Hammond AO-29 project

[1blueheron]

I have a Hammond AO-29 sitting on my bench.  Looking to do something interesting with it.  It is the Permanent Magnet version and was paired with an AO-44 reverb amp which I have already converted into an amp copied from Sluckey's PeeWee project.

Today I removed most of the guts.  This thing comes with 5-  7pin sockets.  Since I don't have any pentode preamp based amps, I am thinking this would make a nice candidate for a multiple gain stage, pentode preamp setup. 

I am thinking fill the 7 pin sockets with 5654/6AK5 or 6AU6's or some of both, use the 12AX7 for tone stack and the 12AU7 as PI?  Replace the wafer socket and put in another 12AX7 for reverb driver/recovery?

I have looked at several Youtube conversions of this amp but nothing really tickled my fancy there.
I'm good with the 6V6 power section. 

Looks like the PT is 320-0-320 according to schematic.  Haven't put a meter on it yet to confirm.  I cleaned up a schematic I found online and have attached for reference but I have not verified it is accurate to the amp I have.  At first glance I would say it is pretty close.

Open to thoughts and suggestions.

[1blueheron]

Here is the unverified schematic:

[sluckey]

I don't like that amp. Here's the verified schematic...

[bmccowan]

Open to thoughts and suggestions

Here are some random thoughts/ideas:

• A lot of amps use one pentode (or one per channel) in the preamp - usually in V1, sometimes in V2
• DanElectro did some multi pentode amps - one version of the Gibson EH-185 uses multiple strapped pentodes - I built one from a PA - Sluckey and PRR helped me a lot with that project
• If you stick to one or two pentodes, there are good 7 pin triodes to play with 6c4, 6av7, 6at7
• It'd be fun to try the Clubman preamp using one of the above triodes for V1, and a 7 pin pentode for v2
  And I'd considerer using the iron in a different chassis
And I wish I could figure out how this bullet thing is supposed to work

[/list]

[1blueheron]

I don't like that amp. Here's the verified schematic...

Sluckey,

The print on your schematic is certainly a bit clearer than mine.  It says it is for the A0-29/35  version.  Here is the one I have for the A0-29/44 version.  As best I can tell the only differences have to do with some resistor values in the output of the A0-44 and the speaker connections.

I am pretty much going to take a scorched earth approach to this.   Looks like I can use the PT and OT.  Is there an issue with the iron that you don't like it or just the way it was all wired up?

I am, thinking I can steal the schematic for a 5E3 for power supply and output section.  Then maybe something similar to what BMcowan is suggesting or something like Tubenits Carolina Blues Rocket dual pentode for the pre-section.

[1blueheron]

Open to thoughts and suggestions

Here are some random thoughts/ideas:

• A lot of amps use one pentode (or one per channel) in the preamp - usually in V1, sometimes in V2
• DanElectro did some multi pentode amps - one version of the Gibson EH-185 uses multiple strapped pentodes - I built one from a PA - Sluckey and PRR helped me a lot with that project
• If you stick to one or two pentodes, there are good 7 pin triodes to play with 6c4, 6av7, 6at7
• It'd be fun to try the Clubman preamp using one of the above triodes for V1, and a 7 pin pentode for v2
  And I'd considerer using the iron in a different chassis
And I wish I could figure out how this bullet thing is supposed to work

[/list]

I will check those out.

[sluckey]

IMO... The chassis is too narrow and long to be repurposed for guitar use. And I don't like that laydown OT. I'm sure the components are quality stuff just like all the other Hammond electronics.

I studied most of the Hammond amps and the AO-29 is the only one I shied away from.

And do you know for sure if the amp you have is NOT the Field Coil version? Take a look at this pic to be sure which one you have...

[vampwizzard]

There is SO much current available in 29.. I had a hard time getting even the 6L6's to bias on the tweed tremolux I built. Took a lot of bias voltage to get a bassman to behave. Do things on a different chassis.. by the time you strip things away youre looking at swiss cheese, and as slucky says its very narrow for anything useful. The bassman sounded pretty damn good when it was done though.

[1blueheron]

IMO... The chassis is too narrow and long to be repurposed for guitar use. And I don't like that laydown OT. I'm sure the components are quality stuff just like all the other Hammond electronics.

