Hammond AO44 conversion to SE guitar amp

[chris937]

I've been living with the guilt of gutting a basket case Hammond M101 organ for about a month now and it's finally boiled over in the form of an amp build. This was assembled over the past few evenings using mostly organ parts.

It's a SE 12AX7 6L6 6CA4 build using Hammond AO44 PT and chassis, ebay'd Stancor OT, and a pair of C12R from a Wurliter 4100. Thought I'd share!

[nosaj]

How's it sound?

nosaj

[chris937]

Pretty darn good. I'd characterize it as a late '50's early '60's Fender-ish amp voicing which is what I was shooting for. I think it handles a little better with single coils than humbuckers.

[nosaj]

So what circuit is it? like a princeton or a super champ?

nosaj

[Chuck H]

That looks like too much fun to play through. The nerve of squandering two vintage 12" Jensens for one 4W amp!?! I like it

Stellar vintage look too, but I would absolutely add a recessed, almost sealed back to the bottom cab (cutout for the amp knobs obviously) and similarly one or two recessed rear baffles on the top. I just don't like the lack of bottom from the fully opened back vintage Gibsons and such. YMMV. Finding appropriate vinyl to cover the new pieces with might be frustrating. You could just make them from cabinet or marine grade plywood and finish them with amber shellac for an "aged and ancillary part" look.

[DRH1958]

If you ever get another AO-44, you could also do this: Paul Ruby Amplifiers You get about 15 watts, push pull. Keep all the same iron, too and the OT is generous with the amount of iron. The only downside is those 6GW8 tubes, a little spendy to replace. I've done one of these and it's a great sounding amp. The triode section on these has a mu of 100, so half of a 12AX7 in each bottle. I'm thinking of adding a 6C4 for the cathodyne PI so as to have 2 12AX7 stages in the preamp. It only adds .15A of heater current, so should be OK for the power tranny.
I like what you did too. Also like the cord storage as a nice touch and nice, retro looking cabs. I like Chuck's idea but might just seal the bottom cab for more bass. Just leave a space at the top for the speaker cord. I hope you kept that output tranny that was on it. It's a great piece of iron. 8k to 8 ohms and it has plenty iron for good low end.
Those organs are a nice source of wire and components if the leads are long enough. And you were smart doing this reverb amp instead of trying to rebuild that main amp chassis. There's too much demo work and it's too long unless you can saw off the 1/3 of the chassis that has all those coils on it.
Nice, neat project!

[chris937]

So what circuit is it? like a princeton or a super champ?

The front end was loosely based on a Harvard (5F10). The power amp was something a local guy did in a super champ-ish build that I really liked the tone of when I had it in for a simple repair. The power supply was just something I whipped up out of parts on hand.

That looks like too much fun to play through. The nerve of squandering two vintage 12" Jensens for one 4W amp!?! I like it

Stellar vintage look too, but I would absolutely add a recessed, almost sealed back to the bottom cab (cutout for the amp knobs obviously) and similarly one or two recessed rear baffles on the top. I just don't like the lack of bottom from the fully opened back vintage Gibsons and such. YMMV. Finding appropriate vinyl to cover the new pieces with might be frustrating. You could just make them from cabinet or marine grade plywood and finish them with amber shellac for an "aged and ancillary part" look.

Thanks! I think it was the perfect match. My plan was actually to build a pair of sister amps from those particular boxes using a pair of AO44's from some M101's I took in a while back. I don't plan on putting a back on it at this point in time. If I ever need to mic it on a tight stage, I may reconsider that stance.

If you ever get another AO-44, you could also do this: Paul Ruby Amplifiers You get about 15 watts, push pull. Keep all the same iron, too and the OT is generous with the amount of iron. The only downside is those 6GW8 tubes, a little spendy to replace. I've done one of these and it's a great sounding amp. The triode section on these has a mu of 100, so half of a 12AX7 in each bottle. I'm thinking of adding a 6C4 for the cathodyne PI so as to have 2 12AX7 stages in the preamp. It only adds .15A of heater current, so should be OK for the power tranny.
I like what you did too. Also like the cord storage as a nice touch and nice, retro looking cabs. I like Chuck's idea but might just seal the bottom cab for more bass. Just leave a space at the top for the speaker cord. I hope you kept that output tranny that was on it. It's a great piece of iron. 8k to 8 ohms and it has plenty iron for good low end.
Those organs are a nice source of wire and components if the leads are long enough. And you were smart doing this reverb amp instead of trying to rebuild that main amp chassis. There's too much demo work and it's too long unless you can saw off the 1/3 of the chassis that has all those coils on it.
Nice, neat project!

I looked into those builds but by that point I had already gutted the chassis. I figured I'd do my own thing rather than backtrack. Since the second M101 is in (mostly) working order I kept the tubes and OT as spares for now. The interesting bit is that the full range AO29 pp output transformer is smaller than the one used on the 6GW8 reverb amp.

I also deconstructed the AO29 but I haven't decided on what to do with it yet. I have a pair of 7868 amps from the Conn and Wurlitzer that are probably up first. I'm going to have a hard time not turning them into SB-12 or B-15 based bass amps.

[DRH1958]

The OT on this AO-44 is the same as the OT on the AO-43 which is the main amp for the L-100 series organs. Probably a cost saving thing, not having to buy another model. But I agree, interesting that a reverb amp has more iron than the main amp.
Not sure if you are keeping any of the AO-29, but here is a link to the service manual: https://archive.org/details/HammondO...delsMM2M3M-100 Has all the schemas. It seems like if you want to shorten the chassis, it will cut off about the same place as where the un-needed tone shaping circuitry is IIRC. I gave up on this model and settled for other, simpler Hammond amps.

[chris937]

The OT on this AO-44 is the same as the OT on the AO-43 which is the main amp for the L-100 series organs. Probably a cost saving thing, not having to buy another model. But I agree, interesting that a reverb amp has more iron than the main amp.
Not sure if you are keeping any of the AO-29, but here is a link to the service manual: https://archive.org/details/HammondO...delsMM2M3M-100 Has all the schemas. It seems like if you want to shorten the chassis, it will cut off about the same place as where the un-needed tone shaping circuitry is IIRC. I gave up on this model and settled for other, simpler Hammond amps.

Interesting, I was unaware that those were shared parts. Thanks for the link-I've already spent hours with the schematics doing fun things like designing effects loops for my working M100.

My plan for the AO29 was it cut it down to a more useable size. I'm leaning towards using a different output transformer and doing an EL34 Marshall build with that.

I think I'm going to go ahead and recap the tone generator in the working organ before I attempt a sister-in-law build. If that doesn't bring back the upper drawbar harmonics missing on most notes, I'll be doing an AO44 Matchless Spitfire very soon.

[chris937]

Follow-up: I gutted the second M100 this morning. A ghost note appeared when I adjusted the bus bars while chasing down some corrosion issues. I'm pretty sure I somehow broke a wire on one of the lower manual's keys that shorted across the bars. That was the end of that.

Anyway, the second AO44 amp build is about to happen. I'm still leaning toward a Matchless build but will decide later on this week. It will most likely have its own thread.