Hammond AO-29 Organ Conversion #2 - To AB763 and Beyond!

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Aug 30, 2017.

  1. kleydejong

    kleydejong Tele-Holic

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    Hey guys, I'm back with another build. I recently completed a similar AO-29 conversion project to something similar to a Hoffman 6V6 Plexi here.

    That build had some challenges. Most of them were my own failures, but I learned a lot. I had originally bought two Hammond AO-29 amps on eBay, so now I want to tackle this second one.

    The AO-29

    Here is my raw material.

    [​IMG]

    [​IMG]

    [​IMG]

    Here is my working schematic for the AO-29 : http://www.stefanv.com/electronics/hammond_ao29_overhaul/schematic.gif. There are a lot of Hammond amps and this may or may not be 100% accurate to what I have. But I'm hoping it can give me some guidance. I'll also be using my experience with my 6V6 Plexi heavily.

    I am also going to try and document the build with a series of YouTube videos as well. Playlist link here:

    https://www.youtube.com/playlist?list=PLsb7JxRJF2FhY8gKDrNkmlGmFwXOSDgVS

    I'm not really sure what the best way is to capture a build, whether a thread like this or some videos is better. But I enjoy making videos, so I'm going to give it a shot. Any feedback is welcome.

    Goals

    My favorite amp is a Hoffman single channel AB763 that I built a while back. I'd like to try something like that again. Hoping to gain a better understanding of that amp sounds so good to me, and how I can hopefully repeat it. So my intentions are as follows:

    - Single input
    - Blackface AB763 two stage preamp
    - No reverb or tremolo
    - Long tail pair phase inverter
    - Push Pull fixed bias 6L6GC output section
    - Solid State rectifier
    - Dead simple volume, treble, mid, bass controls
    - Point to point wiring
    - Reuse as much of the original AO-29 as possible. At this point this includes the chassis, transformers, possibly the tube sockets, possibly some of the terminal strips.
    - Build some kind of head cabinet

    I'm going to start simple with these objectives hoping to nail that Blackface Fender tone that I love so much. Then from there I will probably consider adding another 12AX7. I'm thinking I'll use one triode stage to add a different V1A voicing (probably Robrob's lead channel) that can be placed in series or parallel with the AB763 V1A. Then the other triode I'm thinking I will place before the phase inverter. I'd like to be able to access the 3 triode AB763 with Reverb style preamp voice. And with some switching I did some research and believe I'm only a hop, skip, and a jump from a Trainwreck Express-ish preamp. So I may give that a shot. But all of this will be down the road a bit.

    Here goes nothing...
     
    Last edited: Aug 30, 2017
  2. sds1

    sds1 Tele-Holic

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    Cool I'm subscribed.

    So looks like the PT you got here provides a 320VAC secondary for HT, is that right? And would be good for any 2x6L6GC build?

    Big chassis.
     
  3. kleydejong

    kleydejong Tele-Holic

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    Good question. With the 6V6 Plexi my rectifier currently shows about 390vac on pins 4 and 6 of the rectifier. 367vdc on the plates of the power tubes. However I was getting some weird stuff where the voltage would jump up to about 445vdc on the plates if I put a pair of 6V6's in. I currently have 6L6GC's in that amp and it is running well and sounding good.

    A thought that I have is perhaps related to current. The original Hammond circuit runs a lot more tubes than my amps will. So maybe having more tubes drawing more current from the PT is affecting the voltages. But this is really stretching my poor theoretical understanding of all this.

    It is also possible that I miswired or am misreading voltages.

    Also it seems that this hammond is marked as model H-AO-29-13 while my other amp is a model AO-29-1B. Could change things.

    Therefore the plan is to get it up and running. Then depending on where the voltages come in with the circuit I install I can tweak the amp to be closer to a Deluxe Reverb with 6V6's or up to a Super Reverb running 6L6's at higher voltage.
     
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  4. kleydejong

    kleydejong Tele-Holic

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    Deconstruction

    One of my goals with this build is to reuse more of the existing AO-29. So I was more careful with taking the amp apart. This involves snipping out a lot of junk circuit. I take care to take pictures of and use masking tape to mark off the leads from the transformers. Best to know exactly where they went before, so I know what to do with them in the future.

    I will clean out the chassis a little bit. Some sandpaper and elbow grease goes a long way. Though honestly, I enjoy the aged patina look.

    For some unknown reason, there is a pretty woeful smell that occurs whenever I desolder something. Maybe it is 50 years of grime and bleh being burned up?

    One decision I haven't fully committed to yet is which way will be front / back. The top of the chassis has the tube sockets and a row of terminal strips that could be pretty handy for mounting components. I'm wiring this all point to point, so I'd ideally like to keep wire runs as short and direct as possible. Also strange is that my V1 preamp tube is currently that 6th tube socket from the right, near the center of the chassis. The spacing just feels a bit awkward.

    I've done one pure PTP build previously. One thing I learned from that build is the importance of making a plan. Really think through how you place out each and every component.

    [​IMG]

    Thinking about it a little bit more, I think I might take out a few more of those tube sockets on the preamp tube side. Then I can evenly space out my 3 preamp tubes more cleanly and just use the overall space more efficiently.

