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'Organ Donor' - Hammond AO-29 to 6V6 Plexi Conversion

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Jan 3, 2017.

  1. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    New year means new projects right? I finished a 5F2A, AB763, and 5E3 last year so I'm brimming with hubris to press forward into amplifier diy glory.

    I recently picked up a few organ chassi (that's the plural for chassis right?) on a whim and want to convert this first one into a 6V6 Plexi style amp. The organ was originally an AO-29 with two 6V6 output tubes. My goal is to take the chassis and transformers as a baseline, and build a brand new amp from there. I'd also love to use some of the old tubes that came with the amp as well.

    The Mighty AO-29

    Here is the AO-29 ready to be decimated $44 shipped to my door.

    [​IMG]

    [​IMG]

    It had a tube compliment that I removed. The 6V6's in particular may be useful. The rest perhaps not so much. According to the schematic:

    http://www.stefanv.com/electronics/hammond_ao29_overhaul/schematic.gif

    The tube compliment is as follows:

    v1 - 6AU6
    v2 - 6AU6
    v3 - 12AU7
    v4 - 12AX7
    V5 - 6V6
    V6 - 6V6
    V7 - 6C4
    V8 - 6C4
    V9 - 12AU7
    V10 - 68A6
    V11 - 5U4

    So here's to hoping the 12AX7, 6V6's, and 5U4 may be functional and useful in a 6V6 plexi circuit.

    Resources

    Here are some links (partially for my own reference) where I am taking inspiration and direction for the final results.

    Hoffman's 6V6 Plexi - http://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf

    A similar conversion thread - http://el34world.com/Forum/index.php?topic=20482.0

    AX84 'October' Project (Club Version) - http://www.ax84.com/october.html

    Layout drawing - http://el34world.com/Forum/index.php?action=dlattach;topic=20482.0;attach=59143;image

    Step 1 - Tear Down

    This chassis was pretty dirty. Lots of dust and grime everywhere. Most of the wires have a gunk on them. But I tried to disassemble it in a somewhat orderly fashion.

    [​IMG]

    Pretty funky amp with all these little transformers. I'm pretty impressed with Hammond for some good build quality. I decided not to try and use the individual components beyond transformers and tubes. From what research I performed I gathered that the old carbon comp resistors are a 'maybe' but with potential noise concerns. The larger capacitors definitely not. Here is the main filter cap, just for kicks. i don't see any leaking or bulging, but I'm not going to use it anyways.

    [​IMG]

    I paid particularly close attention to the power transformer and output transformer and which wires went where. I used masking tape and a sharpie to help me remember later down the line.

    [​IMG]

    Everything else is coming out though. Was pretty time consuming going through it all. Especially the little doohickey's that hold the tube sockets to the chassis. Are these rivets? Sorry, I have not really seen this kind of thing before. I have been using my drill to drive a hole through the heart of the rivet and then use a screwdriver to finnagle it out. Is there a better process to get these out more effectively?

    [​IMG]

    Going to keep disassembling. At some point I will do a thorough cleaning of the chassis and transformers. Then we'll talk layout and get cracking!

    Any input would be much appreciated. I'm pretty much just an enthusiastic noob who has yet to shock himself.
     
    finom1 likes this.

  2. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    cool project kdj. On the rivets, I like to use an oversized bit to take the head completely off of the rivet then a small tap and the rest will fall out.
     
    Darkness, flyswatter and kleydejong like this.

  3. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I have one of these as I just tore down a Hammond M111....very clean. I am thinking it will make a Dr. Z Z-28....maybe with trem and reverb. There is a cool build on YouTube with the AO-29 where he uses the pentode 6AU6 in an input preamp stage. He also builds a guitar pickup using an Altoid box to hold that odd little two coil tranny to the right in your first picture.
    The AO-35 reverb amp from this organ will become a Carmen Ghia...exactly what Zaite used for his first Ghia builds.
     
    flyswatter likes this.