I studied most of the Hammond amps and the AO-29 is the only one I shied away from.

And do you know for sure if the amp you have is NOT the Field Coil version? Take a look at this pic to be sure which one you have...

Sluckey,

99.9% sure I have the smaller PT, the non-FC version.  It looks like the one in the picture on the right.  Black transformer,  no center wire connected on the output connector.  Don't recall the large resistor when I gutted it.  I had the entire organ and the speakers just had the normal 2 wire connection terminals.  There are no external coils on any of the speakers which would indicate they are field coil driven.  They look to be standard PM alnico type speakers.  I understand some of these organs were converted from FC to PM in the field, which would certainly be a possibility but I feel pretty confident this matches the schematic of the A0-29/44 I attached.  I will look for more specifics when I get home and can take a closer look.

[PRR]

• Open to thoughts and suggestions

  Here are some random thoughts/ideas:
  

  • A lot of amps use one pentode (or one per channel) in the preamp - usually in V1, sometimes in V2
  • DanElectro did some multi pentode amps - one version of the Gibson EH-185 uses multiple strapped pentodes - I built one from a PA - Sluckey and PRR helped me a lot with that project
  • • If you stick to one or two pentodes, there are good 7 pin triodes to play with 6c4, 6av7, 6at7
    • It'd be fun to try the Clubman preamp using one of the above triodes for V1, and a 7 pin pentode for v2
      And I'd considerer using the iron in a different chassis
    And I wish I could figure out how this bullet thing is supposed to work
    

_______________ Open this for Quoting so you see the code ________________
__________ Main error I found was unmatched open/close list tags ___________

Open to thoughts and suggestions

Here are some random thoughts/ideas:

• A lot of amps use one pentode (or one per channel) in the preamp - usually in V1, sometimes in V2
• DanElectro did some multi pentode amps - one version of the Gibson EH-185 uses multiple strapped pentodes - I built one from a PA - Sluckey and PRR helped me a lot with that project
• • If you stick to one or two pentodes, there are good 7 pin triodes to play with 6c4, 6av7, 6at7
  • It'd be fun to try the Clubman preamp using one of the above triodes for V1, and a 7 pin pentode for v2
    And I'd considerer using the iron in a different chassis

And I wish I could figure out how this bullet thing is supposed to work

[shooter]

yup, i now have to click "quote", paste, then click "quote" again AND add a / just before the Q, THEN add a space, otherwise 4outta5 my font changes to 5pt! 

[AmberB]

I like the 6C4 tube as the phase inverter for the PI section, like the way Bogan does it.  You can drive that with a high gain triode, or a pentode, a 2 tube version of the Ampeg PI tube, using 7 pin sockets.

[bmccowan]

Thanks PRR and Shooter: I don't know if its the best way, but copy/pasting multiple pairs of symbols allows more than 2 bullets at the same level.

• This is the first bullet
• This is the second one
• and then I copy-pasted the symbols to get this bullet
• and this one

Anyways - back to the subject - I have an AO-14D amp on the waiting shelf. Its pretty similar but all octal preamp and 2x6V6. I have no real plan yet but was going to pull the iron and other useful bits, giving the chassis to my neighbor scrapper. I imagine I am going to run into a similar situation - more PT than I want/need for the simple amps I usually build. Maybe I should use that for a 6L6 amp?
Amber, are you referring to a Bogen PA? model(s)?

[1blueheron]

Thanks PRR and Shooter: I don't know if its the best way, but copy/pasting multiple pairs of symbols allows more than 2 bullets at the same level.

• This is the first bullet
• This is the second one
• and then I copy-pasted the symbols to get this bullet
• and this one

Anyways - back to the subject - I have an AO-14D amp on the waiting shelf. Its pretty similar but all octal preamp and 2x6V6. I have no real plan yet but was going to pull the iron and other useful bits, giving the chassis to my neighbor scrapper. I imagine I am going to run into a similar situation - more PT than I want/need for the simple amps I usually build. Maybe I should use that for a 6L6 amp?
Amber, are you referring to a Bogen PA? model(s)?

I took a look, the CHB100 and CHB35A both use 6C4 for PI.

