Hammond AO-39 info needed

I've got a Hammond AO-39-2 I would like to use as a simple power amp. I don't really need to make it a self contained guitar amp, just a basic power amp that I might stick in a nice old radio wooden cabinet or something for general use.

Firstly, is there any difference between an AO-39-1 and an AO-39-2? I can't find any reference to this online.

I would probably be looking to use the line level output of something like a guitar preamp pedal or guitar amp line out to feed the Hammond.

The speaker connection is obvious. For the input can I just wire a jack to the "sig input" wires (red and black) and use it that way or should I fit a capacitor inline or use something like a 500k pot to adjust the input signal?

Also I have read that these amps have the high frequencies limited somehow as standard. Is there a simple way to undo this?

Most of the info I find is for converting this type of amp to something else, like a guitar amp, but I would like to keep it as simple as possible if it will work like that, with any volume or tone adjustment done at the input source.
 
First, there is very little difference if any at all in the -1 and -2 versions.

You are correct in your thinking of needing a line level input as this was the power amp for the A series organs. But you can't input on the black and red because the signal input to these has already been inverted in the preamp. That 12AX7 is just the driver stage for the 6BQ5s. This is why the amp is a good candidate for a conversion because it's constructed this way. That 12AX7 is not the phase inverter.

I've attached the schematic. See how the red and black each go to one half of the 12AX7 and each plate of the 12AX7 connects to one 6BQ5 via C2,3? So each half of the 12AX7 is driving one 6BQ5. You will need to provide an already phase inverted signal to use the amp as is and to input the signal as you describe.

So to do a conversion, you re-wire that 12AX7 as the phase inverter and add a preamp. An EF86 works nice and the power tranny will handle the extra tube fine.


AO-39.jpg
 
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Yes I can see how it works now. I'd need to do a bit or rewiring but I was just wondering if I might run across some sort of preamp out of something else that already does the work the original Hammond used to do, maybe something with various inputs or nice tone control to make an interesting general use amp.

I was unable to get the full works out of the Hammond organ at the time so I just settled for the Rola 12" speakers and the power amp without really knowing what I would do with it. Perhaps I might run across something else that could work as an interesting preamp and bung the whole lot into some kind of interesting cabinet.
 
The problem you face with wanting to just add a preamp on the front of this amp is that all the preamps I know of don't come with a phase inverter at it's output, so you are faced with somehow adding a phase inverter to the preamp. It might occur to do a stereo input and just use this as a two channel amp but you run into problems with the two 6BQ5s running their outputs into one output tranny. With both tubes sending the positive AND the negative halves of it's signal to one OT, you run into problems. So the only solution I can think of is to either turn the existing 12AX7 into a phase inverter or add one to the end of the preamp you get.

Just a note to think about for more gain from the existing AO-39 amp, remove R8 and C1 on one half and R9 and C10 on the other half of the 12AX7. This gets rid of that filtering you were wondering about. Then increase the grid leak resistors, R3,4 from 33k to either 500k or 1 meg ohm. This will increase the gain considerably. See the input given as 1.9 volts but only 260mV getting to the grids? Doing the above two changes will give you the maximum gain as the amp sits right now. So a much smaller than 1.9 volt input should drive the amp fairly well. But you still will need to split the signal(add phase inverter).
 
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big speakers

Yes, as I see it now the easiest way for use as a mono amp is to use the existing 12AX7 as an inverter. I would like to find a nice old radio cabinet or something to bung it in, keeping it simple by only needing an on/off power switch and a vol pot on the input.

That's good info about the filter/gain part. I was unsure what was going on there and I probably wouldn't need the filtering anyway for my purposes.

The two 12" Rola speakers that came out of the organ are probably too big for what I want to do with the amp so I might try them as guitar speakers. The numbers tell me they're 1963 and 1964 and they appear to be 16ohm. Excellent condition.
 
Try both speakers because sometimes they used these two speakers as a set. One was constructed for lows and one for higher frequencies. You can tell by the size of the spider. The larger one will be the lower freq one and the smaller one is higher freq. The smaller spider restricts the cone movement so it lacks lows.
 
I was wrong about the speakers. When I hurriedly tested one with a multimeter it read about 12ohms which made me think they were meant to be 16ohm but when I tested them both properly later they read about 9ohms each.

They both have the ribbed cone design and seem to have larger magnets than most of the pictures I can find of 12" Hammond speakers. Maybe they were replacements at some point?

The numbers identify them as 1960's Rolas and they look like what you'd find in many old guitar amps. I've hooked them up to a small 10w 4ohm guitar amp output and even without being in a box they sound identical and lovely.
 
At the moment I don't have the numbers to hand but research eventually showed what they were and the old organ I got the bits from looked to be the right vintage.

I finally installed one of the speakers in a guitar amp that I wasn't too happy with and it totally transformed the sound. The amp was solid state 100w into 4ohms through a no-name 12" speaker that was all magnet and no quality. It was a very mid range, clangy sounding amp. The Hammond speaker made a huge difference. I know it's only probably a 25-ish watt speaker at best but I don't push the amp hard anyway and the Hammond Rola just sounds amazing in it.

Ebay seems to dictate that these speakers are realistically "worth" about 20 quid when salvaged from old organs, and maybe double that if you add the word "VINTAGE" to the description and aim them at the Hi-fi crowd but I'm not selling these. They are by far the best sounding 12" speakers I've ever had in any guitar amp. I'll just have to remember not to push the volume too hard.
 
Those AO numbers means you have original OEM speakers for Hammond by Rola. And regarding your resistance measurements from before one time reading 12 ohms and the other 9 ohms, if you look at the schematic, it says both speakers are 8 ohms, so your output tranny is for 4 ohms since they are wired in parallel. You might need to check the battery in your meter for possibly needing replacement. Those speakers should read about 6-7 ohms. Or they could also have been replaced with other Hammond speakers. But that schematic dictates a 4 ohm load. Usually these amps can handle the mismatch OK but for best operation and max power out, you should match the speakers to the OT.