  4. flyswatter

    flyswatter Friend of Leo's

    Jan 12, 2013
    Quebec
    Nice! I'll be watching this thread because I am currently working on a "FrankenAmp" that makes use of a similar old Hammond chassis I was given last year. In my case, I stole the transformers but found it more convenient to use a chassis (and cab) from an old broken Vox Cambridge I've had sitting around for a couple of years.

    In mine (and it looks, yours), the wires are colour coded very similarly to current Hammond (the transformer company) classic transformer line. So their data sheets are useful in working the old organ transformers into the new build.

    Mine are as follows:

    PT: Black primaries to power switch and AC white; reds and yellows to the rectifier pins; greens to the heaters; the two colour-striped wires (centre taps, I think) to ground.

    OT: Blue and brown primaries to the power tubes; red to the power stage; Green and black secondaries to the output jack.

    I'm also using the choke on mine. If any of these doesn't perform well (they're in rough old shape cosmetically), I'll replace with new Hammonds; but making use of these old boys can really cut down the cost on an improvised build.
     
    kleydejong likes this.

  5. dan40

    dan40 Tele-Holic

    822
    Aug 19, 2015
    Richmond Va
    Looks like a great project! Here is a link to several really nice Marshall style layouts that may come in handy. They use to be available through the Metroamp Forum but unfortunately a lot of them were lost when their site crashed last year. Since you are doing a 6v6 build, the 1987 (50 watt) layouts may be of some use. https://drive.google.com/drive/folders/0By1xsHfamQ8iaFlTdjdaSFNBOGM
     
    flyswatter and kleydejong like this.

  6. Pullshocks

    Pullshocks TDPRI Member

    66
    Dec 17, 2012
    Seattle
    Thanks for posting this. I also have an AO29. The OT looks OK but I discovered the PT is fried. Blessing in disguise, it was the higher voltage type used with the field coil speaker.
     

  7. jondanger

    jondanger Friend of Leo's

    Jan 27, 2011
    Charm City, MD
    Cool project. I have an AO-29 to 6V6 Plexi conversion that I had Mark Moser of Frugal Amps build for me, then I build head and speaker cabs for it. I had Mark use the original OT, but then decided that I wanted to be able to use 6L6s in it, so I swapped in an Allen TO-26. That PT will support 6L6s, especially if you opt for a solid state rectifier. Nothing wrong with the original OT though.

    That Hoffman page seems to be one of the most comprehensive resources on this conversion. Good luck! I'll be watching.

    Here are my threads on the cab build and some troubleshooting.

    http://ww.tdpri.com/threads/6v6-plexi-headshell-and-2x12-cab-build-complete.630615/

    http://ww.tdpri.com/threads/6v6-plexi-input-cap.627199/
     
    kleydejong likes this.

  8. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Great information here, thanks! I am glad I tried to identify what each lead was doing and mark it with a piece of masking tape.

    And on the value note, these are pretty great. One of these AO-29's I got for like $50 and it will supply a chassis, transformers, and a set of old tubes. If even one of those things are in good shape for the build it will come out ahead.

    Then I really like how Hoffman lets you individually itemize a kit, so I took the 6V6 Plexi kit and removed all unnecessary components. I think the small parts were about $180 - and I got them as a Christmas present. So my overhead for this amp is going to be very low!
     
    flyswatter likes this.

  9. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Thanks for this. Those cabs look AWESOME and the clips are sweet. Getting me pretty excited to fire this up.

    Good to know about 6L6's. The original amp has a tube rectifier with those 6V6's, so it makes sense to me that if I use the solid state diodes with the kit that I may need to use JJ 6V6's or 6L6's to handle the higher voltages.
     

  10. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Made a little more progress this weekend. I'm getting wrecked pretty hard by the chassis. Snapped my cheap Skil drill bit in half trying to drill a hole through a rivet. Might need to go to the hardware store and get some new bits. I would prefer to get the chassis all setup and ready to go, but I had some time so I started on the component board.

