AO-43 Conversion schematic needed

[shaun]

Hi All,
I have a lovely clean  AO-43 amp. I want to rebuild it as close to original as possible (new caps etc) and add a preamp tube and guitar input. However, I cannot find a schematic. I have the layout diagram, which has been posted in this forum in a couple of places, and I have the L100 series manual, which also has the layout, but I cannot find the actual schematic anywhere.

In the conversion, I want to keep the reverb (which seems to be built within the power amp chassis) but dispense with the vibrato and percussion units, so I need a schematic in order to trace where the extraneous elements begin and leave off in the power amp. I've seen AO-43 conversions done in various ways online, but none show the original schematic, and this is probably not going to be a simple undertaking. Any help finding the correct schematic would be much appreciated.

[sluckey]

The AO-43 is called the Reverberation and Power Amplifier in the L100 service manual. Schematic is on Figures 5-1, 5-2, 5-3, and 5-4. Look on the right side of the page. Pay attention to the separator dashed lines.

[RWood]

Here is the three-part that I used, but I converted mine to a Fender 6G3:

[PRR]

Paste-together scans:

[shaun]

Thank you all. Mine came from an L101, so I assumed it would be different.

Time for fun, although the old saying is true: be careful what you wish for. This will be a big challenge.

[sluckey]

The AO-43 is the same, regardless of which organ it was in. The service manual I mentioned has much better quality schematics. All these partials in this thread came from the service manual and they have lost a lot of quality in the process. I extracted the same page these copies came from but it's too big to post here. The L100 Service Manual is easy to find. I suggest you get it if you want to restore the original AO-43.

However, the AO-43 is a kinda chopped up circuit. You have stuff coming in and stuff going out. There's not a straight path through the chassis. If you intend to convert this to a guitar friendly amp I suggest you rip out everything except the power supply and power amp. Then build your favorite guitar preamp.

[shaun]

Actually, Sluckey, that was my first instinct, but it looks so pristine, and it has a reverb unit in there!
But you're right; whenever I try to retain an old system, I end up crying tears of frustration. So I might follow your suggestion. I like the Hoffman Stout 18 w/reverb for this.

[shaun]

Hi Sluckey,
a quick question, if I may. I really like your Vox AC 30.4 drawing. Do you think it would be possible to build an amp using just the normal channel of that schematic, and using only two EL84s? I'd like to add spring reverb if possible.

I can't find a Vox 15 schematic using an EF86 and w/reverb, so I was wondering whether I could meet the challenge of modifying your drawing of the 30.4. But I might just end up with a lot of problems I cannot solve, which seems to happen a lot lately.

Thanks for any advice.

[shaun]

Actually, I just saw this from a previous thread in this forum, and it is close to what I wanted. Although no EF86...

[sluckey]

I really like your Vox AC 30.4 drawing. Do you think it would be possible to build an amp using just the normal channel of that schematic, and using only two EL84s? I'd like to add spring reverb if possible.

Anything is possible. This may be a better starting place...

     http://sluckeyamps.com/AO-39_conversion.pdf

Or even better...

     http://sluckeyamps.com/Dual_Lite_(British).pdf

As for reverb, you should keep in mind that the AO-43 uses a cap coupled reverb circuit and a hi-Z input tank. That tank will not work with a transformer driven reverb circuit.

[shaun]

Thank you! The Dual Lite is probably the way I'll go.

As a newbie, I'm often confounded by the old iron; this particular A043 PT puts out 343vac coming out of the PT secondary side, and 414vdc on the B+ side of the 5U4 rectifier. Nothing I do seems to tame that voltage - I tried 150 ohm resistors on the secondary leads of the PT before going into the 5U4. I've tried a various resistors on the B+ line coming off Pin 8 and going to the first filter cap. These efforts seem to make little difference, so I wonder if I simply have to pay up for a new PT. Seems a waste of good iron, but I've spent many hours trying to understand how to tame PTs generally, to no avail.

You seem to have made good use of Hammond PTs, but they are labeled as putting out 315vac at 120v. Very interesting. I must be missing something.

[shooter]

fwiw

As for reverb

https://el34world.com/charts/Schematics/files/Vox/Vox_berkely_ii_super_reverb_v_8.pdf

one I found (in a vox:)  replace V2 with your EF86.

