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 😛
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?
My first thought would be that you probably don't need to do anything to them. Maybe disconnect NFB?
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.
Since this is a guitar amp conversion this thread belongs over in Instruments & Amps where I will now move it. 😀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/HammondOrganServiceManual-EarlyModelsabcSeriesB3A-100RtDG
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.
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.
Here is where a volume could go. Disregard the pot. with the .0047 cap by the output tubes as this is a treble cut control. Look at your amp chassis.The input will be the RCA jack on the end by the first 12ax7, not where it says "sig. input" on the terminal strip. Disconnect the twisted yellow/brown wires that go there and remove the light bulbs.
I've had a little time to think about this. In the second schematic(AO-35) I posted where that "cut" control is, that's where a PPIV master vol. would go if you don't like the location of the first vol. The first location might introduce distortion where the PPIV will not. Also, on the AO-39, you could convert that first 12ax7 into a LTP phase inverter and you could input the preamp on the grid of the first triode since that amp is already only a power amp.
Thanks boobtube your suggestions have worked fine, thanks for the schmatic! I am working on the volume control at this time, I will post as soon as it is solved. Thanks MrCurwen and Alan0354 for you valuable input alsohttp://www.diyaudio.com/forums/members/mrcurwen.html!
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