    This is my first project working with turrets instead of eyelets. My first impression is that eyelets are simpler and more straightforward. Turrets are a little more complex, but perhaps more elegant. I like how you can be a little more orderly by keeping components in the holes and then jumpers wrapped around the outside of the turret. I don't know if there are standard practices, but I enjoyed working with turrets as I progressed through. I tried some funky jumper wrapping and in some spots it got kinda overkilled. Towards the end though I really enjoyed the turret board process. The only thing I really didn't care for was how it felt like I needed to use a lot of solder to fill the inside of a turret. I instinctively try to get that nice Hershey's kiss look on the back side, but perhaps I should not be using that much solder inside a turret.

    I am also using a new soldering iron. I got a Weller 40 watter. It is very effective, gets hot quickly, and is quite efficient for heating turrets. However I feel like the tip is degrading very quickly. It is already chipping away. And it almost seems to get too hot as you can see some singing of the black board itself around the base of some of the turrets. I have to be mindful about how long I keep it on there. I also feel that I need to start using heat sinks as thus far I have not used them.
     

  11. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I started putting stuff on the board as I normally would, but then I saw a video about wrapping bare wire around turrets for the jumpers. Doing so seemed pretty hard with components on. Then the light bulb went off. Page 3 of the Hoffman project PDF (http://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf) has a guide for just the jumpers. So I did those first. Went much more smoothly.

    [​IMG]

    Eventually got to here:

    [​IMG]

    There are some connections on the bottom of the board, and one .1 cap I didn't put in yet wanting to make sure it was the only one left ;).

    Going to get back on the chassis and try to get that all ready to go before I proceed with anything else.
     
    RollingBender likes this.

  12. flyswatter

    flyswatter Friend of Leo's

    Jan 12, 2013
    Quebec
    Looks like your are making good progress. Doing turret boards does take a bit of practice. I generally try to dry mount components and wires to have a mechanical connection before the solder goes on -- but getting too religious about it on a crowded turret can create more hassle than it's worth. As long as the solder ends up shiny and the component won't pull off when it cools, it should be alright.

    Some of the places where you've wrapped in figure 8s might be places where you'd want to trim the legs on the resistors instead. Once or twice I've had to replace parts on a build (for instance, changing a bias resistor, or replacing a wire that I trimmed too short to reach its destination), and it is useful to be able to do so without a big nightmare getting things unsoldered and unwrapped.

    Cheers.
     
    kleydejong likes this.

  13. RollingBender

    RollingBender Tele-Holic Vendor Member

    728
    May 14, 2011
    SW Minnesota
    kleydejong...
    Cool project! I have the same Hammond sitting in the doorway of my workshop. Ordering the bits and pieces tonight to turn mine into a variation of a Deluxe.
     

  14. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    A touch more progress. This is my second project attempting to rehabilitate some old, non-functional amp. The chassis work is my least favorite part. But I managed to get the rivets out by using a drill and going at them from the inside.

    [​IMG]

    Had a bit of a go with some sandpaper to scuff off some of the rust and grime, however I kind of enjoy some of the natural patina - so I'm not going to go hog wild in terms of derusting and buffing / polishing. Still some good metal in there.

    [​IMG]

    Pretty much ready to proceed with mounting the hardware.

    Here is a mockup of how I plan on mounting everything in the chassis. Any input or feedback here would be welcome.

    [​IMG]

    Specifically I went back and forth a little on the position of the choke and cap can on that side of the OT. Would it be better to try and position them closer to the component board - possibly underneath it?

    Also I am planning on using some kind of cover on the rear panel so I can firmly connect the IEC, Fuse, and speaker out. I was thinking I might try to find a thin strip of wood as that would be easy to work with.
     
    flyswatter likes this.

  15. I found a 3 foot length of aluminium flat bar at the local Canadian Tire. Was 2.5 inches wide and about 1/16 inch thick. Using it to reface my Hammond reverb amp conversion. Check your Home Depot
     

  16. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Got pretty busy for a few weeks. Finally had some time this weekend to get some work done. I also had a friend come over who wanted to learn how to solder, so it was pretty convenient that I had a project going.

