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JCM 900-4100 head Red Plating Issue

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  • #1

    JCM 900-4100 head Red Plating Issue

    Hello !

    The case is, I have a jcm 900 4100 head with Red Plated in all the power tubes. This is the history about what happened with my main amp ( this jcm 900 head 4100).

    In a rehearsal with my band, 10 months ago something happened with electricity, maybe a surge and it damaged some devices in the house. I was playing when this happened and 20-30 seconds after the incident, the main fuse in the jcm 900 was blown ( 4amp) replaced twice and always blew the fuse.I made some measures in the power tranny and it does not deliver the right voltaje. So, I ordered a new power transformer for classictone and after 6 months it arrived to my country ( this took a long time because some friend did the favor to carry this to my country by sea)

    Last weekend I had the time to change the transformer and voila! It does not blow fuse , but the sound is good for 10 minutes and after that all the power tubes went to red plating and the sound goes down and shows some grid or little distortion in the trail of a sound , for example, if you play a chord , it vanishes with some distortion, only when is red plated. I played with the old 5881 and red plated like hell but after half out of the test, the tubes fuses 500ma were blown! Before it I checked the voltage in the union of r27-r26, according to the schematic, it must be -50 volts but I always obtain milliVolts dc readings.

    Yesterday I replaced the tubes with 6l6 and new fuses and it sounded but after 4 minutes the same behavior! Diminishing volume, all 4 tubes red plated and grainy trail in sound and check bias voltage and my dmm only show - 30 milliVolts dc. approx. I checked my Digital Multimeter and it seems to be ok.

    Do you have any suggestions to fix my amp? I need some directions or advice because I don’t know what’s next to do!

    I have some electronics skills because I used to work with electronics when I was in the university, but after that I studied Computer Science, Right now I am unemployed ​. I really want to have local tube amp technicians here in my county but there`s nobody here with education or demostrated skill reapairing tubes amp, so I always service my amps, little things like change broken components, some basic tests, biasing, change tubes. Im guess I have fundamental knowledge, so I know some little things but I am not an expert.

    Attached you will find some pictures and schematics and specs from power transformer, and JCM 900 4100 head

    Please Help me!

    Attached Files
    Tags: None
  • #2
    Run the amp without the power tubes until you can get the -50V bias voltage at to hold steady.
    This will prevent you from damaging more power tubes and blowing the 500mA cathode fuses.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​
    • Likes 2

    Comment

    • #3
      As g1 suggests, you almost certainly have a bias voltage issue, so pull the tubes and focus on that. Some marshalls will show signs of arcing around pin five of a power tube, but if as you say all four are red plating, the problem has to be upstream and common to all four. The bias circuit is pretty basic. What is your range of bias voltage coming off the adjustment pot? A little spritz of DeOxit in that pot wouldn't be a bad idea.

      I would have a look at C13 and C14, both 10uF/100v caps. If one is leaky it could pull down your bias voltage.

      https://www.thetubestore.com/lib/the...-Schematic.pdf
      Last edited by Randall; 09-06-2020, 04:18 AM.
      It's weird, because it WAS working fine.....

      Comment

      • #4
        Hello!

        I have checked and r27, r28 and r29 are fine, I followed the same trace and took C12 and according to the multimeter it is fine (48.5nF capacitance), slightly out of its value but I think acceptable. The one that is shorted is the D1 diode, I already took it out and on both sides it marks 0.065 volts, so it is in short. What else could generate the short D1 fault? Could it have damaged some other component?

        There is also something that has caught my attention and it is the area where the voltage input is, as it seems an arrangement of diodes D6 and D7 for filtering to ground, they are also in parallel with r36 of 22ohms and C1 ... the fact is that all that according to my meter are connected to ground, but I took D7 out to test it and it's fine, it's not open or short.

        Could it be that only D1 is the one with the problem? Where else do I have to look?
        Another thing that I measured was the resistance at the speaker output, I put the test leads between the positive and negative of the jack and it marked that it was shorted,I mean, the multimeter gives me continuity. Is this correct?

        Comment

        • #5
          Originally posted by g1 View Post
          Run the amp without the power tubes until you can get the -50V bias voltage at to hold steady.
          This will prevent you from damaging more power tubes and blowing the 500mA cathode fuses.
          ok, I will check that, but first I will replace D1 and will put D7 and C12 back again in the board in order to do your suggestion. Thank you all!

