Boggs here with my first post... and it may be a doozie...

[boggs]

Hey, all! Boggs here from RockBeach Guitars in Rochester, NY. Up to this point, I am a self-taught guitar designer/builder and would like to start learning a little about tube amps with a project. I am the "inheriter" of a HeathKit AA-23 mono amplifier (25 watts). I'll describe its basic parts below as I am awaiting a manual/schematic I have on order...

Tubes used are:
V1- 6EU7
V2- 6EU7
V3- 12AU7 (with hum balance pot alongside)
V4- 7591
V5- 7591
V6- 5AR4

Transformers are:
input: 54-118 549-6407
and
output: 51-82 549-6407 tapped for 4-8-16ohm.

Control pots are simple volume, bass, treble.

I would like to convert it into a guitar amplifier. Anything strike you as to what would be doable? Suggestions? Too much for a first post? Should I wait for the schematic to scan it and post it first? I can read a schematic, but I do not have any training in amplifier design/theory except to know that I do need to be extremely careful of the power supply caps (to discharge them safely) and voltages in the amplifier performing bias setups etc... I am, however, trainable... Any takers?

Boggs

[xtian]

Repurposing an amp is what I'd call an advanced level project. Start with simple and well solved. Ax84.com Projects, for example.

[David Root]

Let's see the schematic first. Was it designed as a PA amp or a hifi amp? With a 12AU7 PI and 7591 power tubes my guess would be hifi. It looks like a monobloc to me at this point.

Changing the 12AU7 PI to a 12AT7 should allow you to overdrive the 7591s, and possibly the 12AT7 too, depending on what you plug in to V1.

But it's not that simple to get a balanced topology that does exactly what you want, there's always more to it. The iron is an unknown quantity at this point. 549 is Midwest Coil & Transformer, now I think Midwest Coil LLC, of Lafayette IN. Used in Alamo amps. Heathkit stuff was pretty good in its day, for the price.

I think you have a solid candidate for a guitar head here! Blues or blooz would be a good choice with the 5AR4 rectifier.

[funkmeblue]

well just like remodeling a old house it would be alot easier to bull dozer if over and start new. Some old gibson amps used 6eu7's in the preamp, I would start there and the 7591 output is kinda rarity now. Does it work, have voltages?

[Phil_S]

Repurposing an amp is what I'd call an advanced level project. Start with simple and well solved. Ax84.com Projects, for example.

FWIW, I am in agreement on this point. If it has two good transformers and the chassis is decent, it will get you a few dollars on eBay you can put towards a proper guitar amp kit. Besides if it's in nice condition, some Heathkit fan deserves a crack at restoring it. Let me ask, would you repurpose an old Fender or Marshall into a hi-fi amp? No, that's the sort of mischief that gets you sent directly to

One of the challenges you will run into when repurposing a hi-fi amp is that the voicing is all wrong for guitar. You will have a steep learning curve to understand what's needed to make it into a good guitar amp. and it could prove quite frustrating.

I think that building a kit is the way for a beginner to go. People who are new to this ofen don't realize how very difficult and often impossible it is to source individual parts cheaper than a kit.

Something from AX84, a Fender Champ type amp or a modest push pull like an 18W Lite are some suggestions.

[Bob-I]

Hey Boggs, welcome to the thing, fancy meeting you here.

Yes, this is do-able, you're good with a soldering iron. Take some detailed pics, locate a schem or draw one up. You'll prolly need new filter caps, and you'll prolly want to lose the 6AU7's in favor of 12AX7's.

Should work out well.

[boggs]

Hey, Bob! Thanks for the responses, all... Bob-I will tell you that I don't ever do anything easy... My first guitar design and build was a chambered body bookmatching the padauk top and back and chambered Cuban mahogany midsection without using so much as a router... It had a jazz style neck humbucker and a vintage rock bridge humbucker both tapped so you could select any individual or combination of coils... The neck was set back at 1.5 degrees... I'm not very bright, but I am sorta fearless... I like the unique factor of reworking the HeathKit and I think it may actually teach me faster about how the thing actually works. I am a design engineering technician by trade so I am not afraid of what I don't know yet. Any day I learn something new is a good day.

When the schematic comes in, I will scan and post it so you can see what I have to work with. Bob, this is the one from CC... I will also take some pictures tomorrow but I need to get some sleep now as I have to get up for work in 4.5 hours... Catch you all later! Thanks again for all of the replies! I'm looking forward to learning! Cheers!

Boggs

[sliberty]

My first question to you is how you define "re-purpose"? Some would say that means changing a few resistors and caps. Others would contend that it means gutting the amp, and just reusing the iron and maybe tubes. Your intention here is a big factor in what gets recommended.