That AO-14 appears to be an earlier version that predates the AO-29.   Looks like it may have had a 340-0-340 PT and a field coil speaker?

[1blueheron]

I took a closer look at my PT last night and did some more research.  Mine is definitely not the big lump, 380-0-380  it is the smaller lump.  Apparently the early models had the 380V tranny with FC and AO-35 reverb unit and the later models like mine had the 320V tranny with PM speakers and the AO-44 reverb unit. 

Earlier AO-29 models were designated as AO-29- 1F, 4H, 7D, etc.

Mine is a AO-29-13.   The PT is stamped on the bell housing AO-24110-1  with an EIA code of 549-6607 Which would make it a Midwest Coil and Transformer built around Valentines day of 1966.  It should be a 320-0-320.  Measurements of the laminations minus the coils/bell-housing are roughly 3.75"L X 3.125"W X 1.75"H.
It is painted gloss black.

The other PT used in the early models appears to have used an ink stamp for the model number and was gold or silver in color.  It takes up nearly the full width of the chassis.

[bmccowan]

That AO-14 appears to be an earlier version that predates the AO-29.   Looks like it may have had a 340-0-340 PT and a field coil speaker?

Yes, Thanks, I think you are right. I just found a service manual at medias.audiofanzine.com Do you have that? could be useful.
And thanks for the Bogen info - they had a lot of models!

[1blueheron]

That AO-14 appears to be an earlier version that predates the AO-29.   Looks like it may have had a 340-0-340 PT and a field coil speaker?

Yes, Thanks, I think you are right. I just found a service manual at medias.audiofanzine.com Do you have that? could be useful.
And thanks for the Bogen info - they had a lot of models!

Yes,

I found manuals at that site and also info at the capatain-foldback.com website.  The service manual you found has better quality scans IMHO.

I found a few more amps that use that 6C4 tube.  It seems that Gibson used it in the Maestro/Falcon GA19RVT as a reverb driver, and also in one of the later models of the GA5t Skylark.   https://el34world.com/Forum/index.php?action=dlattach;topic=21568.0;attach=63617;image

[bmccowan]

I have a LectroLab made Harmony 304A that uses it in V1. V2 is a 6AT6 - single ended 6V6 amp with a 10" speaker. Very cool little amp that allows the power tube distortion to shine.

[1blueheron]

I think I have made a decision on what this will become.   I think it will match up pretty well with Tubenits Tweed Overdrive Special.   PT should be pretty close.  Hoping to do a voltage check on it later tonite.   Anyone foresee any issues with a 6v6 version of the TOS?

[bmccowan]

I'm sure it can be done with 6V6. Tubenit will help you out, no doubt. I built a minimalist HoSo56 and he gave me lots of advice and drew up modifications to his schematics that cut out some of the features I wasn't interested in.
Sounds like fun.

[AmberB]

I have an AO-29 chassis that is stamped on the one end of the chassis, H-AO-29-7
It has a pair of 6V6s and a 5U4 rectifier tube in it, the tube designations are stamped into the chassis.  I see no reason why an AO-29 wouldn't do well with a PP pair of 6V6s...
I converted it to a guitar amp, but it's not finished, yet another unfinished project.  You can play a guitar through it but it has an obnoxious buzz that I have to find...

[drew]

I think I've seen at least one forum post where a guy sawed several inches off one end of one of these chassis to make it shorter.  Still skinny, though.

[vampwizzard]

Im not as keen as some of the other members here but every build ive seen (including mine) with an AO-29 wound up going to 6L6's because the available current on most of our designs threw the voltage WAY up and made using 6v6's impossible.

https://el34world.com/Forum/index.php?topic=20482.msg218508#msg218508
https://el34world.com/Forum/index.php?topic=24756.msg267559#msg267559

and others. Most just do it with the 6L6's.

[1blueheron]

Im not as keen as some of the other members here but every build ive seen (including mine) with an AO-29 wound up going to 6L6's because the available current on most of our designs threw the voltage WAY up and made using 6v6's impossible.

https://el34world.com/Forum/index.php?topic=20482.msg218508#msg218508
https://el34world.com/Forum/index.php?topic=24756.msg267559#msg267559

and others. Most just do it with the 6L6's.

