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badraven 8th January 2013 10:47 PM

Donor Amps for Guitar Amps
Been wondering something

I read and hear about people from time to time that supposedly take some tube amp and build it into a "Great" guitar amp. If this is true, how do you know what amp will be a good donor and which ones to avoid? And before someone says it yes, it obviously takes someone that understands electronics and looks at the schematic etc.

How about an old Organ or something like that? How about anything older with large transformers?


kevinkr 9th January 2013 12:06 AM

:cop: Moved per forum policy to instruments and amplifiers. Please note forum subheader in Tubes / Valves and here.

AmpKiller66 9th January 2013 12:25 AM

Typically you want to look for something with enough gain stages to be useful (count the preamp tubes- typically you want at least 3 12AX7s or similar) and enough power to be heard in your given application- look at the power tubes used to determine this if it's not stated outright. Old tube PA systems/ Integrated amplifiers are often ideal candidates for modding- I've made a pretty badass guitar amp out of a Bogen CHB35A that I got from a church yard sale for $5! The amplifier from an old organ can also work in a pinch, and I've used them for parts before. If it's cheap and has tubes, it's worth scavenging in my experience.

Printer2 9th January 2013 12:29 AM

Pretty much anything 4W and up will make a good guitar amp. Depends what you want out of an amp. Something for home use or something that will also be used in a band situation. Older? Older than what? Not much new tube equipment being made that you would want to convert. Getting into the 30's and early 40's may be a little more difficult as the tubes are less current (relatively speaking). Something with 6L6's, 6V6's, 6BQ5's/EL84's would be excellent as they are standard guitar ware. NBut then there are the amps with 6AQ5's, 12AB5's, and a whole host of others. Best to find out what the output tube is and check E-Bay and other tube sellers to see how available and the cost of the tubes.

Printer2 9th January 2013 12:32 AM

Two 12A*7 's will do, a 5E3 has less gain on tap and has no problem getting dirty.

badraven 9th January 2013 02:58 AM

I appreciate the input. As for what I want it for. Well home recording mainly. I don't play publicly very often, not that I don't want to, just don't get the opportunities these days since I am not in that scene anymore and can't seem to find a single person near me that is interested in practice or jamming or anything really. Of course I want an amp powerful enough to play in a band but I can mic a small good sounding amp so that isn't an issue. I'm looking for quality not volume.

I want a Blues Deluxe or a Deville or a Belair or something like that but I just don't have the money for it and won't until well into spring/beginning of summer when work picks up again.

I have looked at kits and it seems to cost more for a kit, not to mention the effort of assembling it and any problems that may arise, than it does to just buy a decent tube amp at retail. And yes I understand that everyone touts their amp as being the coolest and best sounding out there.

And used amps seem to cost just as much or more than new amps.

Don't get me wrong I'm not trying to snivel here, just pointing out fact.

I see amps for 2,3 even 4k and wonder what the hell could possibly make an amp worth so much money. I understand you get what you pay for but in my mind there is a line where you go from "Here, I spent the time to make something that will last you a lifetime" to "Here sir please lean over this counter while I shove this tree stump in your poop-n-heimer and break it off.

Like anyone, I occasionally run into someone with "junk" laying around. Now that I am learning about electronics and modding pedals and amps and such I am thinking maybe something on the side to adapt and mod and experiment with might be a good course if I can identify a good candidate when I see it. So that is pretty much what I am after.
Currently there is a fella about 60 miles from me offering a free organ if I will come get it. about 14 months ago I disassembled an old Hammond organ with tons of tubes and several large transformers and such. But I didn't have the slightest idea how to make use of them. So, when we moved it all went in the trash.

Of course four months or so from now I imagine I will get a Fender Hotrod or something like that and then a who new round will begin.


Enzo 9th January 2013 10:48 AM

Ever buy a six pack of beer and take it home? Ever walk into a bar and order the exact same beer, and pay three times the price for it? You just paid for the beer, and the waitress, and the barkeeps rent and his insurance and the large TV on the wall so you can watch the game, etc etc. Expensive boutique amps have a couple things going in their price. One is that they ask it in the first place. But if you have ever built an amp, how many hours of labor went into it? Now what would you have to pay per hour for a skilled craftsman to build such an amp? What did the parts cost? Now when you sell the amps, you need to make a profit on the parts, so you mark them up as well. We have a local bakery called the something or other pie company. They make great pie. They also charge like $15 for a pie. They might be able to justify every cent of that price, but a pie is just not worth $15 to ME. SO those amps may not be work 2-3-4k to you, but if someone has the money and really likes the sound of an amp, sale made. No one forces you to buy those. And that beer? The local tavern I might attend charges one thing, but the fancy club the local lawyers and professionals attend charges $3 MORE for that beer. I do not have to go there.

Everyone who converts some old thing into a guitar amp thinks his thing is really great. And it is... to him. There is a lot of basic work to building an amp. You need a chassis, need to mount tube sockets, wire them up for heaters. Going to need a power transformer and the power supply circuits for B+, bias, and of course those heaters. All that before you even start to build the amp circuit itself. Now look at some old Bogen tube PA head. Chassis, check, power and output transformers, check. Tube sockets already wired, check. A lot of that basic stuff is already done. You can rip out the amp circuit and build to your own desire from there and save a lot of work. Or you can recognize the amp circuits are similar enough to guitar amps that a few smaller changes result in a reasonable guitar amp. The power amps are all wired in VERY similar fashion, so in many cases, you can leave the power amp stage alone, and concentrate on the preamp.

So old Bogen and similar amps are great for conversions. But a guitar amp itself is also a good candidate. The difference between a Fender Bassman and some Marshall is not spacious, a few changes inside, and your Bassman sounds real different. But you can find a VERY sturdy Traynor BassMate for cheap, and it is very similar to a Bassman, but costs less. The Traynors are VERY strudy and have tons of room inside to add things or move them around.

Free organ? Might be a potential amp there. Check out what kind of organ. A little air powered chord organ won;t help you much, and if you want to play with tube amps, an old Thomas transistor organ again won;t help much. But regardless, you'd wind up with at least a lot of wire.

Robert Kesh 9th January 2013 11:19 AM


Originally Posted by Printer2 (
Two 12A*7 's will do, a 5E3 has less gain on tap and has no problem getting dirty.

Well, kind of, but try it with 12AU7s, it will be pretty clean. Especially as the 5E3 has only two common cathodes in any signal path

Printer2 10th January 2013 12:38 AM


Originally Posted by Robert Kesh (
Well, kind of, but try it with 12AU7s, it will be pretty clean. Especially as the 5E3 has only two common cathodes in any signal path

Didn't want to sound like you needed two 12AX7's, the 5E3 will get dirty with a 12AY7. The point was three holes is not necessary, two will do, heck if you have a single channel amp with a single 12AX7 and a 12AU7 you still can get crunchy.

Printer2 10th January 2013 12:40 AM

Badraven - Pick up the organ amp. It will probably do what you want with a little bit of work.

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