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  • Marshall JMD problem

    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?
    Last edited by arten; 03-29-2019, 09:06 AM.

  • #2
    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.

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    • #3
      Re: Marshall JMD problem

      Another way to check power tubes is to pull the two outer tubes (thatís one pair)..then if needed do the same with the two inner tubes (thatís 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 Iíd 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.

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      • #4
        Re: Marshall JMD problem

        And how do i check the power inverter?


        Sent from my iPhone using Tapatalk

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        • #5
          Marshall JMD problem

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

          Put a known, good 12ax7 in there.

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          • #6
            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.
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            • #7
              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

              Click image for larger version

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              • #8
                Re: Marshall JMD problem

                I think I would replace the tubes with a matched quad and bias properly....thatís very important.
                Then re-assess whatís going on.
                That may very well solve the issues youíre having.
                But at the very least, you need tubes.

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                • #9
                  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.

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                  • #10
                    Marshall JMD problem

                    Ok so i swapped the tubes for in el34lís.

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

                    Itís 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?


                    Sent from my iPhone using Tapatalk
                    Last edited by arten; 04-05-2019, 01:07 PM.

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                    • #11
                      Re: Marshall JMD problem

                      Did the amp have EL34 tubes to begin with or was it modded to use different tubes?

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                      • #12
                        Re: Marshall JMD problem

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


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                        • #13
                          Marshall JMD problem

                          Gotcha...the way you worded it, it seemed like you changed tube type.
                          Last edited by Gtrjunior; 04-05-2019, 02:14 PM.

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                          • #14
                            Re: Marshall JMD problem

                            It has no preamp tubes. The preamp is digital modelling. Thereís 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 itís way too much


                            Sent from my iPhone using Tapatalk

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                            • #15
                              Re: Marshall JMD problem

                              Only one way to find out.

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