    I also have currently left the heaters wired as the original. They're not perfect. They untwist near what will be my phase inverter kind of randomly. But I might leave them for convenience.

    Some more close ups. Rectifier on the left. One of the power tube sockets on the right.

    [​IMG]

    The two power tubes on the left. Then two more 9 pin preamp tube sockets. These would be V1 and the Phase Inverter if I don't move things around.

    [​IMG]

    Output transformer on the right. You can see the yellow and green secondaries go to some weird output thing on the bottom left. It has two connections for 3.2v filaments. The schematic indicates an artificial center tap via two 100 ohm resistors to ground. I need to remember to do that.

    Then I believe that yellow wire from the secondary of the OT is being grounded via a black wire going to the shell of the Rectifier tube socket? So the green would be my speaker out.

    [​IMG]

    Not 100% sure what the impedance is... I think the schematic indicates 8 ohms.
     
  5. Wally

    Wally Telefied Ad Free Member

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    Subscribed.
     
  6. kleydejong

    kleydejong Tele-Holic

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    Due to the more custom nature of the circuit and the fact that I want to do a PTP layout means I spend more time planning than I think you otherwise would need to. I will say this though, I think this approach has taught me a lot and helped me transition from being able to follow a kit build with a 'paint by numbers' approach - to being a little more self-sufficient. The learning curve is higher, but I think it is more of a 'teach a man to fish' instead of 'give a man a fish' end point.

    So at this stage I am finalizing the circuit I want to build. Then I draw a schematic from scratch. Then I draw a layout from scratch. I found two schematics that have been very helpful for my initial direction for this build:

    http://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf

    https://www.scribd.com/document/352505894/SLuckey-AB763-Deluxe-Lite

    Mapping out my estimated layout. This is actually a second draft. I did a first draft and also drew out a layout on paper with all of the individual components roughly placed. I was thinking I might use the terminals on the rear wall, but have moved away from that idea. Those terminals are actually exposed on the rear of the amp, which made me uncomfortable. Instead I'm going to try a line of turret strips down the center. Then another smaller set near the pots for the tone stack.

    [​IMG]
     
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  7. kleydejong

    kleydejong Tele-Holic

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    I'm still committed to starting with the two triode basic non reverb AB763 circuit. But I will install V2's tube socket and its heaters. I believe I will want a third preamp tube for experimentation once I get her up on her feet.

    I'm debating adding a third gain stage before the PI with a master volume and possibly switchable cathode. The idea is to turn the master down a bit to simulate the reverb and tremolo laden AB763 similar to a Super Reverb. Or I can turn it up and switch the cathode to a 10k bias resistor with no bypass cap to achieve something closer to a Trainwreck Express preamp.

    I'm also debating on what to do with that remaining triode. I am thinking about using Rob's lead channel with series or parallel switching. Or I'm also intruiged by a Princeton style single triode tremolo with a solid state cathode follower as discussed here:

    http://el34world.com/Forum/index.php?topic=9255.0

    Something maybe like this:

    [​IMG]
     
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  8. muscmp

    muscmp Tele-Afflicted

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    excellent build! i'm keeping an eye on this.

    i have an ao43 amp, ao42 preamp, the reverb pan, the connectors and, two heppner speakers that i wish to convert into a head and cab project. i saw everything work when it was still in the 1965 L112(i believe), hammond so i just may piece it together and use pedal(s) in front.
    play music!
     
  9. kleydejong

    kleydejong Tele-Holic

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    I reviewed an uncle doug video on identifying PT leads.



    It led me to chart this diagram for my PT. Interesting as it seems to be a little different than my other AO-29 PT.

    [​IMG]
     
  10. kleydejong

    kleydejong Tele-Holic

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    It seems that I do not have a bias tap. Doing some research led me to this thread on creating a fixed bias tap when you don't have one.

    http://music-electronics-forum.com/t2545/

    Further review of those suggestions led me to Doug Hoffman's 50 watt Plexi project. It appears to me as if that amp takes a bias tap directly off the HT, and the bias circuit leading to the power tubes should be pretty good for my purposes. Perhaps with some minor tweaks to a few components to get my desired B- in the right ballpark.

    http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf
     
  11. D'tar

    D'tar Tele-Afflicted

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    King Fan, kleydejong and Snfoilhat like this.
  12. kleydejong

    kleydejong Tele-Holic

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    Brilliant, thanks.
     
  13. kleydejong

    kleydejong Tele-Holic

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    I drew a layout. I find with a PTP build it is pretty essential. If nothing else it pretty clearly shows where things will get bunched up and help plan where I need turret strips located.

    [​IMG]
     
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  14. kleydejong

    kleydejong Tele-Holic

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    Next I did some work on the chassis. I used some sandpaper to remove some grime and rust. I maybe could have spent more time with this, but I'm mostly just looking to get it a little bit cleaner. I like some of the natural relic'd patina. How much do kids pay for 'distressed' looking jeans these days?

