Howto Adjust The Preamp Drive Level

This was submitted by AlGoff to the OriginalHammondLeslieFaq

In organs using either the AO-28 or AO-29 preamps (this includes the B-3, C-3, A-100, ?M-3, ?M-100, amongst others) driving a Leslie (or similar) speaker, the output level can be adjusted as to the amount of distortion ('growl') desired. The preamp drive level is set by adjusting a trimmer swell capacitor accessible behind a removable plug located on the cover of the box containing the swell capacitor. (On some spinets there is no access plug, and the whole expression capacitor cover box must be removed to access the trim screw.)

Here's a picture which shows the plug you need to pull. (The plug is labeled "Y" --- and it's upside-down because the picture is of an A-100.)

      http://theatreorgans.com/hammond/faq/files/bri-pics/73288_11.jpg

Here is the expression control on a M3 with cover on and off.

The Procedure

• Select the A# preset key, upper manual with drawbar setting 80 8808 008 (first brown drawbar and all four white drawbars pulled out to position 8). The expression pedal should be fully depressed (maximum volume), Vibrato and Percussion off.
• If you regularly use pedals, pull the first (16') pedal drawbar out to position 8.
• The preamp trimmer on the organ should be adjusted fully clockwise.
• For a tube amplifier Leslie adjust the volume control fully clockwise. For a solid-state Leslie (760, 722, 860, 900, etc), all amp controls should be almost or fully clockwise.

The procedure (requires two people):

• One person should play a full 5-finger "C" chord on the upper manual; use all 10 fingers if you don't normally play bass. Don't chord in the lower octave, but play a realistic full chord. If you play bass with the left hand, play bass on the first octave of the lower manual with the first four drawbars out to position 8. Play the low C-pedal if you are using pedals.
• While the first person plays the above, the second person should begin turning the trimmer slowly in a counter-clockwise direction, no more than 1 turn. The organ / Leslie should be getting louder as you turn the trimmer, but should not yet be distorting.
• If you like a clean organ sound then the final setting should be slightly less than the point where the Leslie starts to distort.

If you like a dirtier sound then the final setting should be slightly more (1/8 of a turn) than the point where it just begins to distort.

The caveats: Increasing the drive level by turning the trimmer more will only cause problems, as distortion can excessively heat the upper driver and woofer and may lead to their premature failure. The classic HammondGrowl is a direct result of correct preamplifier drive adjustment, as well as a well maintained organ and Leslie. Most organs should require a trimmer adjustment no more than 1 to 1 1/2 turns open.

To reiterate: driving your amp to distortion puts a lot of energy into your Leslie's JensenV-21 horn driver. Replacements are hard to come by, and the modern substitutions are not met with universal acclaim.

The test: To test your freshly adjusted organ and Leslie - If you have it adjusted for a slight distortion at maximum volume, the distortion should disappear or be greatly reduced by simply pushing in the first two drawbars 2 clicks to position 6. It should still be very loud and very clean, but should have no distortion. The desired result is to have some HammondGrowl on demand, and a clean organ sound at other times.