June 4th, 2012, 12:24 PM
Actually, I got me two of these. One is in working order, the other was last seen smoking in a non-smoking area.
This is the working model. An AU-36b. All I've done to it thus far is replace the 2 prong power cord with a proper 3 prong cord. Caps could probably use an update, but it's working with a low noise floor and it sounds great. I'm using it with my turntable and iPod (yes it's mono)
The non working model is shelved until I move in a couple of weeks (and until I figure out what to do with it. There's a big old ceramic resistor in there (125 ohms @ 25w I think) that went, taking a tube socket and some other resistors with it. I really have no need for a second one of these so I'm thinking about putting all of those octal sockets and 6L6s to good use and building a Bassman in there. (Currently has tube rectifiers)
The iron is HEAVY. Not sure if it would translate over to a guitar amp or if I should knock some weight off and put in smaller transformers.
Any thoughts amp gurus?
June 4th, 2012, 01:49 PM
Bigger = Better where transformers are concerned. That working one is too pretty to mod, but I blown-up one is fair game in my book. It'll make one helluva Bassman.
June 4th, 2012, 02:37 PM
One nice thing is the full schematic is inside the boddom plate, so I'll have to take a shot of that the next time I have it open. I intend to rewire the mic input to accept a 1/4 or 1/8 in so that I can run the iPod in there but I wanted to hear what it sounded like first.
My other option is to run a second input in where the Phono is and put a switch in there to select between them. The iPod sounded good into that input.
The one that I'll be rebuilding looks just as good as this one, but I've yet to assess the full extent of it's troubles. Most of the parts seem to be in good shape and I assume the trannies are fine.
Now, what to do with 4 6L6s?
June 4th, 2012, 02:56 PM
How does it sound now as a guitar amp? The good one I mean. I would be inclined to repair the non working one and restore it as is, rather than try to turn it into something it is not, minor mods being perfectly fine.
June 4th, 2012, 06:41 PM
I don't know yet. I plan on trying it out that way before I gut it too severely. It might also be cool as a flip-top like the Ampeg B-15 if I can find an old cab in need of a head! :)
June 5th, 2012, 01:52 AM
I would highly recomend you do some studying on it before you make a move.
I used the attached link to an artical on converting tube amps to guitar as a reference for a long time when I first got started.
I've converted a many of them. The first things I do is:
1-Install a 1/4" input jack and wiring. The two mic volumes are not needed for your purposes unless you decided you needed a master volume--which might come in handy with 100 watt output(4 6L6's). If you only need one volume, one of the volumes can be removed and a 1/4" jack installed. Or if you did use one of the volumes for a MV, looks like you have plenty of space to drill a 3/8" hole for an input.
2-Install a 1/4" speaker jack on back panel to the Ohm tap you need--4, 8 or 16 Ohm. Most old PA's have those three connections. 8 Ohm is most common.
3-The four 6L6's in your power section is the same as a twin reverb or from 75 to 100 watts output. I would guess at this point, you can leave that part of the amp as is. Your focus should be on the preamp section to get it wired for guitar input instead of mic input. It looks like your preamp tube might be a 6SC7 or something similar--I can't tell from the pictures. Having the schematic is a plus! The existing tone stack(Bass/Treble) might be OK, you would just have to get you a working input installed and test.
That is just general, the details is in the wiring. The attached link will help you on that. Platefire
June 5th, 2012, 09:19 AM
The tube compliment is 6sj7, 6sc7, 6n7, 4x 6l6, 2x 5u4
I had the chance to run a guitar into it this morning and it sounded pretty good. Maybe in need of a little clarity on the top end response, but balanced and warm. I used the phono input which bypasses that first preamp tube (the mic input uses that tube) and I think that I'll probably want to re-wire the mic input for guitar rather than phono, although on the repair piece I may just wire them both to one input and switch it for different gain/volume levels
June 5th, 2012, 10:13 AM
It would be great if you could scan the schematic and post that. I'm wondering if the 6SC7 or the 6SJ7 is the first preamp tube V1.