I imagine that some Ampeg model would be most closely matched to the tubes you've got there. You might consider searching through those schemes to identify a candidate.

[Phil_S]

... I am sorta fearless... I like the unique factor of reworking the HeathKit..."
Boggs

Boggs,
I think your enthusiasm is not lost on us. I'd guess that most of us are the tinkerer type. We are curious and are willing to go out on a limb now and then. I see you mention that you are a design engineer. I'm not quite sure what that is and how much electronics background that gives you. I'd argue that building a guitar is not nearly as dangerous as working on a tube amplifier. I think you probably know, but will say it anyway, the circuit, even when unplugged from the wall, can carry a potentially lethal electrical charge due to the cap reserves, and, of course, working on the live circuit has it's own set of concerns. I suppose you can run a chisel through your hand when you are making a guitar, but that just doesn't carry the same lethal potential. Please remember this in the midst of your enthusiastic spirit.

If you are so motivated, by all means, go for the project and see what you can do with it. Good luck.
Phil

[M Fowler]

Hello Boggs and welcome to fun bunch of builders.

Mark

[xtian]

I suppose you can run a chisel through your hand when you are making a guitar, but that just doesn't carry the same lethal potential.

You've never had your tie sucked into the planer, or your hair wrapped around the lathe?

"I'll give you five--no...THREE good reasons to learn tool safety."

[Richie]

The tubes you list, as was posted, look through some of the Gibson amps schematics. Quite a few use those 6EU7 and 7591 tubes. But a little warning too. Some of the models that used those tubes did not sound very good. And not the tubes fault as much as it is in the amp design.

Once you know the pinouts and operating points. You can design around those. The 6eu7 is a dual triode,similar to waht a 12ax7 would be, but uses a different pin out etc. just have to look up the tube data. 7591 are expensive and more of a rare type tube.
Some Ampeg amps used the 7591 tubes too I think Univox. As i'm sure their are others out there.

If your just learning to build, build something that has been built before,once you get the idea and hang of it, then move up to other things.
That way, you have info and help to begin with. You could build a small JTM45 type amp with that setup. be kind of different..

[boggs]

I do work with some pretty leathal equipment at Xerox in my profession so I do have a healthy respect for electrons... just as healthy as a spinning router or table saw or radial arm saw or coping saw or planer or...

I have been in contact with Andy Fuchs and he has also offered to help give direction in this. He actually has a couple of my guitars right now that he is going to record through one or more of his amps...

Anyway, silky cleans are the most important to me. Having gain control to be able to add some grit would be nice. I am not looking for a screamer... and master volume with bass and treble would be good. Being able to plug a guitar with acoustic pickups into it and have it sound reasonably nice would be a plus. It will likely be pushing a 1 x 12 open back cabinet I built with a Carvin British Series speaker (I think made for them by Eminence). It will feel like I accomplished something to be able to play a guitar I designed and built into an amp I "built" driving a speaker cabinet I built... Either way, I would like the amp to have enough harmonic detail to show off the chambered body guitars and exotic woods I build and use... Here are pictures of the amp. I reduced them 50% but I do realize they are still pretty big but I didn't want to lose too much detail.

[img1368]http://www.angelfire.com/ny5/boggs.com/ ... -23-1s.jpg[/img]

[img1368]http://www.angelfire.com/ny5/boggs.com/ ... -23-2s.jpg[/img]

[img1368]http://www.angelfire.com/ny5/boggs.com/ ... -23-3s.jpg[/img]

[img1368]http://www.angelfire.com/ny5/boggs.com/ ... -23-4s.jpg[/img]

Thanks again for the return enthusiasm and concern for my safety!

Cheers!

Boggs

[Phil_S]

Real nice. It's in great condition. You might give consideration to allowing for all mods to be reversed, to put the amp back to stock. Some colletor might pay big money for that someday. In particular, I'd look to drill new holes as a last resort, and then I'd do it on the rear if I had to.

I wouldn't be too concerned about what tubes just yet. You'be got a typical mix of octal and noval sockets. 6EU7 is the same as 12AX7 with a different pin out. Both can easily be had. I'd see if I could get a schematic and BOM showing the capacity of the transformers. Then you really will know with what you are working. Whatever voltage comes out of that PT will give you some limiting parameter. Find this first. Then you can decide what to do.

There is plenty of room in there. You can do almost anything with it.

[boggs]

I have a second pristeen chassis and front for it so I could essentially keep the original stock and get new sockets and such and start from scratch so to speak and just use the transformers...