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Sudden Loud Buzz on my Tube Amp

I have a Luxman SQ-N100 integrated amplifier (12 watt EL84 based) that has just begun to intermittently buzz at a high volume.

The tubes all look good -- I added two NOS Raytheon 12au7 tubes a month ago, but doubt that's the cause.

I can't predict when it will buzz, but it's very loud and the volume control has no effect on it. If I turn off the amp it continues for a moment. Switching the inputs makes no difference.

Nothing has been altered in my setup and the amp is fed power through a conditioner.

Thoughts?
pjudice
atmasphere's avatar
atmasphere
11,784 posts
 
Possibly an intermittent cathode/filament short in one of the 12AU7s.
pjudice OP
5 posts
 
I suppose I should change them back to the old ones and see if it happens again.
yogiboy's avatar
yogiboy
5,655 posts
 
What about the EL84 tubes? Have you replaced them?
pjudice OP
5 posts
 
Thanks for your input guys. This sort of mysterious problem is the worst part of hi-fi.

I think Atmasphere nailed it. I replaced both 12AU7 tubes and the problem is gone, gone, gone.

I always figure that a tube is either working or dead, which is ridiculous of course.
knghifi
1,668 posts
 
I had a similar problem with a tube mono amp. One input/driver tube was bad and it made a constant loud buzzing noise.

This is debugging 101, always roll back your last change when a problem surfaces.
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