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Marshall JMD problem

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  • #16
    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.

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    • #17
      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?


      Sent from my iPhone using Tapatalk

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      • #18
        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 itís not necessary. Iíve tried both and canít tell any difference so...

        Also...I didnít 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. Thatís asking for problems.

        Sorry I didnít know more about this particular amp. But at least somebody that did was able to help you figure it out.

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        • #19
          Re: Marshall JMD problem

          I havenít biases the tubes yet because I canít find anywhere on the web where I should put the probes or how to do it. There doesnít seem to be a biasing manual anywhere...

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


          Sent from my iPhone using Tapatalk

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          • #20
            Re: Marshall JMD problem

            In that case I would recommend not playing the amp until itís 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.
            Iíll 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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            • #21
              Re: Marshall JMD problem

              Yeah I have a multi meter and Iíll look into YouTube as well but I havenít had the time yet


              Sent from my iPhone using Tapatalk

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              • #22
                Re: Marshall JMD problem

                Biasing the JMD is not too difficult. The bias ports are on the board. You will need to pull the chassis. Locate CON-6. There are 3 pins sticking up. The middle pin is the ground. The two outboard pins is where you measure millivolts with the meter. Next to CON6 are two mini pots that are used to adjust the bias for each pair of tubes. Adjust the associated pot until you get 76 per side with your meter set to mv DC.

                Just follow the normal safety precautions of not touching anything in the amp with your bare hand, and use one hand at a time, and an insulated mini screw driver to adjust the pots...and so forth. Be careful that the probes of your meter do not touch the wrong pins. In the case of the JMD you do not want any static electricity jumping from you to the amp. Get rid of any static electrons first by zapping your self of any static electricity by touching anything to ground.

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                • #23
                  Re: Marshall JMD problem

                  Originally posted by Lake Placid Blues View Post
                  Biasing the JMD is not too difficult. The bias ports are on the board. You will need to pull the chassis. Locate CON-6. There are 3 pins sticking up. The middle pin is the ground. The two outboard pins is where you measure millivolts with the meter. Next to CON6 are two mini pots that are used to adjust the bias for each pair of tubes. Adjust the associated pot until you get 76 per side with your meter set to mv DC.

                  Just follow the normal safety precautions of not touching anything in the amp with your bare hand, and use one hand at a time, and an insulated mini screw driver to adjust the pots...and so forth. Be careful that the probes of your meter do not touch the wrong pins. In the case of the JMD you do not want any static electricity jumping from you to the amp. Get rid of any static electrons first by zapping your self of any static electricity by touching anything to ground.
                  Thank you so much!

                  You were right, it's not too difficult though it was difficult to get it perfecly on 76 each but after a couple of tries i got it!
                  Last edited by arten; 04-06-2019, 09:52 AM.

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                  • #24
                    Re: Marshall JMD problem

                    today I was playing my jmd through my hx fx in 4cm. From some patches I did get no sound but that seemed to be a helix problem. I downgraded the hx and turned the amp back on.
                    It made a louder and louder buzzing or humming sound, than a big POP and now it doesn't make a sound any more.

                    Any idea's what happened and where I should look?

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                    • #25
                      Re: Marshall JMD problem

                      Check the fuses.

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                      • #26
                        Re: Marshall JMD problem

                        I checked the fuse and it's intact.

                        I also noticed that one of the powervalves which I replaced two weeks ago seems "burned". It's the same valve that seemed damaged on the original ones.

                        I can turn the amp on and it starts humming a little, but switching it from standby to powered doesn't do anything.

                        I've added a picture of the tubes below. It's hard to take a good picture because of the reflection, but there seems like a burn stain on the glass and on the plate at the top of the valve

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                        Last edited by arten; 04-28-2019, 08:49 AM.

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                        • #27
                          Re: Marshall JMD problem

                          The fuse can look good but still be bad. Check it with a meter.

                          You can check voltages at the tube socket without tubes installed and the amp off standby. Do this and see if there is anything anomalous with any the pin sockets at any of the sockets.

                          The slight hum when you first switch on the mains is normal.

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                          • #28
                            Re: Marshall JMD problem

                            I just tried the amp with the headphone and direct out and that works as it should, so the problem is definitely with the power amp section, so I think I can rule out the fuse no? without the fuse the amp wouldn't do anything correct?

                            To check the voltages:
                            - what do you mean with 'the amp off standy"?
                            - what should the normal read out be for every pin

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                            • #29
                              Re: Marshall JMD problem

                              It appears that the tube in the same socket as before may have red plated causing the fuse to blow. It could be coincidence or it could be an ongoing problem. Check the voltage at pin 5 in that socket vs the others. You will need to replace the fuse first. Also make sure you are using slow blow fuses. If it is a fast blow fuse you will keep blowing fuses for no other reason. Pin 5 should be about negative 50 volts. Pin 3 is the plate voltage which will probably be in the high 400s. Pin 4 is the screen grid and should be about the same as the plates. These are DC so set your meter appropriately.

                              There are the mains fuse and the HT fuse. The HT fuse blows if there is a tube problem, leaving the digital preamp working but killing the tube power amp.

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