Marshall JMD problem

A

arten

New member
I recently bought a Marshall JMD 100 watt combo and it has 2 problems:

- It makes a loud hissing sound that get's louder by each preamp you select (from clean to high gain), which makes sense, but it really is a too loud hiss
- It makes another noise that I would compare with when you would be filming with a lot of wind. This one comes and goes and is not regular but almost always there.

When I use the headphones or the XLR out there is no noise so that would lead me to suspect the power tubes?

Am I right in suspecting the power tubes or could it also be something else?
Could I test this by pulling tubes one by one to find the faulty power tube or is there something else going on?
 
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Re: Marshall JMD problem

That would seem to indicate that the problem is in the power amp. It could be tubes, including the phase inverter, but could be other things, such as the power supply to the power amp, a leaky cap, and so forth. Let's hope it is a tube. Try the PI first before replacing all four power tubes.

You can't pull power tubes one at time, because they work in pairs as a team. If you have a spare EL34 that works you could probably swap it in in place of one the stock tubes one at a time to see if it is one them being faulty.
 
Re: Marshall JMD problem

Another way to check power tubes is to pull the two outer tubes (that’s one pair)..then if needed do the same with the two inner tubes (that’s a pair).
You can at least narrow it down to one of two tubes that way.
Then as LPB suggested, test with a known good power tube.

Just a guess but the wooshing sound to me indicates filter caps needing to be replaced. They are inexpensive enough, but I’d suggest letting a tech replace them if needed. They can carry enough voltage to kill you if handled improperly.

And as suggested....check the PI first.
 
Marshall JMD problem

The phase inverter.
It’s the 12ax7 tube that is typically furthest away from the input jack.

Put a known, good 12ax7 in there.
 
Re: Marshall JMD problem

Does it change in intensity if you play around with your tone controls. If it does that would suggest it's in the P.I./ Power amp area.
 
Re: Marshall JMD problem

Sorry had a couple of busy days.

Some more observations:
- if i put the amp on, I here some sort of irregular "ticking" sound, like ticking against glass
- stupid that I didn't check before, but one of the tubes is not glowing, as you can see in the image
- it seems like the tube most on the right is glowing harder than the ones in the middle
- I'm not getting the "wind in mic blowing" sound anymore, but the noise level is really high and increases more when I select a higher gain digital preamp
- from time to time, I get another buzzing sound, sounds a bit like a really silent micro wave oven humm
- the controls do not affect this noise, the preamp volume does a little but the master volume increases it a lot when i turn it up

so as one of the power tubes is not glowing, I assume that I can be sure I need to replace them.
Do you guys think there's something else wrong or could all of the above be caused by this?

the image seems to be upside down, so the tube on the "right" which is not glowing is actually on the left :)

IMG_0214.jpg
 
Re: Marshall JMD problem

I think I would replace the tubes with a matched quad and bias properly....that’s very important.
Then re-assess what’s going on.
That may very well solve the issues you’re having.
But at the very least, you need tubes.
 
Re: Marshall JMD problem

The bias is fairly easy to set on these, but if your a newb to this sort of thing have somebody experienced do it or show you how to do it safely.
 
Marshall JMD problem

Ok so i swapped the tubes for in el34l’s.

The random noise seems gone but still incredibly high noise that increases with each gainier amp model from the preamp.

It’s not a buzz or a hum but a ccchhhhhhhh type of sound.

If I open up the gain on a higher gain preamp model the amp starts squeeeling with feedback.

Any thoughts?


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Re: Marshall JMD problem

No it has el34 tubes standard. I just took the original Marshall branded ones out


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Marshall JMD problem

Gotcha...the way you worded it, it seemed like you changed tube type.
 
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Re: Marshall JMD problem

It has no preamp tubes. The preamp is digital modelling. There’s only one PI tube.

But would that affect the noise? I know every amp has noise and that it goes up the more gain you have but it seems to me it’s way too much


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Re: Marshall JMD problem

A ssshh type tone in the back ground is normal with increasing gain. Squealing feedback on models 14-16 with high gain settings is to be expected unless your guitar is quite a way away from the speakers. Model 16 is the JVMs high gain channel set to the red gain level. Models 1-4 should be almost noise free though.

Did you engage the noise gate? The noise gate activates by turning the modulation knob above 0. Then you can set the parameters of the noise gate by using mod depth to just shut down the noise without affecting the sustain.

You can down load the JMD manual if you don't have one.

The PI tube might be contributing noise.

Where is the bias set with the new tubes?

Here's some setting to try:

Clean , model 1, gain 8, bass 6.5, mid 7.5, Treble 7, channel vol 9, Reverb 3.

JCM 800 crunch, Model 6, gain 9, bass 9, mid 4, treble 2, channel vol 6, reverb 0

Modded 800 overdrive, Model 10, gain 8, Bass 7.5, mid 5.5, treble 2.5, channel vol 5, reverb 0

Boosted plexi lead, Model 13, gain 6, bass 6.5, mid 4, treble 5, channel volume 4 reverb 0, delay a smidge analog.

Make sure you turn on the FX loop even if you don't use it. Set it to about 2/3 on the knob in the back. This makes the amp 100% tube sounding.
 
Re: Marshall JMD problem

The noise gate does make it a lot more quiet.
Still want to replace the PI tube though because that must be at least 5 years old as well, maybe even 10 as it is still the original.

Would the Groove Tubes GT-12AX7 be suitable for this because that seems the only 12ax7 my local shop seems to have. Or better to order another brand online?


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Re: Marshall JMD problem

The GT will be fine.
Depending on what you read online, some people will say you need a balanced tube for the PI, some will say it’s not necessary. I’ve tried both and can’t tell any difference so...

Also...I didn’t see that you responded to the question about whether you biased the new tubes....VERY important that they are biased correctly. You cant just slap a new set of tubes in without biasing. That’s asking for problems.

Sorry I didn’t know more about this particular amp. But at least somebody that did was able to help you figure it out.
 
Re: Marshall JMD problem

I haven’t biases the tubes yet because I can’t find anywhere on the web where I should put the probes or how to do it. There doesn’t seem to be a biasing manual anywhere...

And I know there are risks involved so I’m not doing anything before I’m 100% sure how to do it


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Re: Marshall JMD problem

In that case I would recommend not playing the amp until it’s been biased properly.

There are lots of YouTube vids on how to do it. Not sure if any of them show your amp specifically though.
I’ll take a quick look-see in a little while.
I bought a bias probe from Eurotubes for about $100...well worth the investment imo. They sell cheaper ones too, or a simple multimeter works too.
 
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