I have read through all the conversion posts I can find on it and yes, many went to 6L6.  I am open to that option if neccessary and have a couple sets handy. One of my reasons in leaning towards Tubenits TOS design is it leaves the option open to go to 5881's which should be able to handle the current if it ends up being too much.  On the schematic his PT is showing 340-0-340 which should be approximately the same as what I estimate I will get with 125V wall current.

{EDIT - untangled quote -PRR}

[vampwizzard]

Certainly. If you're mentally prepared for it you'll be in much better shape. Youve done more than I have for sure.. looking forward to the build.

[tubenit]

On the schematic his PT is showing 340-0-340 which should be approximately the same as what I estimate I will get with 125V wall current.

The schematic you posted shows a PT that is 275-0-275.   I've built TOS amps using 275-0-275 and 300-0-300 & have used 6V6's and 5881/6L6's.

The schematic Sluckey posted shows a 320-0-320 PT. 

I think if the first B+ node is under 425v, that you'd be fine using JJ6V6's or 5881/6L6's.

With respect, Tubenit

[1blueheron]

I fear I have authored confusion....   Too many different schematics and chassis being discussed at the same time... But that's all good.

So here is what I actually have and measured this AM.

Red wires on PT measured to ground.  354V and 356V AC respectively with 121V line current measured at the wall.  This is unloaded.

I would expect to be right at 500V DC coming off the 5U4 Rectifier from a purely mathematecal perspective.

Not sure how much it will drop with all the tubes loaded in.

[shooter]

Not sure how much it will drop with all the tubes loaded in.

gueses;
460 with 6V6
440 with 6L6
420 with KT88's self biased @90% 

[Adrien]

I built a 6V6 Plexi using the AO-29 transformers a while back and ended up with about 420V B+ using the 5U4.  It works ok with JJ 6V6s, it's been running for a couple years with no issues.  I remember the heater windings being a bit high too...the PT is really not ideal but might as well use it if you've got it.

I think I've seen at least one forum post where a guy sawed several inches off one end of one of these chassis to make it shorter.  Still skinny, though.

That was me...it's not too bad if you're handy with an angle grinder.  It turned out ok and it's a nice small amp, but it took a lot of planning to fit everything in there.  I would suggest using a more appropriate chassis if you can.

Here's the schematic with my as-built voltages, hope this helps.

[1blueheron]

I built a 6V6 Plexi using the AO-29 transformers a while back and ended up with about 420V B+ using the 5U4.  It works ok with JJ 6V6s, it's been running for a couple years with no issues.  I remember the heater windings being a bit high too...the PT is really not ideal but might as well use it if you've got it.

I think I've seen at least one forum post where a guy sawed several inches off one end of one of these chassis to make it shorter.  Still skinny, though.

That was me...it's not too bad if you're handy with an angle grinder.  It turned out ok and it's a nice small amp, but it took a lot of planning to fit everything in there.  I would suggest using a more appropriate chassis if you can.

Here's the schematic with my as-built voltages, hope this helps.

Adrien,

Thank you very much for chiming in.  That will be useful. Can I ask you which AO-29 PT you had on your donor chassis? Was it the black one with AO24110-1 stamped on it?

[Adrien]

Just took a look through my build pics and yes it’s the AO-24110-1.

[1blueheron]

Adrien,  How did you treat the Green wire off the PT that was originally attached to the pedal volume switch?  Did you just bolt it to the chassis?

[tubenit]

354V and 356V

OK, let's say 355-0-355 and you want to use 6V6's.  These are my guesstimates

355 x 1.1  (5Y3GT) = 390v which would be fine with JJ 6V6's as well as many other brands.

355 x 1.2  (5V4 or 5U4GT) = 426v   

As an FYI,  I converted a Bogen yrs ago and it ran 476v on JJ 6V6's for 7 or 8 yrs with no problems.  It was for a guy that plays mostly acoustic so it wasn't used for gigging out.   

Tubenit

[Adrien]

Adrien,  How did you treat the Green wire off the PT that was originally attached to the pedal volume switch?  Did you just bolt it to the chassis?

