# Crackling Noise Coming Out Of Amp



## vds5000 (Apr 14, 2008)

Hey there,

An acquaintence of mine just bought a used Marshall head and has noticed a crackling noise coming out of the speakers as soon as he turns the 'Standbye' switch on. It goes away after about 30 seconds. This noise happens whether a cable is plugged in or not. If he puts the amp back on standby and then turns it back on again, the noise reappears for another 30 seconds.

I'm guessing just a run of the mill bad power tube, but I can't say this with 100% certainty.

Any thoughts?


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## Wild Bill (May 3, 2006)

vds5000 said:


> Hey there,
> 
> An acquaintence of mine just bought a used Marshall head and has noticed a crackling noise coming out of the speakers as soon as he turns the 'Standbye' switch on. It goes away after about 30 seconds. This noise happens whether a cable is plugged in or not. If he puts the amp back on standby and then turns it back on again, the noise reappears for another 30 seconds.
> 
> ...


Could be a number of different things! Crackling usually means some sort of arcing. It could be tiny arcs inside a power resistor in the power supply, one that drops the voltage and decouples each filter cap stage. When you throw the standby switch voltage goes through theses resistors to charge each and every filter cap.

Or it could be a tube reacting to the shock of getting voltage on the plate. Maybe the spacers between elements have a bit of play in them and can flex.

Or a coupling cap leaking a bit as it charges. Or a plate resistor sparking a bit inside.

Or even an old standby switch that has a bit of carbon dirt on the contacts inside!

Without actually crawling in the amp we're talking crystal ball here...

:food-smiley-004:


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## vds5000 (Apr 14, 2008)

Wild Bill said:


> Could be a number of different things!
> 
> ...
> 
> ...


Hey Bill,

So we meet again! My buddy just tried taking a plug and sticking it in and out of both the effects send and effect receive jacks several times (while the amp was off). He claims this resolved the issue. Does this make sense?


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## Adicted to Tubes (Mar 5, 2006)

That makes perfect sense.The jacks can get dirty and sometimes plugging in to them a few times makes the contacts better.However,I would use some contact cleaner on the jacks to make sure.

www.claramps.com


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## Wild Bill (May 3, 2006)

Adicted to Tubes said:


> That makes perfect sense.The jacks can get dirty and sometimes plugging in to them a few times makes the contacts better.However,I would use some contact cleaner on the jacks to make sure.
> 
> www.claramps.com


Yep! What he said!:smile:


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## vds5000 (Apr 14, 2008)

Ok, apparently the crackling has returned. He did spray contact cleaner into all the jacks (2 inputs, 2 speaker, effects in/out). The volume of the crackling does not change when increasing/lowering the master volume and the crackling itself now only lasts for the 1st minute.


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## Wild Bill (May 3, 2006)

vds5000 said:


> Ok, apparently the crackling has returned. He did spray contact cleaner into all the jacks (2 inputs, 2 speaker, effects in/out). The volume of the crackling does not change when increasing/lowering the master volume and the crackling itself now only lasts for the 1st minute.


Well, we're back where we started - it's not a simple problem!:smile:

It needs to be put on a tech bench, where the tech can mutter curse words under his breath as he tries to find the problem!

Me, I'd start by replacing any power resistors in the filter cap string and then looking at how old are the filter caps and if I can hear any filter cap hum. If I do, then the caps are a likely culprit. As I said before, there's also plate resistors and coupling caps. Often using an oscilloscope can narrow things down to one stage.

One thing's for sure, it doesn't sound like he's gonna find the trouble himself!

:food-smiley-004:


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