Hammond AO-63 conversion

[sluckey]

Still following with interest. Keep us posted.

[jmccanna]

Hi,

I remain very interested in this project. I am still sometimes getting a bit of hum. I was fortunate to find a speaker cab with four 12" Jackson drivers in it that was in great shape. It sounds great with this amp when driven by the little Akai preamp. The AC15 preamp looks intriguing to me but for the tremolo. And I wonder what anyone's thought might be to build a Marshall style preamp?

In essence, since I have this preamp chassis that came from the organ I want to build two preamps on it and use them alternatively to drive the amp. Not sure how to figure that out yet. Gotta get through the next week or so before I will have sufficient time to get to it. Suggestions are welcome. I am intrigued by emulating this amp which is on the local Craigslist here in Seattle.

https://seattle.craigslist.org/tac/msg/ ... 56579.html

[sluckey]

That Craigslist amp looks like an AC15 preamp, Marshall 18W preamp and 18W power amp built on a Hammond AO-43 chassis. I have something very similar, in fact I would not be surprised if it was identical. Very simple. Take a look at my Dual Lite...

http://sluckeyamps.com/dual_lite/dual_lite.htm

[jmccanna]

Hello,

Again, thanks. A path forward seems obvious now - I was leaning towards a Marshall style alternate anyway.

I was going around in circles trying to decide what to do next. Your amps look so "put together" and professionally laid out.

While, I do not have the capability of making a chassis and such I do have some sheet metal left over from the various things I have taken apart. Maybe at least a face for the switches and pots is in order with some of that. Then the amp case.

Thanks.

[jmccanna]

I decided to poke around during a free moment to try to narrow down the hum I was hearing. Then, I had a "Doh!" moment. I was wondering if I could isolate the circuit causing the hum. After messing around a bit I decided to unhook the Akai amp I have been using as a preamp and plug directly into the Hammond main amp with my guitar. Virtually no hum. Plugged the Akai back in got hum. "Doh". I am pleased to say it is not the Hammond but now I have a problem with my Akai.

One thing at a time.

This hum stuff is hard man.

James

[jmccanna]

So, the hum is intermittent. It is there even if just the Hammond is powered up with the guitar inserted. I tried changing guitar cables same hum. I disconnected everything including the reverb. The hum actually lessens a bit when I turn the organ level pot all the way up. When I turn the level down it gets louder midway and then lessens towards all the way down. The reverb level pot makes no difference. Everyone's favorite Uncle Doug has this youtube up and describes the 60 cycle culprit is often a tube? I do not have a tube tester so it is unclear if the original tubes are bad at this juncture.

https://www.youtube.com/watch?v=GrVtX0QGNls

[jmccanna]

Update on Hum:

I did some old-fashioned poking around, tube-swapping, chopstick poking, DMM measuring and found the problem. I noticed that V6 (V306) seemed to be the source of hum when I was pulling and placing tubes. I took my DMM and measured the resistance of the resistors there. One coming off pin 7 of the reverb recovery input R345 was supposed to read 3.3M and it did not come close to that in circuit. So, I removed it and tested it out of amp and it read correctly. I simply went in and reflowed all the connections for the tube pins there and anything connected to it. The hum is gone now.

So, next step is to reconnect the Akai preamplifier and see if it works now.

[pompeiisneaks]

Glad to hear it's coming in along.

Phil

[jmccanna]

Hi,

I have been struggling with balancing an emergency family situation with getting through this project. It has been piecemeal for sure.

I have decided to start real simple with a Vox AC15 inspired EF86 preamp and then add more later. I would like to use the existing preamp chassis through which all the original tonewheel generator tones, vibrato tones went through before going to the main power chassis.

I have attached photos with the tube sockets labeled, the topside of the pre-chassis and the underside of the existing connection boards. I have tried to trace it PTP but then get called away to deal with the ongoing family emergency. I have connected the preboard to the main power amp with the existing connection wires and am getting good voltages at the tube sockets and the tubes warm up nicely. It is the input/output connections that I am having a problem sorting out.

I would like to use the existing prechassis because it has six sockets on it that are available for the preamp and/or preamps I settle on.

I have attached pictures with the sockets labeled, a topside photo of the prechassis and an underside of the connection boards.

My attempts to trace the existing circuitry has been challenging. I am wondering if I would be better served to carefully strip out the unneeded wires and start the preamp from scratch using the existing sockets except for the heater connections?

As always, I appreciate the experts here and am open to your suggestions.

James

[jmccanna]

Progress Update: Made a mistake on my drawing! Be right back.


James

[jmccanna]

I think I got it right this time.

I traced the modifications I did to my Akai M-8 conversion when I adapted it to a guitar amp. Here is what I did to the Akai and it is what I am trying to emulate for the Hammond amp:

1. I am connecting to the input pin 9 on the EF86, the output pin 6 of the EF86 to a potentiometer between it and the shorted jack for the 12A.
2. When plugged into the EF86 its signal routes through the 12A jack (no plug connected into 12A jack) to the 12A.
3. I control the level of gain from the EF86 with the intermediary potentiometer.
4. If I plug into the 12A jack the EF86 circuit is shorted.

I think I did this right.

I have attached the corrected schematic and the original Akai M-8 schematic I worked off of.

[jmccanna]

Update:

Just in case anyone was too embarrassed to tell me how wrong I was on the uploaded schematic I made several mistakes on it and recognize it now. Took a look at it this morning after looking at the Akai M-8 amp and went DOH!

My schematic drawing skills leave much to be desired. I am going back through everything now.

[sluckey]

Let us know when you get it straightened out.

[jmccanna]

I had some time finally over the last day or two to get back to it. I managed to get audio out of the preamp this afternoon which is great because I was getting nothing at all before. The sound is awful, low-level and distorted but I am getting audio!

I need to go back through and take a look at the solder joints and make some adjustments to the values of the plate resistors. I suspect I have the wrong values for the resistors on the plates for sure. I have been using parts available to see if I can get anywhere at all. At least I got sound and now I just need to fix it!

[jmccanna]

Hello all,

At the risk of seeming an idiot - a common occurrence - I have hand-drafted a schematic that I think may work. I am actually quite nervous about presenting this because I am so new at this. I am trying to emulate a signal path similar to the Akai M-8 I have used as a preamp for the Hammond.

The hand-drawn schematic differs from the Akai M-8 schematic. You will notice on the Akai schematic it allows me to pull a pre-amp signal off of J5 and that appears to be connected to the cathode of the 2nd half of the 12A. This is not in my hand-drawn schematic though I would love it to be. When I was testing the Hammond main amp using the Akai, I was pulling the preamp signal off of J5.

Does my hand-drawn schematic make for a sensible preamp?

I scribbled a bit on the "Akai" page to indicate the signal path I am trying to create. My scribbling is bad I know but on the Akai page R11 references a 500k resistor that connects to the grid input and to ground and R12 is 3k but it connects to the cathode of the 1st half of the 12A. I scribbled in in violet a 500k pot between the plate of the EF86 and the grid of the first half of the 12A and then a second 500k pot between the plate and grid of the 2nd half of the 12A. That way I can monkey with the drive level from the EF86 to the 12A and then to the amp. The output of the 12A would go to the Hammond amp which is working very well at the moment.

I am getting about 330v of B+ at the preamplifier chassis which if I understand correctly will come in to the circuit from the right and then the resistors in line will bleed off some voltage.

Thoughts, comments, criticisms or suggestions?

James
Kingston, WA