Need help with a Hammond AO35 conversion

[twiggybush]

http://captain-foldback.com/Hammond_...atics/AO35.JPG

I'm looking for ideas with this. I searched all the threads, some were wacky others to involved. So far I replaced the front end with a 1meg/56k input. After v1a I replaced the 1 meg to ground with a 250K pot cuz 1 meg had no sweep. Tied a .1uf tone pot to the volume control. It's all drilled,fused,pilot light and jacks installed. I tried replacing the 2.2meg coupling resistor with a 220K, huge mistake. Just total distortion. As is, it's clean and breaks up at 7 on the dial. Compared to my Marshall Class 5 with one EL84 the AO35 has far less volume, overwhelming bass and little treble. The distortion is ratty. I'm not sure if it's the PI or the 6bq5's making the distortion sound bad. I'm hoping to find a way to make the 6bq5's have more output and make the thing brighter overall. As simply as possible of course. Oh to dream (:

[olddawg]

Check out the Ceritone site and look at the layout for an 18 watt Lite. I would get a tag board and rebuild the amp using a known good design. Also make sure you have fresh tubes to swap in and out. Maybe consider a solid state rectifier.

http://www.ceriatone.com/

[loudthud]

I understand your reluctance to tear everything apart to make an 18 Watt Lite. There are many things you would have to improvise and that takes experience. If you want to try to get the amp to play before taking that big step, let's first see where you are and go from there.

Please list the parts you have changed or removed. Use the "circuit designator" ie R1A, C2 so everybody knows exactly what you are talking about. The circuit has many parts to kill treble and limit gain. For starters remove C1, C2, C5 and R15. That should improve the sound a bunch.

[twiggybush]

Now we're talkin. I'd seen all those other sites and they are quite a change from the original. I kinda inherited this thing when I bought an M3 Hammond recently. My first post pretty much said where I am. I'm up for removing C1,2,5. Makes sense. I've been eyeballing the feedback resistor R15. Gonna try it. As she is now, I probed V1a and the output was clean. I probed V1b and noticed the onset of this gritty distortion. Seems high frequency based. I'm leaving the tone pot disconnected for now. I'll be back!

[twiggybush]

Thank you Loudthud! Your suggestions woke this thing up. In hindsight I see now that a stock AO35 is a phono input amp to accomadate the piezo crystal transducers on reverb springs. That's what all those small value caps are about. My volume is better, the icky distortion is gone. She now sounds very Marshall like. Very articulate high end. She's in balance with the overwhelming bass I was experiencing. She sings and sustains. Finger tappings a breeze. Best I've heard. So from the top here's what I've got. R1=56K, R1A =1 meg, C1, C2, C5 are gone. R6 is replaced with a 250K pot. 'R15 is gone (feedback disconnected)'. This was all so simple. More clipping than soldering. She's clean till about 3, at 10 shes very distorted but the trebles are articulate so I might increase my volume pot to 500K. If I was to snit I'd think she'd be capable of a bit more clean volume. A tone contol is kinda redundant as the guitar tone control will do the same thing. I have drilled the hole for it though. I'm all ears if you have any other thoughts.

[loudthud]

An Organ amp is more conservatively designed than a guitar amp so power output is somewhat limited, primarily by the transformers. A solid state recitfier would give a small increase, but it would stress the old filter capacitors. Don't try that until you replace all the filter caps with new 450V rated parts.

The first tube in the signal chain, V1A is not biased to handle a signal as large as a guitar. This will limit headroom because that stage is before the volume control. Clean tones will be a little distorted no matter how far you turn down the volume. You would have to turn down the volume on the guitar. The following changes are suggested:

Remove R3
Change R2 to 1.5K 1/2W
Change R4 to 100K 1/2W
Change R5 to 22K 1/2W
Change C3 to 10uF 450V
Optional... connect a 22uF 35V cap in parallel with R2. Minus side to the ground side of R2. Smaller values will help clean up any muddy bass. Values as low as 0.47uF may be tried. The cap doesn't have to be a polarized electrolytic. A film cap can be used if it will fit.

The biasing of V1B will give a somewhat dull but high gain tone. If you want a brighter but lower gain tone, change R7 to 1.5K and R6 to 100K. Again you can connect a capacitor across R7 like R2 above.

