Stromberg-Carlson AU-35 PA Amp for Guitar - diyAudio
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Old 5th March 2011, 02:45 AM   #1
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Default Stromberg-Carlson AU-35 PA Amp for Guitar

Let me just start off by saying this is going to be my 1st attempt to take an old PA amp and try and use it for guitar. I have a basic knowledge of how things work but I just want to make sure I am going about this right....

I have a 1951 Stromberg AU-35 all original with 3 mic inputs "xlr", 1 phono input, 2 separate master volume controls, 3 separate mic volume controls, 1 phono volume control, bass & treble control. Each channel puts out 25 watts. I would somehow like to set this up so that I have 1 input for guitar through a 1/4" jack and also leave the 3 mic inputs the same. I would also like to have 1 8ohm output 1/4" jack and also leave the 2 octal jacks on the back. Is there any way I can use both volume controls through the 1 output jack so that I can use the full 50 Watts and control each individual? I am guessing I can just tap off one of the xlr inputs for my input jack and tap off of the 8ohm pin on the octal jack for my output 1/4" jack? If I just tap off the 1st octal jack will the 2nd volume control increase the output as well because it does look like they are wired together somehow or would I be only able to get the 25 watts per channel? I figured the 2nd Master Volume might be handy going through 1 output so it would make the amp break up for a little distortion. Here are some photos and schematics. any help is greatly appreciated.
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Old 5th March 2011, 03:52 AM   #2
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Default Schematic

Schematic
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File Type: jpg AU-35 Sch.jpg (128.9 KB, 82 views)
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Old 5th March 2011, 04:40 AM   #3
Gilgy is offline Gilgy  United Kingdom
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The schematic gave me eye strain but I imagine the easiest thing to do would be to buy a couple of octal plugs and wire the 8 ohm connections to inline 1/4" jack sockets. Same with the inputs really, XLR to jack adaptor.

As for just using one output, it looks like both power amps (labelled channel 1 and channel 2) are hard wired to the power supply so you would need a speaker load on both octal plugs to avoid damaging the amp.
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Old 5th March 2011, 04:45 AM   #4
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Quote:
Originally Posted by Gilgy View Post
The schematic gave me eye strain but I imagine the easiest thing to do would be to buy a couple of octal plugs and wire the 8 ohm connections to inline 1/4" jack sockets. Same with the inputs really, XLR to jack adaptor.

As for just using one output, it looks like both power amps (labelled channel 1 and channel 2) are hard wired to the power supply so you would need a speaker load on both octal plugs to avoid damaging the amp.
Could I run a wire from the 4ohm pin "so output would be 8ohms" on each octal plug to the 1/4" jack so that the amp will have a load on both channels or can I not bridge the 2 channels like that?

Last edited by ecguitarworks; 5th March 2011 at 04:50 AM.
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Old 5th March 2011, 04:50 AM   #5
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Default Slightly Better Schematic....

Slightly Better Schematic....
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Old 5th March 2011, 09:02 AM   #6
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Wow! Unique design. Technically, it has stereo capabilities, but is only using mono inputs. That design reminds me of the old sears silvertone 1485 guitar amplifier. It used 4 6L6 tubes with two seperate output transformers. The speaker cabinet used six 8 inch woofers where 3 were run off of one output transformer, and the other 3 were run off of the other output transformer.

It was mono but had two seperate channels and outputs. I don't know what your electronics capabilities are, but I could tie the two channels together, and parallel the two output transformers together. Dynaco ST-70 ties (bridges) the two outputs together in this configuration for mono operation. A Bogen MO-200 also wires their mono amplifier in this fashion.

Last edited by century tek; 5th March 2011 at 09:11 AM.
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Old 5th March 2011, 02:00 PM   #7
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Quote:
Originally Posted by century tek View Post
Wow! Unique design. Technically, it has stereo capabilities, but is only using mono inputs. That design reminds me of the old sears silvertone 1485 guitar amplifier. It used 4 6L6 tubes with two seperate output transformers. The speaker cabinet used six 8 inch woofers where 3 were run off of one output transformer, and the other 3 were run off of the other output transformer.

It was mono but had two seperate channels and outputs. I don't know what your electronics capabilities are, but I could tie the two channels together, and parallel the two output transformers together. Dynaco ST-70 ties (bridges) the two outputs together in this configuration for mono operation. A Bogen MO-200 also wires their mono amplifier in this fashion.
Reading a schematic, not so good. Soldering and doing the psychical work excellent Another Idea I had was to run a jumper wire from the 4ohm pins on each octal socket to my output jack so that there would be a load on each socket and give me 8ohms at the output jack. Would this harm anything by doing this?
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Old 5th March 2011, 07:19 PM   #8
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It won't give you 8Ω's jumping both 4Ω pins, just 4Ω paralleled. If you want 50 watt, 8Ω's, just parallel the two 8Ω jacks together. You can also tie the two master volume controls together into one so you only have one master volume control.
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