Amp making odd nosie...

  • Thread starter Thread starter TelePaul
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TelePaul

TelePaul

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My Marshall AVT is making a weird noise...it sounds like something is loose inside it, and for whatever reason, when I play a 'C' note I get this sound like two glasses hitting off ecah other. I dunno what it is...it's not a big deal but it's a little annoying. And I don't wanna open the thing and shock myself to bits.
 
It could be the 12ax7 is microphonic. Try replacing it.
 
Farview said:
It could be the 12ax7 is microphonic. Try replacing it.
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Forgive my ignorance but isn't that the tube? And dont these amps store electricity for ages after they're unplugged? Is it safe to open it up?
 
I'm not sure how the AVT lets you access the tube but there should be a panel or little door that lets you simply change the tube.
If not .... then it's still ok to open up but don't stick your fingers into any capacitors which are the specific part that'll zap you.
An AVT probably doesn't have any very big caps anyway although I couldn't swear to that.
 
I might open it up lata and take a peak. Would it be possible to open the back and switch on as normal to see if I can identify a component moving?
 
well, it's not neccessarily the tube ..... it could be a mechanical noise of something rattling against something else. If the amps very old though, I'd check that tube out.

And you can certainly open it up and turn it on (make sure to have the speaker connected!!!) ...... but I doubt very seriously that you'll actually see anything moving ..... sound vibrations don't usually make components move a large enough amount to be visible.
I don't know your level of expertise so I'm a little reluctant to suggest you go poking around in there but what I would do if it were mine would be to open it up, hook my keyboard up to it so it'd play that note continuously, and then use a non-conductive plastic rod or something similar and poke the various things I saw touching each other to see if the noise stopped. It's not impossible that simply moving two components apart would stop the noise. It could be components rattling against each other ..... it could even be loose screws on the speaker or speaker baffle ..... a loose handle ...... all sorts of things could make a rattling noise and they can be hard to track down since the noise might sound like it's coming from somewhere else.

Your caps are gonna be those cylindrical can shaped components. On that amp they might not be very big although on actual full fledged tube amps they can be quite large. But they're the things you don't want to short out with your fingers ... (*ouch!!*)
 
Tell you what, I'm gonna eat a sandwich. Then i'm gonna move them amp into a more neautral room without so much glass or a wooden floor, just in case it's something else vibrating. If it's still humming, I'll open it up and look for obvious stuff out of place. The amps not even two years old yet.
 
How did you knwo it was a 12ax7? I mean, do differnt tubes make different sounds when thy're microphonic?

I think that if you have your chasis opened so you could see the tubes, turn it on and play your magic C, you could see which tube is grumbling because of the light show that will ensue.
 
cephus said:
How did you knwo it was a 12ax7? I mean, do differnt tubes make different sounds when thy're microphonic?

I think that if you have your chasis opened so you could see the tubes, turn it on and play your magic C, you could see which tube is grumbling because of the light show that will ensue.
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I think it only has one valve....it's like a 'hybrid' amp.
 
right ..... it just has a preamp tube to get a little tube sound .... and that's almost always a 12ax7.

And it absolutely could be something in the room ..... so moving it to another room is a good idea. It's amazing how rattles can sound like they're coming from somewhere else.
 
Lt. Bob said:
right ..... it just has a preamp tube to get a little tube sound .... and that's almost always a 12ax7.
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Yeah i checked the manual...it also has an EEC name I thinks.
 
Had a thought guys...I can't hear the sound when i listen through the headphone out...does that soudn more like it's coming from the speaker?
 
Have you tried tigthening all the screws in the cabinet and speakers yet? If you use it a lot then that might be something you might want to do every once in a while. They don't really tighten stuff up that snuggly when it manufacturerd and after it settles in a bit nuts and screws will naturally loosen up some.
The fact that it does this on a particular note makes me think that is all it is it will usually do it on the next octave as well.

But if you need to probe inside I'll call 911 for you if we don't hear back from you before the day is done :D
 
TelePaul said:
Had a thought guys...I can't hear the sound when i listen through the headphone out...does that soudn more like it's coming from the speaker?
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the likely hood of a physical rattling just increased exponetially... but i would still try tapping the tube lightly to rule it out totally...
 
TelePaul said:
Forgive my ignorance but isn't that the tube? And dont these amps store electricity for ages after they're unplugged? Is it safe to open it up?
Click to expand...
That is a tube. Tubes are normally changed, they don't last forever. Are you sure the tube in inside where you have to take the whole thing apart?

Just make sure it's unplugged and don't touch anything but the tube and the chassis. You will be fine.
 
TelePaul said:
Had a thought guys...I can't hear the sound when i listen through the headphone out...does that soudn more like it's coming from the speaker?
Click to expand...
Yup, could be the cabinet.
 
The tube is probably held in place by a tube restraint, which is a little wire hook that prevents it from slipping out.

The noise is more than likely the wire rattling on the glass tube.
 
Thanks everyone. I opened the top up and played a big A5. I found something that was certainly contributing to the noise, though I don't know if it's because the screws were loose:

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I went around everything I could see with an electric screwdriver and tightened it all up. There's a small noise now but it was nothing compared to what it was. EZs suggestion seems a liekly one.
 
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