Bought a new Marshall amp less than six months ago. Everything was fine until a couple of weeks ago, when I turned it on and began making a static sound along with intermittant crackling. The first time it happened, it went away after about five minutes. Now it's back and won't go away.

It's not the guitar or cables, because it does it even when I unplug those.

I haven't touched any of the knobs since the first time I set it up, so it's not due to turning up the gain or anything. But the static & crackle do increase when I try to troubleshoot and increase the volume and/or gain.

Nothing else in the environment has changed - no new electronics, I haven't moved the amp, etc.

Thanks for any suggestions.
Which Marshall amp? If it is a tube amp if could be a bad tube. 
Gibson LP traditional and DC standard, SG standard, MIA Standard Strat, Schecter Banshee 7
EVH 5153, Orange TV50H 2-2x12's
Line 6 M13
Tubes go bad all the time. It could very easy be a tube. Assuming you bought this brand new?
Gibson LP traditional and DC standard, SG standard, MIA Standard Strat, Schecter Banshee 7
EVH 5153, Orange TV50H 2-2x12's
Line 6 M13
try plugging it into different outlets throughout the house, just to see if it does it everywhere.

but yeah, could be a tube. i kinda doubt it though for some reason. newer amps shouldn't blow tubes in a few months.
pencil test the tubes, and plug the guitar into the effects return and see if there is noise.
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AggieKJ the pencil test is to lightly tap the tubes with a pencil with the amp on to see if it's microphonic. You'll hear the tapping coming through the speakers if it's microphonic. Even if tapping the tubes doesn't show any results it doesn't mean it's definitely not a bad tube.

If the amp has an effects loop it'll have jacks on the back for send and return. This is for any effects pedals that you want to be after the amp distortion. Normal use is that you have your guitar plugged into the front of the amp, the amp does it's distortion thing, you have a cable going from 'send' to the input of your pedal chain, your pedals do their pedal thing, and then the output of your pedal chain goes to the 'return' and the amp actually amplifies everything. This is mainly for time based effects like delay or reverb. Distortion type pedals usually go in front of the amp instead of in the effects loop.

Anyways, he's saying to plug your guitar into the effects return and see if you still get noise that way - if the noise goes away when plugged into the effects return then that would mean the noise is somewhere in the gain section of the amp and would help identify the problem.

I had a problem with a similar crackly noise a while back that turned out to be from the pins on the tubes being dirty and not making good contact. I pulled out the tubes, shined them up with some steel wool, dripped a little rubbing alcohol in the tube socket, pushed the tubes in and out a few times, let the alcohol dry, and it was good as new. This was an amp I'd left in our practice shed for a long time and I'm sure it was super humid in there. I kinda doubt a 6 month old amp would have that problem.

A bad or dirty potentiometer (the knobs) can cause crackling as well, especially if it gets worse when turning a knob. Usually if that's the case you should be able to find a spot on the knob where the crackling goes away. A bad pot would need to be replaced, but if they're just dirty you can spray some electronic contact cleaner in the pot, work the knob back and forth, and let it dry. I kinda doubt that's the problem being a new amp, but if you want to give it a shot:

Last edited by The4thHorsemen at Jun 22, 2017,
Thanks so much for th detailed replies, man. Unfortunately I tried everything above and the problem persists.

Anyone know how to contact Marshall in the USA?? I did register my amp online as soon as I bought it, so it definitely should be under warranty. Unfortunately, none of the paperwork lists a number to call for warranty issues, and I also couldn't find anything online. Just a number for Marshall in England.

They sure don't seem to want to make it easy to talk to you about warranty issues.