Hammond AO-63 conversion

[pompeiisneaks]

I've not seen that type of input arrangement where you have the 68k screen resistor, and two grid leaks? With all of those in parallel like that you'd end up with a smaller than 100k grid leak no? Youv'e got 68k and 1M in parallel with 100k and parallel ends up slightly less than the lowest resistor usually. It seems like you're combining two grid leaks on accident. I'd say ditch the 100k for that first input segment. Otherwise the rest seems fine to my quick look, but I'm no guru

~Phil

[sluckey]

That's a LOT of gain! How is it connected to the AO-63?

[jmccanna]

Hello,

Thanks for the feedback. I am just pleased that no one blew me out of the water for how dumb this was! Small victories I guess. I think I have mis-drawn it. What I was trying to do was emulate what I physically did with the Akai amp.

With the Akai I sat over the amp and thought through the point to point process and just rerouted wires and connections to get the EF86 input through to the 12A with a potentiometer in between. All point to point logic. But when I tried to commit it to paper it got confusing.

I just went back and looked at the Akai again and see that I eliminated the 100k to ground and now remember doing so before when the circuit in it did not work. (Note to self - make notes as you go because memory is fleeting) Thanks! I still have not actually built this.

[jmccanna]

That's a LOT of gain! How is it connected to the AO-63?

Hi,

Thanks. I agree it is a lot of gain and was hoping with the two potentiometers I would be able to adjust the actual output pretty finely. It is connected to the amp coming from the preamp chassis (picture attached) with an RCA connector to the main amp input.

I have left the main amp with its onboard 12AX7 alone so there is a ton of gain. Possibly too much.

This leads me back to the way the Akai amp has the preamp out at its J5 off of the cathode. This is not in my drawing because I could not figure how to terminate the plate of the 2nd half of the 12AX7 if I was actually outputting via the cathode as done on the Aka. Using the J5 output would closely emulate the Akai as a preamp I have been using. That would be ideal so if a suggestion or two gets me there that would be awesome.

I have attached a photo of the prechassis that came right out of the organ. I would love at some point figure out how to take advantage of the other sets of pre tubes to have multiple options for the preamp stages.

James

[jmccanna]

Hi,

I had a thought - and yes - it hurt! I could not figure out how to terminate the second half of the 12A in my handdrawn schematic. Then I had a thought, to emulate the Akai amp pulling a presignal off of the cathode of the 2nd half of the 12A would I just connect the plate to B+ and not to any output? Could I then add the circuitry like the Akai and output like it does at J5 on its schematic?

James

[jmccanna]

Hi,

I am wondering if this incorporates/addresses the concerns expressed by Pompei and Sluckey? Would this make a sensible preamp?

James

[jmccanna]

Well, I got it working quite well. I went back to being really simple with just an EF86 driving the amp to start. I need to tidy things up and sort out why the reverb is not working again. The reverb amp works if I go into it directly but the reverb send does not appear to function correctly. Gotta go study how to test a reverb tank now.

Learning lots as I go but I do have a functional and good-sounding amplifier at this point. Thanks for your help all along.

James

[pompeiisneaks]

One test is to just shake the reverb gently, if you get that jangly springing sound at the speaker, the output is working, and your spring tank is working, if not then either the spring isn't or the recovery stage isn't

[sluckey]

Well, I got it working quite well. I went back to being really simple with just an EF86 driving the amp to start.

How is that EF86 circuit connected to the AO-63? If it's not plugged into the organ input that's probably why you don't have any reverb.

Look at the schematic. It's pretty straightforward. Follow the reverb path. There are several simple things that could break the reverb. You must be plugged into the Organ input to ever get any reverb. The reverb cables must be connected to the proper jacks. That #15 lamp must be plugged in securely (you'll never see it glow). You must have a jumper on P302 pins 1 and 3 (unless you are using that reverb switch assembly). And the reverb speaker must be plugged in. Check these simple things before you drag out the test equipment. I certainly would not suspect the reverb tank just yet unless you have been treating it roughly, or dropped it on the floor.

