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scrimpu 28th July 2012 08:05 PM

Hammond AO-29 conversion
I have a Hammond AO-29 amp that I would like to convert for guitar, I would like to keep it as close to stock as I can for now. Does anyone know how I can eliminate the expression pedal volume control? I would like to replace it with a simple volume pot but its quite a complex circuit that I have never encountered before.

Kay Pirinha 29th July 2012 11:38 AM

The AO-29 amp was used e.g. in the M-3 organs, right? So we have the schematics here. It's quite a complex circuitry with many components which are of no use in a simple guitar amp.

The expression (= volume) pedal actuates the differential variable capacitor between V4A and V4B. You'd eliminate everything between C9 and R37 and replace it by a 1 M audio taper potentiometer, hot end tied to C9, wiper to V4B, pin 7. To start with, use terminal "A" as the input.

Have fun!

scrimpu 29th July 2012 02:36 PM

Yes, its from a 1957 M-3. I have removed al the vibrato percussion stuff and have wired terminal "A" as my input. I was unsure how to couple V4A to V4B since they are not phyisically connected in the expression pedal assembly, its just metal plates with a gap inbetween. Also this amp has a field coil speaker I want to change to a PM, as I understand all I need to do is put a 700ohm 30+ watt resistor in place of the coil, is this correct?

indianajo 30th July 2012 03:24 PM

yes, a 500 to 1000 ohm 30 watt resistor should be fine to replace the field coil.

scrimpu 30th July 2012 08:38 PM

I have wired up a pot between C9 and R37 and I have sound but only with the pot turned down to almost zero, it gets slightly quieter as I turn it up and goes silent about half way. How is it that more resistance equals more volume? Also whats the point of the 1.5 volts on pins 3 and 8? I have not seen this before. I am starting to think I will have to gut this thing and start over, these hammond amps are way more complex than need be.

indianajo 31st July 2012 03:40 PM

What pins 3 & 8? I/O on Hammond stuff is labled with letters until they went to AMP connectors.
Organ players ride the gain a lot, since pushing the keys harder doesn't do anything. The variable capacitor gain on the Hammond amps is WAY more reliable than the pots used on swell in early Baldwins or the CDS cell, light bulb, and cardboard baffle, used in other brands. Why don't you buy a variable capacitor from or some other M scrapper? You don't have to attach a pedal to it. Just put in a wood box so you don't kick it on stage and bend it.
You can change the overall gain by fooling with R25 and R26 on the schematic.

ab0ez 3rd August 2012 03:52 PM

My best advice is to completely gut it and start over with a clean sheet of paper.

scrimpu 4th August 2012 02:47 AM

Sorry I was refering to pins 3 and 8 of V4 on the schematic, I see now its part of the bais circuit, I was up way too late and must have been looking at the schmatic wrong. As far as the volume control issue Im thinking I should just couple V4a to V4b and place volume circuit elsewhere.

Kay Pirinha 4th August 2012 11:45 AM


Originally Posted by ab0ez (
My best advice is to completely gut it and start over with a clean sheet of paper.

My thoughts also!

He'd better remove everything besides the transformers and the output stage, buy one or two more 12AX7s, some potentiometers, have a proven schematic from the web and start constructing.

Best regards!

scrimpu 4th August 2012 08:00 PM

Anyone know the specs for the expression control variable capacitor? I dont see the values for it on any schematics.

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