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electronics: guitar amp "fizzes out"
Posted by: mrbigstuff
Date: May 26, 2019 10:41AM
I'm hoping that the confluence of guitar people and electronics geeks can help me with this issue.

Vox modeling amp, 50 watts with variable power knob. This amp sounds fine when a note is played, but then halfway through, before the note dies out, there is a "fizz" sound. Almost like radio interference - but it's not, it just has that sound. This particular amp has a tube in the preamp section, too.

Any ideas? It's almost worth it to bring to a tech, but not quite....
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Re: electronics: guitar amp "fizzes out"
Posted by: deckeda
Date: May 26, 2019 11:13AM
Play songs with only short notes?


If it holds the note at low volume it's probably the power supply. If it never sustains, it might be that preamp tube.

Then again, I'm totally guessing. But yeah, test at different volumes etc.
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Re: electronics: guitar amp "fizzes out"
Posted by: lost in space
Date: May 26, 2019 11:20AM
First thing I'd do is look for swollen capacitors.



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Re: electronics: guitar amp "fizzes out"
Posted by: Will Collier
Date: May 26, 2019 12:23PM
Quote
lost in space
First thing I'd do is look for swollen capacitors.

Yeah, that's the most likely failure vector for pretty much any piece of electronics. But try swapping out the preamp tube first, mostly because that's very quick and easy.
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Re: electronics: guitar amp "fizzes out"
Posted by: Filliam H. Muffman
Date: May 26, 2019 01:05PM
Not a guitar person, but the question about different volumes is a good start. I couldn't find the exact model based on what was listed to look up a circuit diagram. Possible tube, capacitor, power supply rectifier diode, bad solder connector.

Pull the tubes to test if you can get them tested locally, or replace if you can get them cheap. Tubes in amps under 10 years should be a little more reliable. Too bad there aren't testers in grocery stores any more like the 60's.



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