    First I finished the chassis. Didn't really need to do much other than drill some holes here.

    [​IMG]

    Got a little creative with a wire mesh cover to mount the input jack and cover a hole on the front panel. I had this old piece sitting in the corner for a while. I think it came off the Epi Valve Junior I converted into a 5F2A.

    [​IMG]

    Holes drilled in the chassis. Will this kind of work dull a drill bit pretty quickly?

    [​IMG]

    Pots mounted.

    [​IMG]

    Rear will only have the speaker out and an IEC power connector. I have never actually been a big fan of old Fender style power cables that connect internally. I much prefer IEC's.

    Power switch (no standby), pilot light, and fuse on the front.

    [​IMG]
     
    Fiat_cc likes this.

  17. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Had a good night's rest and started working inside. I quickly realized that the chassis is quite narrow front to back and I may have miscalculated my spacing. Specifically the location of the component board as it relates to the input jack and pots.

    The input jack is quite close - but I tested it with a cable plugged in and nothing touches. I may use some electricians tape to make sure though. It is tight.

    [​IMG]

    The pots actually sit on the component board. I had to finnagle it a little to get it all to fit snugly.

    [​IMG]

    It is at this point that I wonder how I should revise my process. I am at kind of a weird place where I have a couple successful builds under my belt, but I still don't quite know if I have my preferred methods figured out yet.

    Specifically now. I did the component board and chassis pretty separate. I installed the wires with plenty of lead to reach their spots - but it was pretty blind. So now that I drop the component board in it gets pretty messy. Lots of stray wires going everywhere.

    And I never quite know when to do filaments. If I do them now they're easier, but then the leads running from the component board to the tubes gets harder. I guess that whatever you do last in a tight chassis is going to be a bit of a pain though.

    [​IMG]
     

  18. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
    34
    510
    Oct 8, 2016
    Melbourne, Australia
    In the build I just did I ran heaters first but I reckon I had to remove them at least twice to acces the pins closest to the board (which are practically under the board in my chassis). I too would be interested in other people methods and order of doing things as I intend doing a lot more of this (18W Marshall next I think).
     

  19. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    In my first build I did the filaments at the very end and thought it was a huge pain in the rear. So I'm doing them first here. The connections to the preamp tubes from the component board will be tricky though.

    Got some work done on the power section. The wires from the PT are pretty gnarly. Perhaps it was cloth once, but now they're very rigid and kinda nasty. Strip easily though, and the wire seems fine.

    The two brown-ish wires that came from the power cord are now routed to the power switch and the IEC as indicated here - http://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf - as black and black / white.

    Then I have two brown-ish / black-ish wires that had previously gone to pins 2 and 8 on a power tube. I'm interpreting those as my heaters. Given the fragile state of the wires I actually chose to run my heaters to the power tube as the stiff wires were already heading in that direction previously. And the pilot light terminals are kinda small, so I chose to wire my heaters as shown below. And then I derped and ran out of the green / white wire I was using - so I'm switching halfway to a yellow twisted pair I had left over from a previous build. Kind of a weird way to do heaters...

    [​IMG]

    The red and orange wires had previously gone to a tube rectifier. I could use a little help here. The reds came from pins 4 and 6. Yellows from 2 and 8.

    [​IMG]

    So if I'm reading the schematics from both the ao-29 and the Hoffman 6v6 project my reds are the same as the ao-29 and so I will run them to the diodes.

    Then the yellows are not used as I am using a solid state rectifier. Then do I just tape them off or run them to a dead end terminal strip so they don't cause any trouble?

    Then after that I have a red / yellow which I believe is a center tap and will be grounded. Then I also have a mysterious green wire that went to a pot of some kind. Was this a bias of some kind? Any thoughts on what to do with this wire in my build?
     

  20. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I think I found an answer to my own question. Information found - http://el34world.com/Forum/index.php?topic=20482.100.

    As I understand the original amp had a field coil speaker. I can replicate the load this would provide with a 700 ohm 30 watt resistor between the green mystery wire and ground.
     

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