Yes it was grounded in my original amp, so I did the same on the conversion. I think it’s the center tap for the 6.3V heaters but don’t quote me on that, it’s been a while.  I can post a couple pics later tonight.

[1blueheron]

354V and 356V

OK, let's say 355-0-355 and you want to use 6V6's.  These are my guesstimates

355 x 1.1  (5Y3GT) = 390v which would be fine with JJ 6V6's as well as many other brands.

355 x 1.2  (5V4 or 5U4GT) = 426v   

As an FYI,  I converted a Bogen yrs ago and it ran 476v on JJ 6V6's for 7 or 8 yrs with no problems.  It was for a guy that plays mostly acoustic so it wasn't used for gigging out.   

Tubenit

I was using the max 1.414 mulitplier for worst case scenario.  I like your figures better!

I should have an additional tube or two in mine compared to what Adrien does in his Plexi so that should help too right?  I have read guys are running the JJ 6V6S at 500V+ without failure.

[AmberB]

Just a note about 6V6GC...I have a Fender silverface Princeton, it puts about 420 volts on the plates of the 6V6s, I've never had a problem with any NOS 6V6 that I've put in it. 
I read somewhere a while back that the 6V6GC will handle more voltage than the tube books rate them for, as long as they're biased appropriately for the current ratings.  My Princeton amp seem to bear this out.

My amp is an AA1164, and the schematic shows 410 volts on the plates of the 6V6s, but the wall voltage was perhaps a bit lower back when these were built, and I'm using a diode rectifier tube replacer for the GZ34...

[PRR]

> a note about 6V6GC...

Never heard of a 6V6GC. I suspect a typo.

[kleyplays]

Sweet thread!  I've done two AO-29 conversions to guitar amps.  One is now a 6L6 Bassman / Plexi thing.  The other has had JJ 6V6's and is a Fender / Trainwreck thing. 

This was the Plexi early on in the conversion.  It was a much more difficult project.  It had a red / orange - ish wire from the PT secondary that I had no idea what to do with.  I ended up using a 700 ohm 50 watt resistor to ground it to the chassis.  The resistor was a heat sink type so I used some CPU thermal compound for computer building and bolted it to the chassis.  It gets nice and warm after 20 minutes of cranked Hendrix.  The amp is a real work horse now, sounds great.  But I definitely had to do a lot of work to get it into shape.  Reading through this thread it explains a lot of my problems.  Way too much current and voltage for 6V6's.  Works much better with 6L6GC's.  The problem I had with that was that it overwhelmed the Output Transformer, so I ended up replacing that iron. 

You can see the PT wires here.  I think this would help ID what type I had.  I believe it was the field coil type. 



I documented the build here - https://www.tdpri.com/threads/organ-donor-hammond-ao-29-to-6v6-plexi-conversion.697430/.

The second was a Fender / Trainwreck inspired build.  It went MUCH smoother.  I know some complain about the chassis.  It is definitely overly wide, but the narrow depth makes it a nice build for PTP wiring.  I loved working in it with this build. 

Documented it - https://www.tdpri.com/threads/hammond-ao-29-organ-conversion-2-to-ab763-and-beyond.763327/

Chassis early on:



Ended up with this:



I mostly use it now as an AB763 clone and keep the trainwreck gain stage out.  I will probably tinker with it again when I'm smarter.  I love this amp. 

If I were to do it again, I would definitely go for the non field coil version and do PTP wiring again. 

[1blueheron]

--{entire message above}--

Wow!  Thanks for sharing your experience.  Will read through your build threads.

[High Voltage]

I've done two AO-29 conversions now. One with the regular PT and one with the field coil version. Both 6V6 PP. I also have 3-4 more AO-29 donors waiting for inspiration.

Feel free to reach out if you want me to verify anything, take voltages etc! Im looking forward to seeing what you do with yours and am following the thread.

[1blueheron]

Stay tuned.  Working on schematic, layout and measuring while finishing up the cabinet for the AO-44 pee wee build.  Progress, schematics an pics should be forthcoming.

[bmccowan]

High Voltage - I and I'm sure others would be interested in hearing what B+ voltages you arrived at, and what methods you used to reduce voltage to appropriate 6V6 levels? I have a 14d waiting in the wings and I think that has the same PT, or similar, to the AO-29 PT for circuits with field coil speakers. I'll likely build a 6L6 amp, but tend to like lower voltage amps. Thankfully I live at the end of the electric service line so my house V is only 115-117 which helps a bit.

