Hammond L100 AO-43 amp project help?

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Hammond L100 AO-43 amp project help?

Postby housekustoms on Tue Dec 30, 2014 2:38 am

Hmmm...seemed like a great idea when I shot a lowball offer to the seller for this gear ($70 - no tubes). I really didn't think my offer would be accepted. The joke may be on me if my wife is home when this gear is delivered.
:cheers:

It is a Hammond AO-43 amp, both preamps and the reverb tank pulled from an L100 organ vintage 1965.

This is the first time I've attempt to bastardize vintage circuitry and try to turn it into a working guitar amp. Part of my continuing effort to wake up the gray matter in my noggin while I still can by presenting myself with a deliberate challenge.

:old:

I have solid assembly/soldering skills, moderate tube amp knowledge, a healthy desire to still be breathing tomorrow; read that as cautious around high voltage :Zap: having received 2 years of basic electricity & electronics training back in the days when this organ was new. I have never routinely practiced the same so am rusty and welcome any and all input, advice or criticism.

If anyone who has been down this path before has any initial pointers I would love to hear them. My intentions are to first see if the components seem healthy. I'll assume to replace the filter caps and will check the resistors to be sure they are in spec still. I'll de-ox the tube sockets, inspect the power cord wiring and move it to three prong if it isn't (I doubt it is). Probably have to round up some wiring or duplicate the cables that connected the preamps to the amp.

These are the things I think of off the top of my head.

The two preamps are not the same it would appear so if anyone understands why that is please chime in. I am downloading the L100 service manual now but I get the sense it is really more about the mechanics of the organ and the tone engines than it is about the amp and preamp circuits. Hopefully I am wrong!

Thanks if you can offer anything more!

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Last edited by housekustoms on Mon Dec 14, 2015 3:25 am, edited 2 times in total.
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Re: Hammond L100 AO-43 amp project

Postby Clanger on Tue Dec 30, 2014 5:30 pm

The two other units are I suspect not really pre-amps, more likely the percussion and vibrato modules as used in the L100 range.

The Power amp section is sorta ok, but the only *sensible* thing to do is to keep the PT and OPT, junk the rest and start again. Not that I'm advocating *sensible*.

The PT's in particular are awesome for a 25-30W guitar amp by the way. BTDT. so worth your $70 for that alone.

Regards,

Clanger.
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Re: Hammond L100 AO-43 amp project

Postby housekustoms on Tue Dec 30, 2014 5:53 pm

Cool beans Clanger,

I appreciate the jump start. I felt the same, even if I heard the rest was crap I got a good working reverb tank, chassis & trannies and it Is a low watt I was aiming to do.

Thanks! :thumbsup:
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Re: Hammond L100 AO-43 amp project

Postby Clanger on Tue Dec 30, 2014 11:48 pm

No worries,

I'm prolly being a bit harsh on the power amp section - you could keep it more or less as it is, in which case what you have is a 15W push-pull output section, preceded by one pre-amp section and phase splitter. But this will only be sensitive at line level, so you need to add a front end for gain and tone control. Perhaps an EF86 pentode would give a good drive, and then maybe one chanell with fender/marshall tone stack channel volume control, and another channel that bypasses the tone control section but with maybe a bright cap across the volume control. That could all be done very cheap and mounted in a separate control panel, like selmers or Gretsch back in the day, thus saving the need to make the chassis presentable or drill holes in it.

BTW the PT has the power to run 6L6's no problem. see my thread on "Ruskin Room Amp" , somehwere on there is a chassis of an amp I bult using same PT.

Regards,

Clanger.
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Re: Hammond L100 AO-43 amp project

Postby housekustoms on Thu Jan 15, 2015 1:35 am

While I am still poking at these dead bodies with a stick...I'm not yet convinced that I am through messing with them and that I can't make something from them. I enjoy the challenge for the moment. But knowing I may yet throw in the towel on the electronics in the chassis', I am also looking at other alternatives that would allow me to at least repurpose the chassis and iron which are good. So I was wondering if someone in the audience might have any thoughts and/or advice on this low wattage amp? I noticed it was made from a repurposed Hammond chassis and iron and it caught my attention as a good possibility.

http://www.trinityamps.com/lightning_project.htm

1. I don't know enough or have enough experience building amps yet to know what being a “Class A amp with no negative feedback” really means as far as using the amp in the real world. Can anyone fill me in?

2. I also noticed this statement “The output section tubes are biased "hot" so you have to turn off the amp if it is to be idle for long as there is no standby switch” so I think it would be good to add a standby switch too or is that somehow mutually exclusive with this type of design?

I'll say thanks in advance for anyone's :2cents: or advice.

Thanks!

:cheers:
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Re: Hammond L100 AO-43 amp project help?

Postby housekustoms on Fri Jan 16, 2015 5:46 pm

It would be really helpful if anyone would be able to answer either of the questions I've posed. Thanks
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Re: Hammond L100 AO-43 amp project help?

Postby PeterS on Fri Jan 16, 2015 7:49 pm

That's probably a good design to build --- not so high gain as to be likely to be trouble.

