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Lab Series L9 making background noise

Started by steinros, September 19, 2013, 02:09:53 AM

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The tone of the amp is loud and clear, but there is a fizzy background noise in it wich is annoying when the guitar sound fades away. When the amp gets no signal, the amp is quiet. I have heard similar noise from other amps, but then from a Fender Super  Champ, and the solution was new output tubes. That can of course not be the answer here. Any suggestions?


Circuit and layout;

There are quite a few things that could cause something like that, and as usual it's a matter of swapping and cross checking to try and isolate with section it's coming from.

Do any of the amp controls have any effect on the unwanted sound?  If so, what?

A speaker with a loose voice coil can sometimes case this sort of effect, so try the amp (at low level) into a different cab, and a different amp into the cab for this amp.

Take a known good lead and plug into Preamp Output and Power Amp Input to check the Fx loop jack switches.

Cross over with a different amp, it's pre into your main, then your pre into its main.

In each case you are looking for changes, or if the fault seems to be present with one crossover but not the other.

No doubt others will suggest more tests to try and localise the problem.
If you say theory and practice don't agree you haven't applied enough theory.


That was my immediate reaction:  rubbing speaker voice coil.