[jmccanna]

Hello All:

Pompeii: I tried several different things and the reverb tank may be faulty even though it was not before. I get a very faint jangly sound when I tap it.

Sluckey: The reverb worked before I started modifying the preamp chassis but it is not working properly now. I am plugged into the organ jack and see that it sends a signal to the reverb generator on the schematic and visibly.

I did a series of tests to see just what was what. Using my digital player I was able to confirm the reverb amp itself was working well, clean and loud. I tried a few resistance tests I found on the internet for the tank itself and they were inconclusive. I visually examined the tank and it seems fine and there were no obvious wire breaks. I will return to that later I guess.

I have bypassed (disconnected) the 12A on the main chassis and connected the EF86 to coupling capacitor just before pin 7 of the first 7247. Sounds good and clean but no overdrive to speak of. It is relatively quiet and full-sounding.

[sluckey]

I have bypassed (disconnected) the 12A on the main chassis and connected the EF86 to coupling capacitor just before pin 7 of the first 7247. Sounds good and clean but no overdrive to speak of. It is relatively quiet and full-sounding.

Well, that's why you don't have any reverb! Read my previous post.

[jmccanna]

Hi,

I saw on the circuit that the reverb drive circuit was connected to the input point on the organ level pot at the same spot as the preamp signal and branched off to the 7247. When I moved the input point for the guitar signal I moved that wire with it too.

[sluckey]

I saw on the circuit that the reverb drive circuit was connected to the input point on the organ level pot at the same spot as the preamp signal and branched off to the 7247. When I moved the input point for the guitar signal I moved that wire with it too.

Ah, OK. Has the reverb worked properly since you moved that wire?

[jmccanna]

Hi,

Well not really but I am not sure it was the move that did it. Now, the reverb amp itself does not appear to be working at all. I need to do some troubleshooting for sure.

I recall I was testing the reverb amp itself and input some signal into the reverb return, I heard some weird sounds and the reverb speaker burped and I saw the cone expand up like a balloon and then collapse with no sound. I immediately shut everything down. But, since then I am getting no audio out of the reverb amp itself.

I will poke around between meetings today.

[jmccanna]

Hello: After much trial, tribulations, mistakes, misunderstandings, learning moments and dumb-assery I got it working the way I want it to. Including the reverb. The reverb flummoxed me all day yesterday as I tried to think it through while working at my day job. . . . . . It did not dawn on me what the problem might be until I was nearly asleep and it hit me. The reverb driver circuit in the original Hammond leaves the organ preamp input at the same place the organ tone generator signal enters the organ preamp. It branches off to the reverb driver circuit just before the organ preamp. After it is a 500k organ level pot, the pot goes to the plate of the preamp and beneath that pot is a 220k resistor to ground. When I moved the input point I left the reverb driver wire attached to the input but did not include the 220k resistor to ground. Once I inserted the 220k resistor to ground the reverb came back.

My circuit is now this: (subject to much tweaking but operable):

Guitar input -> EF86 (on prechassis) -> 10k pot (change value?) -> grid of 1/2 plate of 12A (on main chassis) -> plate of 1/2 to 500k pot (change value?) -> 2/2 grid of 12A -> (this plate not used as output) -> output from pin 8 cathode -> 25uf cap -> amplifier. I want to tweak the values of the cathode resistor on the EF86 to bring the gain down a bit and probably the 12A as well. It sounds good when played quietly and gets really loud when I turn it up.

This is major progress for me.

Thank you for the patience you have shown me as I stumbled through this. I learned a lot. I will tweak this some more and clean up the wiring and then start on a simple cabinet. I am a big believer in repurposing things. I was given an old set of home-made speakers by a friend and will be using the 1/2" plywood from it for the amp cabinet. I will use a separate speaker cabinet driven by the amp.

Thanks all!