TECH TIP

Hammond Organ Chorus Modification

Here's a simple modification that will increase the depth of the chorus effect on tonewheel Hammonds. All it takes to do this modification is a resistor, the value will be between 15k to 75k or so - depending on how it sounds to you - and a couple of feet of hookup wire. To do the modification you will solder a resistor to one of the pins on the vibrato delay line. The other end of the resistor will go to a convenient ground in the system somewhere, to a lug under one of the preamp chassis securing screws for example. Referring to the first figure,the depth of the chorus on unmodified organs is set by the value of the resistor located inside the vibrato switch assembly. The vibrato switch assembly is the long metal box under the drawbar base that has the rotary chorus/vibrato knob emerging perpendicularly and at one end. The chorus effect is created by summing the vibrato'd with non-vibrato'd signals across this resistor.

The value of this resistor can be reduced by paralleling another resistor with it - at the vibrato line box - not inside the switch assembly. The new resistor is externally mounted to the pins on the vibrato delay line as indicated in the second figure under the "Modifications" column. If your chorus is too deep (too much vibrato), you'll have to increase the value of the resistor inside the switch. This will require disassembly of the switch to change out the resistor with one of a higher value. 68 kOhms ought to do it.

The vibrato line boxes on the different models are somewhat different. While they have similar topologies, they differ in numbers of components, impedance, etc.. Before you actually solder anything in place, you might want to experiment with the amount of vibrato mix. Parallel a 100k potentiometer, wired as a rheostat, to the pins indicated on the second figure. If you need a ground to the preamp, run a wire to the preamp chassis somewhere. Tweak the rheostat until things sound nice and either replace it with a fixed value or just leave it in place.

I've tried this mod only on one of the models shown above. On all of the other models, I got the pinouts from looking at the service manual and schematics.

I'm interested in getting some feedback from those of you that try this out.
Brad Baker, bpb@sprintmail.com

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