Hammond organ amp conversion to guitar amp.

[DustyBadlands]

Hello all,
I picked up a Hammond Amplifier model: H-AO-43-1 Tubes: 2 - EL84 1 - 5U4, 2 - 12AX7, 1 - 12BH7 that I would like to covert into a guitar amp.The plan is to gut it and use the iron and tube sockets.

Possibly a half power rocket type or a Matchless lightning. I did find a ceriatone lightning schematic and layout that might work.The lightning PT is 290-0-290 where the Hammond PT is 320-0- 320. with the same 6.3V and 5.0V secondaries. How can I work the Hammond PT into this circuit? Plus I'm not sure of the Hammond OT specs or if it would work with the Lightning.

Any suggestions and or schematics that might help would be greatly appreciated. Thanks. dustybadlands

[David Root]

I must be missing something. This amp has a 320-0-320 PT in it? Or you want to put one in it?

That PT with 5U4 rectifier is kind of pushing it for a pair of EL84s, but would be fine with a pair of the Russian uber-EL84s. Generally most people would not run EL84s much above 340 Vp.

Before you gut it take a look at just modifying it, do you have the schematic? I'm curious as to where the 12BH7 fits in.

The OT should be OK for the Lightning, which is also a 2xEL84 I think. (not a big fan of EL84s so I could be wrong). You could work the Hammond PT into the Lightning by using the 5U4G, but I wouldn't. I'd use the full 440V or so and put in suitable Russian uber-EL84s, some of those can handle 500Vp without problems. That plate voltage would turn ordinary EL84s into Roman candles pretty quick.

[passfan]

I'm with David, see what you got first. If you haven't torn it apart yet a slight modification to the input preamp tube should allow you to plug in a guitar and give you an idea. You might be surprised as well. The AX84 web site has a bunch of building block schematics for preamp sections and output sections that can be picked through for ideas using the tubes that you have.

[M Fowler]

The AO-43 has been done before and posted on 18watt.com I believe.

I would download the schematic from Captain Jack's website and see what values you have. It would make a nice Bassman type amp with TMB controls with the 320-0-320. Change to octal power section.

[DustyBadlands]

After a late night surf on the net trying figure out what to do with this AO-43 I came across this Trinity amp project. http://www.trinityamps.com/lightning_project.htm Looks like the AO-43 chassis That I have.

I do not have the schematic for the AO-43. I'm sure its out there. I had no luck finding it.
The first thing I did with this amp was to modify the first 12AX7 input preamp tube to plug my guitar into. (The 12BH7 had something to do with the reverb in the organ circuit I believe). Plugged into a 8 ohm speaker cab. I was not happy with the results. So after looking at this thing sitting on my bench for a few days I decide to....as Dave says not to do. LOL... Gut it! Could not help me self. The rectifier,power and pre amp tube wiring is still intact. Just ripped out the tag board. Measured the PT 320-0-320 a lot more voltage than the typical 18 watt amp circuits I 've seen out there. So my journey begins. The plan is to change the 12BH7 over to a 12AX7 and use that as the first gain stage.

Looks like I'll start with this trinity lightning circuit and see where it leads me. I have no idea how this amps sounds.The amp had all of the tubes and included 2 Amperex EL84's in it when I bought it.

Thanks for the input. Victor

[txbluesboy]

I googled "hammond AO-43 schematic" and came up with this. I'm not registered there yet so I couldn't look at it, but appears promising
http://www.diyaudio.com/forums/musical- ... rsion.html

[txbluesboy]

I haven't found it yet, but a schematic of that 12BH7 reverb unit would be interesting. That tube would probably make a better 1 tube reverb than the 12AX7 scheme thats been popular.


Edit: After a little research it appears that it has to little gain for the recovery stage.

[M Fowler]

http://www.captain-foldback.com/

I think the A0-44 amp schematic is the same? These are the reverb amp.

Not sure but the picture may be the A0-44 reverb amp and not the main amp.

[M Fowler]

The A0-43 is the Hammond L-100 from what I can find.

[DustyBadlands]

Wow! Thanks for all the info! Should keep me busy looking over this stuff.
Here are a few gut shots of what I'm starting out with. Thanks again. Victor

[Structo]

That is a lot nicer than the Hammond amp I got.

You should be able to use most of it.

The problem I had was that the B+ was too high.

Mr. Hammond was known to be thrifty.
He would use the tube load to load down the PT to lower the voltages.

Mine unloaded was around 760v! If I recall correctly.

I ended up using a stud mount Zener diode to drop about 30 volts off of that.
My amp was a 2 x 6V6 amp which I eventually converted into a Rocket.

A while back I had to rob a couple parts out of it so it is not currently operational.

Have you measured the B+ on that yet?

[M Fowler]

Yeah that looks like a good platform to start with.

The voltages is too high will have to use the reverse zener diode like Tom suggested.

Mark

[DustyBadlands]

Found this on Ebay if anyone is interested.Hammond A0-43.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... K:MEWAX:IT

[M Fowler]

That one started out high priced didn't it.

[DustyBadlands]

Agree! Maybe he will take a low offer.