Marshall JCM 2000 is making strange noises

[Securb]

I have had this problem with my Marshall JCM 2000 for a while. It happens intermittently so it has been ongoing.

The issue only happens when playing the amp at high volumes for a long time, so I did not notice the case playing at home. The amp will start to sound like a speaker is blown. This only affects frequencies below 440 hz. If I hit an A chord, the amp sounds great anything below that gives the blown speaker sound.

I took the cab apart and tested all of the speakers. I also glued a loose support I thought could contribute to cab noise. The cab sounds great with my JC120 and my Carvin. No issues with the cab or speakers.

I am going to drag the amp home today from the practice studio to check it out. My initial thought is a tube has gone microphonic or bias drift. There is a set of Mullards in the amp right now that have been in there for around 7 years that get worked hard.

I am going to check the tubes and bias and might throw in a set of GTs I have sitting around. Is there anything else I should check out? Has anyone else run into this problem?

 

[devastone]

How old is it? If changing tubes doesn't fix it, the filter caps might be the next thing to look at.

 

[Securb]

It is 20 years old. I was thinking caps might be my next move.

 

[NegativeEase]

If tubes are good and nothing rattling resonantly -I would change caps.

 

[JMP/HBE]

90% of tube amp problems are tube related.
If you have not changed tubes in 20 years thats a good place to start.

 

[Securb]

90% of tube amp problems are tube related.
If you have not changed tubes in 20 years thats a good place to start.

They are about 7 years old. But this is where I am going to start.

 

[NegativeEase]

Forgot to mention ONE critical thing B.

Move the amp to another place in the room facing a corner and make sure it isn't the placement in the room creating a big modal buildup

Ive never seen that cover 5 semi tones like you describe -but I have seen 2 or 3

 

[Securb]

Forgot to mention ONE critical thing B.

Move the amp to another place in the room facing a corner and make sure it isn't the placement in the room creating a big modal buildup

Ive never seen that cover 5 semi tones like you describe -but I have seen 2 or 3

This is the thing. The last time this happened, I brought the head home, plugged it into my 2x12, and I had no problems with it for almost a year. My JC is in the shop so I brought my Marshall back to the practice space around two weeks ago. The problem just reared its ugly head again. We have sound dampening and rugs all throughout the room, so I don't think it is modal buildup. Plus, once it starts happening, it continues even when I back off my volume pedal.

Weird enough it happens right when I hit Ab on the E string or lower. a fretted A on the E string or open A5 or higher and the sound goes away. I slide up a single fret and the sound is gone, I have never seen anything like it.

 

[devastone]

The fact that it is lower notes and and only seems to happen at higher volumes is what makes me suspect filter caps, the low notes and high volumes pulls harder on the power supply. Assuming you rule out tubes and rattles first of course.

 

[ErikH]

Something else to check is the voltage reading of the power at the rehearsal space. You could be dealing with dirty power or fluctuating voltage vs. at home. See if this happens at home again before changing tubes and if It doesn’t, get those outlets checked.

 

[Securb]

Something else to check is the voltage reading of the power at the rehearsal space. You could be dealing with dirty power or fluctuating voltage vs. at home. See if this happens at home again before changing tubes and if It doesn’t, get those outlets checked.

We do have to check this out because my bass player is also having mystery problems. I might put a voltage meter on the outlet next rehearsal and monitor the voltage as we play.

 

[Securb]

I picked up one of these voltage meters so I can monitor the juice as different gear is being used and the heating is kicking in. It was $9 on Amazon

 

[chadd]

I had a similar issue with my Mesa Road King. The fix ended up being to replace some resistors.

 

[Demanic]

I picked up one of these voltage meters so I can monitor the juice as different gear is being used and the heating is kicking in. It was $9 on Amazon

$9? I'm getting one.

Sent from my SM-A115A using Tapatalk

 

[JMP/HBE]

2 Prong has no ground >

 

[Securb]

2 Prong has no ground >

Why do you need a ground to check voltage?

 

[devastone]

You don't.

 

[Securb]

You don't.

I know, it was a rethorical question

 

[JMP/HBE]

 

[devastone]

You don't need the ground plug to measure the voltage, to plug the amp into, yes you do.