Despite the comment in question (1), that is not a class A amplifier, it is cathode-biased class AB. The power stage is a lot like classic Vox designs. Hard to explain the real-world effect on the sound of cathode bias and no negative feedback, but let's say it's not a hifi amplifier.

There's no reason not to add a standby switch. Even more important, make sure there's good ventilation for the power tubes if you build a box for it. You can also increase the 120 ohm cathode resistor to say 150 ohms if you want less heat and longer tube life - possibly with some sacrifice of sound quality but I would experiment a bit.

It's a good and simple design, I would suggest build it with a tidy layout so that it's easy to make changes -- it's a good platform for experimentation.

You may find 18watt.com interesting --- all kinds of variations on similar designs.
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Re: Hammond L100 AO-43 amp project help?

Postby housekustoms on Fri Jan 16, 2015 9:36 pm

Thanks PeterS!

It sounds like a good plan to follow then and thanks for taking the time to look through the Lightning Amp info and provide solid feedback. Your tips/suggestions will be put to use.
By modern standards this isn't a very high tech approach but I plan on doing a mock up using a breadboard with Fahnestock clips. It is kind of old school like me I guess :old:. Once I like the result then I'll commit it to a turret board and build a well vented cabinet for it with 2x12 high efficiency speakers. I've got a bunch of Canarywood left over from a bathroom remodel and it is a pleasant wood to work with.

I'll need a faceplate of some sort and had a few thoughts on that.

1. My first choice would be to have one made with silk screened labels. Is there a business anyone has been pleased with that will make one-off faceplates for amps for a reasonable price like under $100?
2. I thought about possibly making one from a dark, stiff wood like a rosewood or ebony thinned down to 1/8". I have a lot of these types of woods on hand for other work I do. But I would still have the issue of labeling things.
3. I also thought about trying to polish the chassis and putting a clear coat on it but I'm not sure a polished turd will look much better that it does now. :Doh: I would also still have the labeling issue.

I would love to hear/see things that others have done for faceplates.

Thanks Clanger :thumbsup: and PeterS :thumbsup: for your replies. I appreciate you sharing your experience. I just started looking at 18watt.com recently as well.

:highfive:
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Re: Hammond L100 AO-43 amp project help?

Postby housekustoms on Wed Jan 21, 2015 2:50 am

Ran the first test of the Hammond amp chassis today after jury-rigging some speaker wiring. Used the same speakers that were in the organ cabinet that my components were pulled from. Brought it up slowly on a variac and got no fireworks so jury-rigged an input into what would or could be used as a phono input and then plugged it into AC and I plugged into it.

No preamp yet so no volume control other than the guitar's controls but it actually sounds quite nice. The tone is quite nice (neutral) through these original paired bass and treble speakers. They are alnico high-efficiency speakers so they are actually louder than I would have expected them to be. They seem to complement each other quite nicely and thanked me for being reunited with their original chassis :D

Now I have to go through the schematics and try to figure out how to get the preamp chassis hooked up and the reverb tank working. No chassis interconnect cables came with the pieces I bought so I'll need to make one up first but getting these connectors seems to be a challenge. Also need to bypass the organ pieces I don't have and don't want. I can see now there is no way to use the vibrato chassis without all the organ's missing related pieces.

Anyway, this is actually further along that I thought I would be able to take it given my greenhorn amp knowledge.

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Re: Hammond L100 AO-43 amp project help?

Postby PeterS on Wed Jan 21, 2015 3:46 am

You may have difficulty adding in the reverb because the organ has a kind of separate signal path for it, not just the usual send-return that's in a guitar amp. You might find it useful/interesting to explore the L100 schematics a bit- you can find them online,including a block diagram that gives you an idea what the various sub-chassis do. The vibrato box is interesting, the workings of it are explained in a patent you can find online. Not sure how useful it is, Hammond enthusiasts have no respect for it compared with the fancy motor-driven chorus arrangement on the bigger organs. Hammond engineering is fascinating, the bigger organs were built to compete with $$$$$ pipe organs, so no expense was spared in making them as good as they could be in both sound and reliability. The L100's get no respect but the quality is still awesome. I have a 1965 L that's 100% original and 100% functional like the day it left the factory.
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Re: Hammond L100 AO-43 amp project help?

Postby housekustoms on Wed Jan 21, 2015 4:31 am

PeterS wrote:You may have difficulty adding in the reverb because the organ has a kind of separate signal path for it, not just the usual send-return that's in a guitar amp. You might find it useful/interesting to explore the L100 schematics a bit- you can find them online,including a block diagram that gives you an idea what the various sub-chassis do. The vibrato box is interesting, the workings of it are explained in a patent you can find online. Not sure how useful it is, Hammond enthusiasts have no respect for it compared with the fancy motor-driven chorus arrangement on the bigger organs. Hammond engineering is fascinating, the bigger organs were built to compete with $$$$$ pipe organs, so no expense was spared in making them as good as they could be in both sound and reliability. The L100's get no respect but the quality is still awesome. I have a 1965 L that's 100% original and 100% functional like the day it left the factory.

