Hammond AO-63 conversion

[jmccanna]

Update Ad Infinitum 2: Right now I have P302 jumped between 1 and 3. The third wire off of that comes from what looks to be a ground point for several resistors and a capacitor off of V6. These are connected with a black wire that runs to Pin 2 of P302. Of course Pin 2 does not connect to anything since I have the plug pulled.

[jmccanna]

I mentioned earlier that I previously had converted a little Akai M-8 (RtR) SE tube amp to a guitar amp. I reconfigured the wiring so I could use either an EF86 through the first stage of a 12AX7 with a pot in between or go straight in to the the first stage of the 12AX7. I had forgotten that the amp has a pre-out in between the first and second stage of the 12A. I plugged my SG into the pre in on the Akai and that pre out in to the Hammond and my oh my I like it!

Gotta lessen the 60 cycle hum coming out of the reverb circuit. All said though it gives me pause and hope that I can put together a really great preamp/tone circuit with the help here!

Thanks,

James

[sluckey]

Chase the hum...

Swap V5 and V6. Does the hum go away?

Disconnect the reverb tank output cable from P304. Does the hum go away?

Use a gator clip lead to put a jumper between chassis and V6 pin 7 (be careful!). Does the hum go away?

Remove V6. Does the hum go away?

Report

[jmccanna]

Swapping V5 and V6 did not work - I tried that earlier.

Remove cable for reverb tank output from P304 lessened the hum a lot but there is an underlying ocean like sound.

Gator clip between V6 pin 7 and chassis virtually eliminated the hum but there was still the ocean sound under it.

Remove V6 eliminated the buzz and there was still a bit of ocean sound.

I noticed when I looked at V6 with a mag glass that the capacitor between pin 6 and pin 3, I believe, had a leg that was really really close to pin 5. I moved it out of the way with a chopstick.

Chase the hum...

Swap V5 and V6. Does the hum go away?

Disconnect the reverb tank output cable from P304. Does the hum go away?

Use a gator clip lead to put a jumper between chassis and V6 pin 7 (be careful!). Does the hum go away?

Remove V6. Does the hum go away?

Report

[jmccanna]

I swapped the three 7247 tubes around and got them in a configuration where the hum is gone. Still present is a hissy ocean-like sound but it is much less than it was before. Tighten sockets? Sounds good though.

[jmccanna]

Last update of the day. I messed around with the reverb level pot. It was up quite high before which seems to be the source of the hissy sound. The ocean sound is gone since I switched around the 7247 tubes. There is no more 60 hz hum.

Go figure?

Have good evening all.

James

[sluckey]

Maybe everything just needed some time to settle in? Sounds like everything is basically working as designed. I would want to let it burn in for a while. Turn it on and let it just idle for several hours at a time over the next few days while you consider things you want to do to make it guitar friendly. You've heard what a guitar preamp can do for it. Give some consideration to replacing that organ preamp with a nice guitar preamp. I think the Fender AB763 preamp or the Ampeg preamp would be good candidates. Either would really wake up that AO-63 and are fairly simple (but a bit tedious) to implement. Don't rush yet. Give the amp some power on time.

[jmccanna]

Thanks for all the help. I am always impressed at the willingness of others to share their knowledge and time. Sluckey - did you get on a jury?

I woke up with a hare-brained idea. I have already converted the Akai M8 to a cool-sounding SE amp but it has limited power. The tone is outstanding. In the Akai, I incorporated the EF86 to precede the 12AX7 (I can use either as starting point) and I get a natural tone that is really sweet to my ears and to others I have shown it to. It does not have tone control, reverb or tremolo but it sounds terrific as it is. I have not yet put it in a case with just the naked chassis/face sitting on my work bench.

What if .... . . . . . . . . . . . . . .

I combined the Akai and Hammond into one head? I love the preamp sound of the Akai through this Hammond. I love the amp sound of the Akai when it drives its own speaker.

I could use the Akai as the plugin point, tap into the preamp signal of the Akai and go to a tone stack and then choose to use the Hammond or Akai via a switch? That way if I wanted a simple clean SE amp sound I would just use the Akai and for other uses the Hammond amp.

