speed control circuit. can I modify this circuit?

[jeffg]

This is a pic of my makita sander polisher showing a pair of diodes that control the speed from 3800 rpm to 2000 rpm. I was wondering if I could add another diode to knock off a few hundred more rpm for polishing. I mainly use the polisher for automotive paint/ final buffing and a lower rpm would be better.

http://321mach123.fileave.com/makita.jpg

The diodes read S3V (an upside [u]L[/u] ) N
The motor is rated at 6.5 amps 700 watts.

Maybe somebody can look up a radio shack number as that's the closest store I'm near.

If anybody's interested I can post what materials I use for the help. I mainly hand polish using a two part system.

Thanks