I agree whole-heartedly about the Hammond organs. I grew up with a neighbor who had a unit whose model number I do not know but in my teens (meaning in the mid 60's) it was drop-dead awesome to me because it had a separate Leslie tone cabinet and sounded like the big rock bands of the era. I have the L-100 service manuals and I'm looking through the schematic now figuring how to get the pre-amp/percussion unit active and hooked up (I haven't the original chassis cables) OR what components I'll need to recreate the reverb specific control circuitry. It doesn't look overly complex and if I am lucky, one of the many vendors for Hammond organ parts and repairs will be able to sell me the reverb related switches (there appears to only be 3 that I need) that I am missing in which case I should be able to get it going.

If not, it appears that a yellow wire coming off of V6, a green wire and the output from R323 and possibly C307 may allow me to hookup the little bit of the preamp I need to service the reverb. My thought is to replace all that with one or two pots of appropriate value which I am hopeful will then give me some control over the reverb intensity and presense (?). I also have some Molex connectors coming from Mouser and so I will be trying to hook up the whole preamp/percussion chassis once I can recreate the missing cables. I would still prefer to have separate reverb intensity and presence pots if possible though.

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As is usually the case, things are rarely as easy as my first thoughts so any further insight into this circuit would be appreciated. This whole project is an exercise in putting to use things I have some knowledge of in order to apply it to something new all in the name of not going senile!!

Thanks for jumping into the fire and offering advice on this little project. Any further insight is cheerfully welcomed!

ps. I will still be building that Trinity Lightning amp as it sounds like another great winter project. I've loved woodworking since my teen years and never mind building yet another amp cabinet.
:excited:
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Re: Hammond L100 AO-43 amp project help?

Postby PeterS on Wed Jan 21, 2015 4:12 pm

I wouldn't bother getting the hammond reverb switches - they'd be awkward to mount on a guitar amp (although also pretty cool looking), and they just give you reverb off/low/high. You'd be better off IMO to just use a pot to attenuate the recovered reverb signal in the same way that it's being attenuated by those switches.
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Re: Hammond L100 AO-43 amp project help?

Postby housekustoms on Thu Jan 22, 2015 1:55 am

PeterS wrote:I wouldn't bother getting the Hammond reverb switches - they'd be awkward to mount on a guitar amp (although also pretty cool looking), and they just give you reverb off/low/high. You'd be better off IMO to just use a pot to attenuate the recovered reverb signal in the same way that it's being attenuated by those switches.


I took your advice and passed on the M100 reverb switch set I saw on eBay.

http://www.ebay.com/itm/Hammond-Organ-M100-Reverb-Tablets-TESTED-/400843770209?pt=LH_DefaultDomain_0&hash=item5d54268961#ht_533wt_1226

I suppose being for an M100 it would be different values anyway as I understand the L100 series is somewhat of an anomaly in their organ line and differs a good deal from the M100 and other previous series organs.

So instead I looked over the schematic and ended up placing a 270K ohm resistor between the junction points on the schematic shown as "yellow" (white on my amp chassis) and "green" and viola I have reverb and it sounds real good. So I am thinking I'll place a 1M pot across these two points and call it good.

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May I ask if this seems reasonable? Also should I be making the connection between the wiper and infinite or between the wiper and 0? Obviously I can try it out and see but would hate to damage the tank as a result of careless haste and inexperience.

Next up and I openly solicate opinions please...what suggestions make sense as far as introducing treble, bass and volume controls? I apologize for such a fundamental question but it seems clear I don't really need the vibrato and percussion/preamp chassis at all so I am left with a bit of a dilemma due to my limited experience.

Thanks for any suggestions and for those that have already been offered.

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Re: Hammond L100 AO-43 amp project help?

Postby housekustoms on Thu Jan 22, 2015 3:21 am

Given what I tried earlier maybe what makes sense is to try a 250K pot and use the wiper to infinite side to vary the effect from what I heard earlier to no reverb at all. Does that seem reasonable?

My question about the tone controls still stands although I think I see what I need to do for a volume control already.

Many thanks again for any suggestions or corrections!
Last edited by housekustoms on Tue Aug 04, 2015 6:06 am, edited 1 time in total.
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Re: Hammond L100 AO-43 amp project help?

Postby PeterS on Thu Jan 22, 2015 11:41 pm

For the reverb I would connect the high side of a pot (1M or 500k) to yellow, and ground the low side. Take the wiper, add a resistor like 220k, and connect that to green. So the pot gives you any amount of reverb signal from 0 to all of it. Now you may have to adjust some values to get the right mix of 'wet' and 'dry' signals --- the 'mixer' resistors are R319 and the 220k that you connected to the reverb pot wiper.

Tone controls... there's two reasons why simple amps tend to have simple tone controls (often just a treble cut): one is that it's simpler. The other is that it doesn't attenuate the signal much. If you want to add a more complex tone control it results in quite a lot of signal loss, which usually means adding a gain stage to make up for that. What's not so clear at this point is whether the amp you're starting from has enough gain to be able to throw much away. And if not, how hard would it be to add more.
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