The biasing of V2 is strange. If you are up for a change, look at the Ceriatone amp. One simple thing that you could do is remove R9 and install a shorting wire in it's place. If you want to experiment with any tone controls, you can install something between the two points where R9 was connected, just remove the short.

[twiggybush]

Wow! Those are some super suggestions. Your right about their being an ever present small amout of distortion on V1a. I could easily live with that. I don't have the 1/2 watt resistors on hand so it would be awhile before I did this. The amp is totally useable as is now but your suggestions are desireable. I'm in love with the bell like harmonics I'm hearing. One thing I thought curious. I had previously changed 2.2meg R9 to 220K and the distortion was uselessly insane, so I replaced the 2.2meg. If I strapped it I'd think it to be even worse? I'll look at the Ceriatone. Thanks for your time and wisdom.

[loudthud]

If you like the way the amps sounds now, make changes gradually so if one of them lets the magic out, you can undo it. If you make too many changes at once, it's hard to know which one to undo. Post your results here, there may be some middle ground or a way to add a switch to make the change switchable.

In the stock amp R9 works with R15 to form a feedback network. With R15 removed, R9's effect is unpredictable so experiment with the value to see what works best.

[twiggybush]

Thanks Loudthud, After playing the amp more I see that I need to do this next upgrade. It never cleans up properly. So I've looked over the amp and it looks fairly easy to do. I'll be back after I order the parts and put them in.

[olddawg]

Not to diminish Loud's suggestions, but if you are going to all this trouble to change out these parts point to point, why don't you just make a tag board and wire it up? It will be much more serviceable and be a guitar amp.

[twiggybush]

Well,,, Loudthud is teaching me things, he approached my original post with empathy. I have many antique amplifiers. Which ones do I throw money at? I'm saving pennies. I believe in make do, use it up, wear it out. These changes are quite simple and will serve the community in understanding what the AO-35 is all about. If I tried to cram a new tag board in, making many more connections, it would probably be a bit more difficult. Yes, I would have a more substantial build which may appeal to some. But where's the fun in that? As is, it will run me $15 to have a cool amp using scraps I have on hand. If I bought a preloaded board with an iffy fit that's $100. Then I'd probably want to do the power caps. Another $35ish. Then what about the tubes? New ones? What about the cab? Tolex, hardware etc. Where does it end? Each to his own. Beauty is in the eyes of the beholder. Olddawg, your suggestion has plenty of company on the internet. It is a valid one. Debatable? most certainly. I hope I've answered your question. Thank you for taking an interest.

[olddawg]

Whatever dude...I didn't say buy a preloaded board. I said make a tag board. There aren't that many parts.

[twiggybush]

Parts came in and I installed them. R4 was already 100K, the schematic is hard to read. It says .1meg. The dot is invisible. The rebias of V1A cleaned things up nicely. I have not changed R7 yet but intend to. The amp sounds louder and fuller now. Probably like a 15 watt amp. It definitely sounds British/heavy metal. I found the Orange Tiny Terror schematic to look very similar to this amp. Same amount of stages. The way she sits now I have good clean volume but my 250K volume pot breaks up at 2.5/10. At 5 it's very saturated and there is more volume beyond 5 but all compressed saturation. I noticed the Tiny Terror uses a dual gang 500K pot, fore and aft of V2B, so that is something to consider. Changing R7 to 1.5K will probably reign things in a bit more. At this point it's just some personal preference tweaking left to do. I'm going to use the tone control method I saw on the Tiny Terror. I found some unique metal box parts I'm going to use to fabricate the case. It'll kinda be more Tiny Terror looking (Not Orange!). I'll post a pic when done in a week or so. Thanks again Loudthud. I never would have tamed this thing without your help. Olddawg I'm sorry if I was harsh on you. I'm not an amp builder, so somethings are not apparent to me. I would not dissuade anyone from putting in a new board. After having had my nose in there I can see it would not be to difficult to do. My approach probably took 5 hours of research and drilling. The actual circuit mod took 1.5 hours. Things are cramped in there. Releasing the tag board mounts helps. The snittiest part is hooking up the input and volume shield wires to the phono plug. In hindsight I'd do that after changing the resistors in V1A. I'd also consider removing the existing phono plug as it's more in the way than of use. There are spare ground tabs on V1A that could just as easily be used. If anyone else is doing this I'd love to hear about their experience.

[twiggybush]

Here's a pic of the finished amp.

[loudthud]

Looks Good!!! Anything you want to change about the sound?