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Old 31st August 2014, 11:39 PM   #1
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Default Baldwin organ amp

I found Baldwin organ amp I'd like to convert to home audio use, mostly as a way to learn more about tube amps. This one didn't come from a console, but I think it was used as a dedicated amp for an external speaker - like a Leslie speaker but with no rotating horn. It was mounted on the wall in the back room of an old masonic temple, so I thought it was a PA amp at first, until I saw the Baldwin tubes. Pics are below - after I cleaned all the bird turds off!

It looks different than most of the other Baldwin amps on the forum in that it has no 12AX7 or other small signal tubes in it. I'm not interested in making it stereo, if that's even possible - I don't know much about tube amps but it seems this one is mono all the way. I don't want to put out the money for output transformers for this one anyway - so I'll either find a mate and use them as monoblocks or power my center speaker with it in my 5.1 setup.

So - what do I need to do with this to make it work in my home system, besides recapping and replacing some deteriorated wire? I've sketched out the schematic as well - as you can no doubt see I have no idea how things are arranged inside the transformers, so I probably have that part wrong. Any help/corrections would be appreciated.

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Old 1st September 2014, 12:57 AM   #2
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I think you have the B+ PSU filter wrong. CLC topology is what I'd expect to find. I also think you have signal the topology wrong. The primary of the O/P transformer will be center tapped and a pair of 6L6s will be connected to each end of the primary.

You show only power O/P tubes and a rectifier. You will have to build driver and phase splitter circuitry.

While they may have been good in their day, those old "bumblebee" coupling caps. have got to go. Replace them with a mix of Soviet surplus K40 paper in oil (PIO) and 716P series "orange drop" parts.

Check the carbon composition resistors for drift and noise. Except in grid stopper positions, replace the carbon comp. parts with carbon film parts of appropriate wattage rating.
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Old 1st September 2014, 01:51 AM   #3
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Might the original driver and phase splitter circuitry have been in the console? (I think I can get access to it) I'll have to learn what you mean by those terms....can you point me to a reference?

I thought I had the transformer connections wrong. What puzzled me was the connection of the 6L6 pins #8 to the OP secondary. Why is that connection there?

So I suppose this amp can't be used as-is (after part replacement) - that's what I'm gathering here.
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Old 1st September 2014, 02:29 AM   #4
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I think the B+ filter is just LC - there aren't enough capacitors in there for CLC. I made a few corrections to the schematic - the power filter cap is connected to pin 4, not 3, of the nearest 6L6, and the + end of the power cap is connected to the OP primary, presumably to the center tap you mentioned.
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Old 1st September 2014, 03:08 AM   #5
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Quote:
Originally Posted by schubert View Post
Might the original driver and phase splitter circuitry have been in the console?
Yes it probably is. But it may be integrated with the organ tone generating circuits (chassis) and inconvenient to separate. You can get some information about phase inverters/splitters from the Valve Wizard

Quote:
What puzzled me was the connection of the 6L6 pins #8 to the OP secondary. Why is that connection there?
The cathodes are coupled through the output transformer with a separate winding. By doing so this provides some local feedback to help reduce distortion.

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So I suppose this amp can't be used as-is (after part replacement) - that's what I'm gathering here.
This is correct. And before I did anything else I would verify the transformers are good. DC resistance checks and preferably hipot testing the OPT with around 500 volts DC, or half that if AC. They look pretty old and weathered.
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Old 1st September 2014, 03:26 AM   #6
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I don't have the equipment for a hipot test - when you mention a DC resistance test, are we looking for infinite resistance between the primary & secondary?
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Old 1st September 2014, 04:25 AM   #7
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Originally Posted by schubert View Post
...when you mention a DC resistance test, are we looking for infinite resistance between the primary & secondary?
Yes, and also from windings to the core. But continuity within each winding.
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Old 1st September 2014, 05:27 AM   #8
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OK I'll check it out tomorrow.
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Old 2nd September 2014, 01:53 AM   #9
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The OP secondary, power primary & B+ secondary are OK, but the OP primary & heater secondary show resistances in the 60 ohm range. There are also measurable resistances between the windings & core in both transformers - it jumped around a bit, averaging on the order of 5Mohm but dipping as low as 100K. I guess this iron is shot, unless I can open it up and clean things out a bit. Well - on to the console, which I should be able to access later this week, and which hopefully has not doubled as a bird roost these last 40 years or so.
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Old 2nd September 2014, 04:31 AM   #10
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Well, this doesn't sound good. Any measurable resistances between windings and/or the core is not good. It may be leakage from humidity or moisture that has gotten into the transformers. Make sure that all wires are disconnected when measuring this. Also be very certain not to touch any bare ended wires with your fingers when making these measurements because your skin resistance will affect high megohm readings. And even a very light touch will do this. On a good transformer the most you may see is a slight up-kick on the highest range from the internal distributed capacitance charging up. But this will quickly return to virtual infinity.

I guess the most you could try is to bake them at low a temperature (100) and possibly drive it out. But if the wires are hard and dry I wouldn't even bother with it as they're probably too far gone. Also, if you remove the end bells, be careful to notice any insulating shoulder washers on the bolts and not loose them. They are there to prevent shorting out the laminations magnetically.

You also have to ask yourself why was this amp hanging on the wall and not in the console where it belonged?
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