    Before:

    [​IMG]

    After:

    [​IMG]

    I never know what to do with the faceplate. Most of my amps look pretty homely. For the most part I'm completely okay with that. In fact I kind of like the 'wolf in sheeps skin' aesthetic. All I really care about is tone and reliability. But in doing research on the Trainwreck schematics I did see some cool wooden faceplates that I kind of want to try with this build. So I took some scrap pine and cut a thin veneer on the table saw. It is a little rough, but with all the hardware screwed on I think it might work okay. I may reinforce the input jack as I secured it directly to the wood - which allows it to be insulated from the chassis ground.

    [​IMG]

    Also the pilot light I have for this build is TINY. Lol...
     
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  15. kleydejong

    kleydejong Tele-Holic

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    Spent a good chunk of time on the amp this afternoon. Input, V1A, Tone Stack, V1B, and most of the phase inverter are done.

    [​IMG]

    Trying to be oganized using solid core wire throughout. Black for signal path. Green for preamp grounds. I'm also going to try grouping my filter caps with the part of the circuit they filter, then use a multiple local star grounding scheme.

    Figure 15.14 here - http://www.valvewizard.co.uk/Grounding.pdf
     
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  16. kleydejong

    kleydejong Tele-Holic

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    I think I have the rectifier situation figured out, but I'd like to clarify just in case.

    1. I took resistance readings on my two red HT leads. Then I found the red / yellow lead has roughly half the resistance to either red HT lead. This means my PT has a center tap.

    2. Because I have a center tap I may use a Full Wave rectifier. If I did not have a center tap I would need to use a Full Wave bridge rectifier.

    3. If my preference is to use solid state rectification, then I can choose to use either 3 1n4007 diodes per side in series (as seen in the AB763 Bandmaster schematic) or I may use 1 1n5408 diode per side. 3 1n4007 diodes rated at 1 amp of current each would be functionally similar to a single 1n5408 rated at 3 amps.

    I drew an updated layout also including Rob's bias circuit.

    [​IMG]

    The only thing I'm wondering about is if for some reason I wished to test a tube rectifier, could I just jumper pin 2 to pin 3 which is currently my B+ / choke DC output? It is like the AC has two parallel paths - one through the diodes and the other through the tube - to be rectified. Not a big deal, but I'm curious if I can do that or not.
     
  17. kleydejong

    kleydejong Tele-Holic

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    My girls took epic naps this weekend letting me get some extended time at the amp. Progressing pretty far!

    [​IMG]

    I need some more 25uf 100v bypass caps, and I also need an IEC power cord port. I actually really dislike a built in power cord, but I know I'm not going to be patient enough to wait for that, so for now I just grabbed a spare computer power cable and chopped off the end and soldered it directly to the power supply.

    I did the filaments last on this build. I've done them both first and last - honestly not sure which I prefer. Doing them first means everything else is moderately more annoying all throughout. Doing them last means the entire build is a breeze, but the filaments are super obnoxious.

    The power supply, bias circuit, power tubes, and phase inverter look pretty rats nesty. Not sure how exactly to fix that with a PTP type build. But I went through the schematic slowly but surely. Trying to maintain good build techniques. Keep all signal wires as short as possible. Try to be efficient with terminal strips, and using unused tube socket connection points.
     
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  18. kleydejong

    kleydejong Tele-Holic

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    Initial startup looks good.

    353vac on pin 6 of rectifier.

    3.7vac on heaters.

    494 vdc on first filter cap.

    dc on filter caps seems to be discharging really slowly upon power down.

    I proceeded to initial power up. I made it to installing the power tubes. Initial startup was okay. As the amp warmed up I could hear some escalating noise. The light bulb current limiter started to shine as well. As I flip the amp 'off' it pretty loud screech occurs for one second and goes away.

    Would positive feedback cause the current limiter to burn bright? I may try swapping the OT leads.
     
  19. kleydejong

    kleydejong Tele-Holic

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    Seems the answer to my question is no. Swapped the OT leads to pins 3 on each power tube. Power on - bulb burns for a second and goes dim - then after about 5 seconds of warm up a loud noise starts to come out of the speaker and the bulb slowly increases in intensity to a pretty bright state, where I power off.

    Going to review my wiring to see if I messed anything up.
     
  20. kleydejong

    kleydejong Tele-Holic

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    If I pull the power tubes the light bulb goes bright for a second and then dims. Stays dim.

    Voltages with light bulb current limiter in line and no power tubes:

    Rectifier

    340vac in
    457vdc out

    filter cap rail

    1 - 457
    2 - 457
    3 - 449
    4 - 435

    power tubes

    1 - -27
    2 - 3.5 vac
    3 - 457
    4 - 455
    5 - -27
    6 - 456
    7 - 3.5 vac
    8 - 0

    phase inverter

    1 - 296
    2 - 48
    3 - 76
    4 - 3.5
    5 - 3.5
    6 - 309
    7 - 50
    8 - 78
    9 - 3.5

    v1

    1 - 284
    2 - 3.7mv
    3 - 2.2
    4 - 3.5
    5 - 3.5
    6 - 287
    7 - 0
    8 - 2.1
    9 - 3.5
     
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