[High Voltage]

High Voltage - I and I'm sure others would be interested in hearing what B+ voltages you arrived at, and what methods you used to reduce voltage to appropriate 6V6 levels? I have a 14d waiting in the wings and I think that has the same PT, or similar, to the AO-29 PT for circuits with field coil speakers. I'll likely build a 6L6 amp, but tend to like lower voltage amps. Thankfully I live at the end of the electric service line so my house V is only 115-117 which helps a bit.

Sure, i'll take some voltages today/today tomorrow and post :)

[bmccowan]

thanks

[1blueheron]

Here is what I have come up with.  Probably a lot that is not quite right.  This is an adaptation of Tubenits Tweed OD Special, CBR dual pentode, and the OD Special Lite Reverb.

I tried to make best use possible of the AO-29 chassis.   I would like to attempt re-using the spring reverb unit form the organ.  Instead of using the single tube reverb unit, with a 12AX7 driver/recovery, I have used 2 6AV6's instead to make use of the factory 7 pin sockets.  The only socket I need to replace is the one wafer style socket and there will be just 1 unused socket.

I moved the reverb unit to the return side of the EFX loop rather than the send side.

I probably don't have all the values for the 6V6 Power section just right.  Criticisms are welcomed.

[1blueheron]

This is what the tube layout would look like overlayed on the chassis photo posted by Kley.  On my chassis I believe V4 is still the 7pin socket like V5.

[High Voltage]

Here are my voltages for my AO-29 with the same PT (AO-24110-1). Note that I have 5 12AX7s in this one which appears to pull the voltages down quite a bit. I just rechecked and unloaded it is 355-0-355. I am using a new JJ 5U4GB.

 

[High Voltage]

High Voltage - I and I'm sure others would be interested in hearing what B+ voltages you arrived at, and what methods you used to reduce voltage to appropriate 6V6 levels? I have a 14d waiting in the wings and I think that has the same PT, or similar, to the AO-29 PT for circuits with field coil speakers. I'll likely build a 6L6 amp, but tend to like lower voltage amps. Thankfully I live at the end of the electric service line so my house V is only 115-117 which helps a bit.

Sorry it looks like I misled you. I pulled them both out today and one is the 310-0-310 large-but-not-field-coil PT and the other is the small one the same as the OP (355-0-355). I do have a FC one as well but not yet converted.

[tubenit]

Some thoughts for you.   DaGeezer and I both thought the FAT triode was sort of a "set and forget" thing.  It did add a little something but later I used the FAT triode as a cathode follower after the 5879 pentode.  The CF after the 5879 helped smooth out the OD tone, IMO.  * see attached schematic

The other thing was the 5654 paralleled with the 5879 was kind of an interesting feature.  I used them paralleled, just the 5654 and just the 5879.  After some time playing all those options, I thought the 5879 alone sounded the best.

The TOS is really not a clean to OD amp.   It's more of a pushed mild OD tone to a strongly pushed OD tone.

DaGeezer and I viewed it as a tweakers amp.  Something you built with the original design and then changed components and topology until it evolved to what you personally preferred. 

With respect, Tubenit

[1blueheron]

High Voltage - I and I'm sure others would be interested in hearing what B+ voltages you arrived at, and what methods you used to reduce voltage to appropriate 6V6 levels? I have a 14d waiting in the wings and I think that has the same PT, or similar, to the AO-29 PT for circuits with field coil speakers. I'll likely build a 6L6 amp, but tend to like lower voltage amps. Thankfully I live at the end of the electric service line so my house V is only 115-117 which helps a bit.

Sorry it looks like I misled you. I pulled them both out today and one is the 310-0-310 large-but-not-field-coil PT and the other is the small one the same as the OP (355-0-355). I do have a FC one as well but not yet converted.

Thanks a bunch High Voltage. Like the voltage doc sheet.  That is very useful.  If its not too much trouble, would you mind sharing what your power supply section consists of?  Did you use a choke or not and what dropping resistor values did you arrive at?