Need some advice about repacing A0-29 with A0-28 in M3

[Joe marolda]

I came across a AO-28 I would like to use in a M3 I chopped. I presently have a rebuilt 229 in it powering the field cool speaker that is in a separate cabinet I build resembling a sub woffer or leslie, it doubles as the bench as well. The bass paddles are gone. I want to swap the 29 with the 28 and lose the speaker. I have a trek II line out box on it and assume I draw my signal off that. Everything looks strait forward seeing that most of the terminals on each amp are the same. My question is what do I do with the last few wires that don't match , the power is another question. The 29 has gray blue brown and the 28 has gray blue yellow yellow . Has any one done this?

[Jaim]

Haven't done it, but thought about it. The AO29 in the M3 also provides a place for the power in. The AO28 does not.
AO28 - gray / blue is power. Yellow/yellow is heater voltage to the pilot light.

You will have to move the AC in to the TWG line panel. See attached drawing.
Run 2 wires from the neutral and run terminals to the AO28 - blue/gray
The volume soft/normal is different on the AO28 - jumper terminals E & F on the preamp. You can wire the function in if needed Will require a mod to the wiring.
Depending where your AO28 came from, the expression pedal may be backwards. If so - under the AO-28 chassis swap the 2 screw terminals that go into the expression unit ie. dog house.

Jim

[David Anderson]

I don't really see the point of this swap. You can get a line out from an AO29, and you can modify it, if necessary, to work without the speaker. Most of the AO29 circuitry is very similar to the AO28. It seems like a lot of trouble for little to no gain.

[Joe marolda]

Thank you David and Jaim, I appreciate you expertise, what I've done to 5 spinets so far is add missing foldback and drop the lower manual an octive. I killed a model M for buss bars and contacts/wire. Ive had my amp guy rebuild every amp and they all sound great. One of the spinets is a M3 that I've owned for over 30 years and I recently chopped and added a fall board, and made a bench-speaker cabinet for the field cool speaker, I love it. I know it's to late to make a long story short but I have enough buss bars and contacts to do up another M3. My plan is to pull the rebuilt A0-29 and save it for that next M3 that comes along. The A0-28 I have no use for as of now exept this idea. The A0-29 puts off more heat, its heavy and I would need to add a load resistor to remove the speaker. If my thinking is right it will be easier to swap amps and set myself up for the next M3 and have a great sounding portable M3. I'm assuming I will get a more robust tone from the A0- 28.

[Joe marolda]

Thanks Jim, I understand the blue and gray are power and how to set that up now. It's the volume soft/normal I would want functioning, my amp guy could do it for me when and if I have him replace the can caps and stuff if I can get him a scamatic or something

[Joe marolda]

My guy is Mark Weiss from New Milford Ct. His business is Amplifier Experts. He's rebuilt only one Hammond amp before I came to I'm and he's interested in learning more about them. If I can get him a plan to modify the volume soft/normal this may be do able, thank you for your advice and info. JOE

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Love spellchecker, cool- coil, I'm- him

[Sweet Pete]

I don't really see the point of this swap. You can get a line out from an AO29, and you can modify it, if necessary, to work without the speaker. Most of the AO29 circuitry is very similar to the AO28. It seems like a lot of trouble for little to no gain.

The AO29 can be dialed in hot or hotter than an AO28.
Also has the same bandwidth to my ears. M3 bass tonewheels and decay circuit are every bit as deep on the AO29;(which also operates the pedal circuit),as the AO28.
If you need to install an AO28, then go for it. Let us know if the M3 matching transformers drive the inputs.
The 'solo' button on this M3 is the loud/soft tab,so if you can work around that with the AO28 then give it a try.

[Jaim]

If I can get him a plan to modify the volume soft/normal this may be do able

The AO-28 volume tab is rather simple. Terminals E & F jumpered is loud. Open is soft. Need to run a shielded pair of wires from Terminals E & F to the switch and rewire it.

Jim

[Tim_at_Jonas]

If you don't want to use the speaker, just pull the PI and power tubes from the amp section, and adjust the value of the first power supply resistor to drop the preamp voltages back to where they belong.