[shaun]

Very nice. Thank you, Shooter.

[sluckey]

As a newbie, I'm often confounded by the old iron; this particular A043 PT puts out 343vac coming out of the PT secondary side, and 414vdc on the B+ side of the 5U4 rectifier. Nothing I do seems to tame that voltage - I tried 150 ohm resistors on the secondary leads of the PT before going into the 5U4. I've tried a various resistors on the B+ line coming off Pin 8 and going to the first filter cap. These efforts seem to make little difference, so I wonder if I simply have to pay up for a new PT. Seems a waste of good iron, but I've spent many hours trying to understand how to tame PTs generally, to no avail.

Until you put a load on that power supply (such as a working power amp with tubes plugged in) all your voltages will be high. Putting series connected resistors will do nothing, as you've proven.

You seem to have made good use of Hammond PTs, but they are labeled as putting out 315vac at 120v. Very interesting. I must be missing something.

The thing you have missed is that both of those schematics used transformers from an AO-39, a smaller amp than the AO-43. Both of my AO-43 projects had much higher voltages so I built a 6V6Plexi and a Supro 6424. I would not use the AO-43 iron to build a 2xEL84 amp.

[shaun]

Thank you, Sluckey. Then maybe I'll steer toward a Plexi. Much appreciated.

[sluckey]

Thank you, Sluckey. Then maybe I'll steer toward a Plexi. Much appreciated.

Forum member "dude" has built several 6v6plexi's in AO-43 chassis. Search the forum or send him a pm. He got experience and probably has a lot of good ideas.

Now I'm gonna throw out one more idea for you to consider... If you want a cleaner sound, this American Dual Lite would be perfect for that AO-43. Just replace the 6L6s with 6V6s.

     http://sluckeyamps.com/Dual_Lite_(USA).pdf

[dude]

Shaun, I have built a few Plexi's from the AO-43. It's a tight build but the use of a cap can or two really opens up space. The 6V6 is a tone monster too, perfect for gigging. I have also made a few small changes so I can run 6L6s and El34's, the PT seems to handle the voltage for the filaments too. I tried to find the spec's on the AO-43 PT but couldn't find anything but the requirements that Hammond posted for the required specs the original AO-43.


I also, with help from a member here have faceplate PDF files for a head cab or combo cab, taking the file to a print shop they can print it on heavy paper that you can sandwich  between thin plexi glass for a professional look. I.m in the process of designing a layout with the software Doug provides here but haven't finished yet, so I don't have a precise layout, yet. If you're interested let me know.


Take care.
al (aka the dude)   

[dude]

I converted mine to a Fender 6G3:

Dick, I love that circuit. I converted an Ampeg J20 it was pretty close to the 6G3 (the J20 was made one year in 2007, was a dud). Much more clarity than it's sister or brother, the 5E3. The only thing I didn't like about the 6G3 was the .003 cap on the normal channel in parallel with the 220K plate. It just took much treble out, sounded too muddy. With that cap off, this is one of my favorite amps. The tremolo is killer, much more headroom then the 5E3 too. I also like the channels jumped, my favorite setting.


al

[shaun]

Wow, Dude, your plexi has got it all going on; it looks great. I've been biting off way more than I can chew in the last few builds, so I'll keep it a little simpler than what you have, but maybe one day I'll be able to create something like that. At the moment, I'm happy if a build actually works, no matter how much hum:).

[jojokeo]

Both of my AO-43 projects had much higher voltages so I built a 6V6Plexi and a Supro 6424. I would not use the AO-43 iron to build a 2xEL84 amp.

I've done EL84s and even 6BM8s without issues

[Ed_Chambley]

Nice looking builds Joe.  Taking the time to cover leads with wire strippings is getting questionable since Teflon tube is easy to get, but I guess recycling the strippings is a good use as well.  Every amp I have ever built looks good when I first complete it, then I mod it til I kill it!

[shaun]

Nice work, Jojokeo. Any chance you can post the schematics?

[jojokeo]

Thanks Ed, on that stuff I just keep rolls of spaghetti tubing on hand and use that rather than left over wire stripping but however the leads are protected is fine. I hear ya on the mods too!

Also, I found a pdf for the AO-43 schematic pieces and layout info that may help Shaun and others?