So, while I love the sound of the Akai pre I do not to discount other options either. Here is the kicker though. I love the appelationary gymnastics folks go through to name their amp offspring. I would call this "The HammAkai"

Eh?

[sluckey]

Sounds like a plan to me. Play it like that for a while. The more you play it, the more ideas you may form about it. You can always modify later on if so desired.

I was dismissed from jury selection this morning during the individual interview. Seems my strong views on the death penalty for a capital murder of a 20 month old baby girl were not in line with the laws of Alabama concerning the penalty phase of a capital trial. I choked out my last words, "Judge, there can be no mitigating circumstances for the murder of a 20 month old baby." The judge repeated those words to be sure the court recorder got it and then dismissed me.

[jmccanna]

Hi,

I am quite sure your sentiment was heard and felt by many there who did not express it like you did. I am going to concentrate on the tone circuit. Wondering about your thoughts on whether the tone circuit is best right at the start of junction of the 2 stages of the 12AX7 pre or later on as a cathode-follower. I watched this set of videos by Uncle Doug to educate myself:

https://www.youtube.com/watch?v=BatwDYFJ9ug

and

https://www.youtube.com/watch?v=uZHM5BwPLRw

The second one has all the information on tone stacks.

Thanks,

James

[sluckey]

I would need to see a schematic to follow what you are saying.

[jmccanna]

I figured a schematic would be needed. I may just hand draw one - my drawing skills are piss poor though. In the next day or two I will get that done. It will be a good exercise anyway to sort through what I did in the real world. I made all the changes to the Akai from studying its schematic and making changes after figuring out the PTP connections. I have attached the original schematic for the Akai though to get started. Maybe that will be enough.

This was awhile ago so if memory serves, what I did was with the Right amp. Essentially:

1. I removed connections/jumpers that went to the tapehead. Removed SW1 altogether and connected the wire to it to ground. SW1 acted as a monitor switch and had to be moved to the left to hear monitor signal (if memory serves).
2. As original, J2 connected to the grid of the 12A at C8 when SW1 was slid over.
3. As original J3 connected to the grid of the EF86.
4. I disconnected VR1 from its connections and connected the plate of the EF86 to VR1 and VR1 to the top of J2 which in turn connects to the grid of the first side of the 12A.
5. I had to redo the connections for J3 so it was in series with J2. I had to remove a resistor that went to ground somewhere in there too to get signal through. The effect was if I inserted a plug into J2 J3 was disabled and the signal went to the grid of the 12A. If I plugged a plug into J3 its signal went through to the EF86 to VR1 through J2 and then to the grid of the 12A.

I think I have this right. Anyway I can adjust the amount of signal from the EF86 to the 12A (the existing pot value is imperfect). VR2 controls overall volume to the speaker and/or pre signal out.

[jmccanna]

The hum and ocean sound is back a bit from yesterday. It is lower if I have the guitar plugged in. It does not change if I unplug the reverb tank though it is definitely coming from the reverb circuit. I pulled and reinserted all the 7247 tubes.

It is there still though. I will let it burn in powered up for awhile and see if it lessens as it fully warms up. Thoughts? Suggestions?

I think I will examine the amp closely following the principles in this article. http://www.lynx.net/~jc/hum.html

That is, unless it makes sense to chase down this hum completely after I complete all alterations such as removing unused wires and components. The hum is present but not terrible now not at all unlike many amps I have heard over the years.

James

[jmccanna]

Sluckey, I hope I have not worn out my welcome here.

Due to size constraints I am losing interest in using the Akai as a pre driver. However, I really like the sound of the EF86 when it precedes the 12AX7. Would it be possible put an EF86 ahead of your Gemini II preamp to drive the 12AX7 there? Then leaving everything on the amp as it is with the reverb on-board and all.

Here is a partial schematic for an EF86 where P1 I think would go to the grid on the first stage of the 12Ax7. The potentiometer would control how much the 12AX7 gets driven by the EF86.

Tell me if I am bonkers - wouldn't be the first time someone did.

Thanks,

James

I would need to see a schematic to follow what you are saying.

[sluckey]

If you like the EF86 sound then just replace the organ preamp V1 with an EF86. It will drive the main power amp nicely. The AC-15 EF86 preamp would be nice.