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Stromberg-Carlson AU-32 distortion in preamp

Westside

Member
2009-10-03 2:58 pm
Hello all,

I've been reading for a while, but this is my first post.

I've got an SC AU-32, which I am updating and optimizing for guitar. So far I have added a grounded cord, replaced most of the caps, (including rebuilding one of the cans) and changed out the inputs and outputs for 1/4" jacks. I added a selector switch to select the output impedance.

It's working and sounds great, if you keep the volume under 3. Above that it starts to distort, much sooner than it should.

I pulled out the o-scope and hooked it up with a sine wave input. Through the input tube (6SN7) all is good, nice symmetrical amplified signal.

The distortion shows up at the input of the mixing tube (6J5). The two input channels are tied together with 1M resistors, tied together, with the junction fed to the mixer tube.

On the input side of the 1M resistor, I see a nice symmetrical sine wave. On the output side of the resistor, I see a clipped signal.

And there I am, out of my depth. I don't see how the signal is becoming asymmetrical across the resistor.

The only caps I didn't replace are in a can with 3 10 uF caps, one of which is circled on the schem photo. This hookup is confusing, since the cathode resistor/cap are connected not to ground but to the B line. Any ideas?
 

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  • 6SC7 output.jpg
    6SC7 output.jpg
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  • 6J5 input.jpg
    6J5 input.jpg
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  • Schem.jpg
    Schem.jpg
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