on the one of the
next measure is rhythmically important because it allows the next octave to
start in the rhythmically same place as the previous octave. When you try to
play the basic mixolydian scale over the same amount of bars you get a
situation where the chord tones start falling on the "ands" and the line is
weaker harmonically. Rhythmically the mixolydian scale when played in eighth
notes descending ends on the "and of 4" in the first measure which gives the
line a clipped feel, it doesn't have the perpetual motion feel of the bebop
scale.
Try playing the F bebop scale in eighth notes over this progression:
Cm7 I F7 I Bbmaj7
If you start the scale descending from F on the 1 of the Cm7 bar you get:
F,E,Eb,D,C,Bb,A,G, F, E, Eb, etc. This ends with F on the first beat of the
Bbmaj7, a good place to end the line. If you tried this with the basic
mixolydian scale you would have to play each F as a quarter note to land all
the chord tones in the same places, this tends to sound awkward rhythmically.
This is just an easy way to internalize the feel of the scale, you will want
to experiment with doubling the scale back on itself too; for example,
(descending) F,E,Eb,(up to)G,(down to)D,C,Bb,(down to)G,(up to)A,(up
to)C,(down to)F,E,Eb. This will break the line up melodically and make it
less predictable. David Baker has a book dedicated to the usage of the bebop
scale, if you use its many suggestions to create your own lines you could
benefit from it.
Once you have the basic bebop scale usage fundamentals down, the Barry Harris
method has a "half step practice model" which I found to be quite valuable
for adding multiple chromatics to my lines, and definitely takes the
randomness out of them too:
Dominant 7th scale 1/2 step rules:
Starting Note, Number of added 1/2 steps (location in scale), Ending Note
1 1 (between 8-7)
tonic
2 0
tonic
2 2 (between 2-8, 8-7)
tonic
3 1 (between 8-7)
tonic
3 3 (between 3-2,2-8,8-7)
tonic
4 0
tonic
4 2 (2-8, 8-7)
tonic
5 1 (8-7)
tonic
5 3 (3-2, 2-8, 8-7)
tonic
6 0
tonic
7 1 (8-7)
tonic
7 3 (3-2,2-8,8-7)
3rd
This chart is the idea of Barry Harris and is used for practicing adding
chromatic half steps into the dominant seventh scale, he also has some
effective rules for adding half steps to major and minor scales. This looks
cold and academic in chart form but is easy to simplify.
The basic premise is to start with the starting note on the beat 1, and
descend scalewise adding each of the named half steps ending on the ending
note from the chart. You will soon see that it can be divided into two
options for even scale tones and two options for odd scale tones:
Starting on the 2nd, 4th, and 6th, your two options in this model are 0 or 2
added half steps.
Starting on the 3rd, 5th, and 7th, your two options are 1 or 3 added half
steps.
His exception is starting on the 1st, which has only the one added note.
Barry's workshop stresses drilling these scales to gain fluency with them and
gives many options on how to create your own ideas from his model. It is
quite effective when you get it under physical control and don't have to
think about it.
I hope this was helpful. If not, the video is very thorough and should help
you.
Brian Oates