December 10,2015 

GemiVibe: a Hammond AO-63 Conversion

The AO-63 is a big amp! It contains a 7591 P/P main amp as well as a complete 6GW8 P/P reverb amp. One power transformer supplies voltage for two separate rectifier tubes and B+ filters for the individual amps. There's big iron on this chassis and it weighs in at 28 lbs.

I picked up this amp back in April with the intentions of building an Ampeg Gemini II. I owned a Gemini II back in the '60s and wanted to try to capture that warm, clean sound again.

I began by ordering some tubes. Then I put it on a variac and slowly brought it up to full line voltage over several days. Everything worked as it should. Even the filter caps seemed to be doing a good job. Those caps are still working fine 8 months later, so I'll leave them alone until they need attention.

I temporarily patched an AB763 preamp into the AO-63 power amp to get an idea of how this project was gonna sound. I really liked it! It showed a lot of promise. I decided no need to mess with the main power amp. So, I carefully removed the V1 organ preamp components and built a Gemini II preamp to drive the power amp. Now it's beginning to sound like the old Ampeg!

Then I got busy with summer projects and chores. I let the project sit on the bench, occasionally firing it up for short periods. I knew I'd eventually get back to it, but I wasn't sure what more to do with it. It was missing something and there was just so much other stuff on the chassis, I knew I had to do more than just a no frills amp. I thought about it off and on for the next few months. It was too hot in the shop to enjoy this kind of work anyhow.

November rolled around and I finally got back on it. I had decided to add some reverb since it also had a complete reverb amp. I dropped the reverb power amp circuit and routed the wet signal from the reverb recovery circuit back into the main amp's PI input, mixing it with the Gemini's preamp signal. After a couple days of tinkering, I had figured out a good place to pick off the preamp signal to feed into the reverb driver. This reverb circuit can go from subtle to over the top Dick Dale. I added a dwell pot to give complete control of the reverb.

At this point I was pleased with the amp and turned my attention to cleaning up all my temporary work and also removing the components for the unneeded reverb power amp. When done, I saw how clean the reverb amp part of the chassis looked. I got the idea to add a tremolo circuit. I chose the 6G16 Vibroverb circuit. This circuit works very well with the sensitive 7591s!

It's a success. Now to dress it up a bit with some kind of cab. I built this kinda plain cab from scraps in my shop. It functions as a good amp cradle too. I'm calling it done! This project turned out much better than I had expected. Here are a few pics...

This is the original AO-63 chassis


...and the original guts.


The conversion is complete. (Click pic for closeup)


Conversion chassis with new faceplate and reverb tank mounted in the back.


Simple utility cab to dress it up a bit.


One last pic...


Big sound!


Download the original AO-63 schematic/layout and the GemiVibe conversion schematic in a single PDF. AO-63.pdf (621KB)

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