A lot of the PA amps have a capacitor from the mic input to the first preamp gain stage. You don't need that for guitar. Attached is champ type schematic. the input shown is more or less your standard fender input grid. That is what you would need to add to your first preamp gain stage in the order as follows: 3 terminal 1/4" switch jack/1 meg resistor to ground/33K in line grid stopper resistor wired just like as shown on attached schematic. This would be wired to the tube preamp terminal where your mic input is going. You will have to de-solder/remove that old mic input wiring first and add this.
Most PA tone stacks I've experianced don't really work good for guitar and need to be rewired for a guitar friendly tone stack but for now I would recomend just trying to establish a good guitar input wiring. Then you will be able to tell better what you got stock and or what you need to mod. Platefire
June 7th, 2012, 08:02 PM
Here's my friends, he probably did a couple tweaks, but said as is it makes a nice guitar amp.
If you'd like I can try and get some more details etc.
*edit, crud, different model, sorry*
June 7th, 2012, 09:29 PM
That thing is awesome! Perfect size too.
December 28th, 2012, 10:34 AM
I know this is Christmas and not Easter, but here's a little resurrection for y'all.
I finally got my parents to bring the other burned out S-C au36 down from Pennsylvania and I was poking around it a bit. I'll get some shots of the busted wire wound resistor, et. al later, but I started thinking about the transformers and wondering if I might do better to harvest parts from the broken unit rather than resurrecting it.
I took some shots of the transformer codes with the hopes that someone might be able to help me identify them:
Here's the OT:
and here's the PT:
All total it has 9 octal sockets and heres a copy of the schematic
January 5th, 2013, 09:55 AM
Anyone know where I can find a 125 ohm 25 watt resistor?
you can see where it falls into the schematic in the middle of this pic
That's from the schematic inside of the amp which is slightly different from the one I posted, but it's also covered in goo so the above is a little easier to read. I'm trying to follow and note the differences.
That resistor is run to one of the can caps which is apparently faulty, because it was "fixed with an old Sprague Atom that may also have gone bad.
Besides that, the only visible sign of trauma is this burnt out octal socket on one of the power tubes
and some of these carbon comps that look fried:
I havent had the time to go through and check them all for values. Surprisingly, the few I have checked have been pretty close to the schematic.
Here's a gut shot of the whole amp:
You'll see the rats nest of an output transformer back there with every conceivable output impedance...
So here's my plan.
1.) begin by finding replacements for that resistor/capacitor network off the power transformer
2.) replace the other electros in there as well
3.) rewire the output transformer to one jack with a selector knob to make it selectable between 4/8/16 ohms
Now, for transformational purposes...
I tried the other working unit with guitar through the phono jack and it sounded good. Not very strong, but a good starting place. The phono jack bypasses the first gain stage though, so I thought I'd start out by replacing the antiquated mic jack with a 1/4 inch jack and drop the phono input altogether.
That leaves me with an extra pot on the front. Any ideas for this? A "presence" knob? A master volume?
January 5th, 2013, 11:15 AM
January 5th, 2013, 11:25 AM
Perfect! I was looking at the 25watt resistors and striking out.
January 7th, 2013, 03:10 PM
I've been trying to understand exactly what that resistor/cap are for in this amp and as I was measuring resistors I made the fascinating discovery that although that big wire wound resistor is physically cracked, the wire is in tact and it's still resisting a dead on 125 ohms.
After digging around a bit more and determining that all of the resistors were close enough to spec for rock 'n roll I got brave (or stupid) and I decided to wire up a speaker, tube up and flip the switch.
Here's what I found.
Before tubes I was getting heater voltages at 5.2 on the rectifiers and 6.3 on the rest.
With the tubes / speaker I was getting no sound but every time I turned the volume input knob I could hear a crackling sound coming directly from the output transformer.
Because the site of the original burnout on the one 6L6 socked was where the blue OT primary came to the tube is it possible I'm dealing with a burned out OT here? This is an amp that would have been used at least weekly for about 60 years.