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Old 17th October 2006, 03:59 PM   #1
PRNDL is offline PRNDL  United States
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Default converting Baldwin organ amp to guitar

I have a tube amp from a Baldwin organ that I'm planning on converting to guitar. It has a pair of 6L6 power tubes, a big power supply iron, and paper output transformer and choke. There's a volume control.

I believe the power supply electrolytics have been replaced, as well as several of the caps in the preamp section (they are blue chicklets). There appears to be several old ceramic disc caps in the tone section that could be replaced, but I'll probably start it up with them to see how it sounds.

The next steps are going to be:
- Putting in a power cord (3- prong)
- The 5-amp fuse and cap appears to be missing
- Adding a guitar connector in parallel to the stereo input

Once it's powered up I'll begin making choices about the tone section, which might include adding a single tone knob (there's not much room).

Does anyone have a schematic?

Any suggestions for conversion would be greatly appreciated!!

Thanks

Here's some photos

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Old 18th October 2006, 06:08 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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Nice find!

That should be a super guitar amp, a couple of 6L6s should give decent power for sure, and that chassis looks mint.

The onlt thing that might be annoying is that it might not have enough gain, guitar is a very low output instrument, and you might need to wire in an extra gain stage- you need about 2-3 12AX7s or 6SL7s.

If you really want to go for it, completely remove the old circuitry and rebuild a Fender Bassman design inside the chassis- this way you get to have an authentic vintage design and it looks original too. You could use the choke under the chassis to improve the power supply too, and install new sockets. Higher res photos and a tube listing would help BTW.

You might want to test the electros with a capacitance meter to see if they are still alive, and ditch any leaky or swollen ones, and also where is the rectifier tube?
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Old 18th October 2006, 08:15 PM   #3
PRNDL is offline PRNDL  United States
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Upon closer inspection, it looks like someone has already rewired the amp.

The new circuit uses blue poly caps, primarily with the single 12AX7 preamp tube.

The volume pot isn't connected, but they don't use them in stereo power amps.

The main issue I see is that it's a two prong power cord and the chassis is grounded to the secondary side of the transformer. Separating the chassis and ground could take some work --- is it necessary?

I'm a bit concerned that the power cord was cut and the fuse is missing.
I recall that some repairmen do this when the unit is broken, so that someone doesn't plug it in without fixing it.

Or, it could be the result of a quick pull.
I sent an email to the seller to check.
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Old 19th October 2006, 01:05 AM   #4
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Connecting the center tap of the transformer to the chassis is common. This is not an issue if there is only one connection to the chassis. In older amps it was common to use the chassis as the ground bus for the amp. Often the parts for each tube were grounded to the tube socket itself. This can cause a low level hum.

If you wind up rebuilding this amp into a guitar amp, you may want to wire each ground connection seperately to a common point on the chassis (star ground) or run a ground bus (I use a piece of #10 solid wire) across the chassis and connect it to the chassis at one point. Connect the third wire in the power cord to the point where the chassis connection is made.

PRNDL - must be a Powerglide
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Old 19th October 2006, 02:51 AM   #5
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wait a segundo, amigos...

Either there isn't a power tranny there, or else there isn't an output tranny.... my bet is that that thing under the chassis is an interstage driver transformer?? Looks kinda small for a pair of 6L6 in p-p??

Quick check of the pin connections will tell...

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Old 19th October 2006, 03:10 AM   #6
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There are questions about missing items. If you look carefully, all of the important parts are there. Follow the yellow and red (discolored) leads from the big transformer (power) they all lead to an octal socket. This is the rectifier tube. The smaller piece of iron under the chassis looks like a choke. The bigger one (in the middle) looks like the output transformer. Yes it is small by todays standards, but it is about right for the 1950's. It is bigger than a Bandmaster OPT. It is fine for a guitar amp since you only need to go down to 83 Hz.
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Old 19th October 2006, 03:16 AM   #7
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Quote:
PRNDL - must be a Powerglide
Yup because PNDLR would have been a Dynaflow.
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Old 19th October 2006, 07:10 PM   #8
PRNDL is offline PRNDL  United States
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I've traced out the preamp section and have attached the schematic.

It is a simple preamp based on a 12AU7 pair with no volume or tone controls.

There might not be enough gain for a guitar, but I figured that I'd hear something. Also, I have a pretty small speaker (a 25W Marshall cab with a 10" Celestion)

I did remove a feedback resistor connected from the output transformer (+ speaker lead) to pin 2 of the 12AU7 (the second gain stage input). The amp wasn't working, and I figured this wasn't needed.

The tubes light up, but there's no sound.

The seller states that it was working.

Does anyone have any suggestions?
What amp is this circuit based on?
Should I try hooking up a guitar preamp with line level?
Perhaps one of the tubes didn't travel well.

Thanks!!

Ron
Attached Files
File Type: pdf ronsamp.pdf (19.6 KB, 173 views)
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Old 19th October 2006, 08:54 PM   #9
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If I were you I'd rebuild the thing. Stick it in a Hammond chassis, and maybe pick up a beefier OPT, maybe the famed TF110-48 at Triode ( http://triodeelectronics.com/tf65wscta48o.html ). A couple 12AT7's might be nice than a 12Ax7 but i'd use what you have. It could be a fun project. Of course it depends on the amount of work you want to put into it for essentially the same thing.
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Old 19th October 2006, 10:23 PM   #10
PRNDL is offline PRNDL  United States
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>> If I were you I'd rebuild the thing.

I've been thinking about that since there's a ton of extra room on the chassis.

I'd like to get it up and running first.
It's pretty close to that, and there's a lot of new parts.
I checked and the capacitor cans have been replaced with new caps.

I'm wondering if a vintage tube tester is worthwhile, since the circuit looks good. There seems to be a lot of them on Ebay for $50 or so, and that would be how much it costs to have them checked by the local tube guru.
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