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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.)

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

The Procedure

The procedure (requires two people):

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.

Trek Solid-State Preamp Organs with solid-state Trek-II preamp should be set almost the same way, but the max drive level on the preamp is adjusted with the volume pot attached to the swell pedal control arm. Loosen the set screw and use a small screwdriver to turn the pot within the arm - do not remove the arm. All other adjustments are the same.

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