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Eric Schold
Posted on Friday, May 07, 2004 - 03:09 pm:   

I've got a 1978 HD 130 which seems to be working fine until twenty or thirty minutes into playing
when it suddendly doubles in output, which I'm sure is the output it should have from jump street. It will then fluctuate up and down with
further playing. Anyone experienced this or have any clues?
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michael kaus
Posted on Tuesday, May 18, 2004 - 06:41 am:   

Sound like a flaky tube. Does it get real distorted as it get louder? If it does, your bias voltage might be flaky. Is this the 12ax7 model or the SS driver model? Mike.
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Eric Schold
Posted on Friday, May 21, 2004 - 08:09 pm:   

Thanks for the reply Michael, this amp has a solid state pre-amp with a single 12ax7 phase splitter. When it gets louder, it gets much cleaner also. The output tubes were recently replaced and biased to try and resolve the problem, but had no effect.
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Steve Kennedy
Posted on Friday, June 11, 2004 - 06:02 pm:   

If the 12AX7 phase splitter tube is replaced with a known good one and the problem still occurs, I would be looking for an intermittent in the power supply or the preamp.

It is also possible that you have a thermal intermittent short in your output transformer which cannot be remedied short of a complete replacement or rewind job.

Steve

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