Hammond AO-39 and AO-35 organ to guitar power amp conversion - diyAudio
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Old 14th January 2015, 05:39 PM   #1
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Smile Hammond AO-39 and AO-35 organ to guitar power amp conversion

I have two Hammond AO-39 and two AO-35 organ power amps that I am looking into converting into guitar power amps with pre and post volume controls. Any rack mount guitar preamp could be used with them. This shouldn't be hard as they are organ power amps to begin with; I'm not trying to convert them into full guitar amps. Do you have any thoughts as to where I could obtain the needed info? Best Regards! Gary DiCenso
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Old 14th January 2015, 07:41 PM   #2
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Well look up their schematics and attach them here.

My first thought would be that you probably don't need to do anything to them. Maybe disconnect NFB?
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Old 14th January 2015, 11:23 PM   #3
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Yep, the power amp part of the guitar amp is the simplest of the simple power amp. Get the schematic of your amp, then look at the schematics of some guitar amps and you can see the trend. It is very likely you can use it as is.
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Old 14th January 2015, 11:50 PM   #4
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Default Hammond AO-39 schematic

Click the image to open in full size.
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Old 15th January 2015, 03:01 AM   #5
kevinkr is offline kevinkr  United States
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Since this is a guitar amp conversion this thread belongs over in Instruments & Amps where I will now move it.
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Old 15th January 2015, 06:14 AM   #6
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You can experiment with the PI balance pot to see where you like the sound most.
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Old 15th January 2015, 01:40 PM   #7
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One thing to consider for the A0-39 is that the signal has already been inverted(split) before the inputs. That 12ax7 is not the phase inverter tube. Hammond used an interstage transformer on the preamp chassis to accomplish this. The AO-39 is only a power amp. The AO35 does have a traditional preamp-phase inverter-power amp architecture. Here is where you can download the entire Hammond service manual: https://archive.org/details/HammondO...iesB3A-100RtDG
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Old 15th January 2015, 03:18 PM   #8
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Default AO-35 schematic

AO35.JPG
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Old 15th January 2015, 04:29 PM   #9
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Thanks guys, I did not know the phase inverter/splitter was a transformer type on the AO-39. Anyone know what to adjust or change out on the AO-35 to make get along nicely with a dedicated guitar preamp? No need for tone controls, maybe just a volume control some where and a jack to plug in a tube preamp.
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Old 15th January 2015, 05:50 PM   #10
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Here is one person's solution. He got rid of the extra filtering at the input. Remember, this is an amp that only processes the reverb signal for the Hammond M-100 series amps so they have extra filtering.ao35_2a (1).gif
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