          Comment

          • #6
            Originally posted by Randall View Post
            As g1 suggests, you almost certainly have a bias voltage issue, so pull the tubes and focus on that. Some marshalls will show signs of arcing around pin five of a power tube, but if as you say all four are red plating, the problem has to be upstream and common to all four. The bias circuit is pretty basic. What is your range of bias voltage coming off the adjustment pot? A little spritz of DeOxit in that pot wouldn't be a bad idea.

            I would have a look at C13 and C14, both 10uF/100v caps. If one is leaky it could pull down your bias voltage.

            https://www.thetubestore.com/lib/the...-Schematic.pdf
            C13 and C14 are not 10uF...Do you mean C11 and C16?

            Comment

            • #7
              The JCM900 serie amps was produced in two versions JCM900 with EL34 tubes and JCM900 with 5881 tubes.
              Check what is your version of the JCM900.
              If in the JCM900 with EL34 tubes instead of EL34 installed 5881, 5881 will glow due to lower bias.

              https://drtube.com/library/schematics/69-marshall-schemas#JCM900
              It's All Over Now

              Comment

              • #8
                Version is 4100 with 5881. This is indicated by the red nuts on the speaker jacks and the screen grid resistors (470R).
                Be careful with capacitor C12. It could have a problem.
                If I remember correctly on pin 5 of the power tubes they appear around -80V in standby mode and when deactivating it that voltage gradually drops to approximately -50V.
                Regardless of this it is a good time to attend to the rear plate and supervise the solders at the output jacks, impedance selector, pentode / triode selector, screen grid resistors themselves and send / return jacks. It is relatively common to find poor or degraded solders in some of these places. Also in the solders of the orange and green wires (both sides) that lead the input signal to the power tubes.
                • Likes 1

                Comment

                • #9
                  "C13 and C14 are not 10uF...Do you mean C11 and C16?"

                  Ok, yes if we are talking about the 5881 version. Same caps, different designations. In other words the two electrolytics in the bias supply circuit.
                  It's weird, because it WAS working fine.....

                  Comment

                  • #10
                    If D1 shorted, the resulting AC will have stressed C11 & especially C16. I would suggest replacing them both in addition to D1.
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    ​

                    Comment

                    • #11
                      https://drtube.com/schematics/marshall/4100-60-02-iss14.pdf

                      Extract all output tubes from tube sockets.
                      Connect the amplifier to the AC main voltage.
                      Turn on the Main and Standby switch.
                      With the voltmeter periodically (for 30 minutes) monitors whether the voltages at C11 (10u/ 100V) and at the 50u/ 500V capacitor between R30 (10k) and R35 (100) are stable or changing (decreasing) .
                      A change (decreasing) bias voltage indicates trouble with C11, C16, D1.

                      1)
                      https://www.guitarristas.info/foros/jcm-900-4100-head-problemas-red-plating/317990
                      Last edited by vintagekiki; 09-06-2020, 10:48 PM. Reason: 1)
                      It's All Over Now

                      Comment

                      • #12
                        Originally posted by Randall View Post
                        "C13 and C14 are not 10uF...Do you mean C11 and C16?"

                        In other words the two electrolytics in the bias supply circuit.
                        OK, Thank you. They will be replaced this monday, cause today the electronic store is not open

                        Comment

                        • #13
                          Originally posted by vintagekiki View Post
                          https://drtube.com/schematics/marshall/4100-60-02-iss14.pdf

                          Extract all output tubes from tube sockets.
                          Connect the amplifier to the AC main voltage.
                          Turn on the Main and Standby switch.
                          With the voltmeter periodically (for 30 minutes) monitors whether the voltages at C11 (10u/ 100V) and at the 50u/ 500V capacitor between R30 (10k) and R35 (100) are stable or changing (decreasing) .
                          A change (decreasing) bias voltage indicates trouble with C11, C16, D1.

                          1)
                          https://www.guitarristas.info/foros/jcm-900-4100-head-problemas-red-plating/317990
                          Thank you, I will do that after replaced electrolitcs caps.

                          I appreciate all the effort you have made to help me. You have even found the link to another forum in Spanish where I have also sought help. Thank you very much, I hope that together we can solve this.

                          Comment

                          • #14
                            Originally posted by vintagekiki View Post
                            A change (decreasing) bias voltage indicates trouble with C11, C16, D1.
                            Also C12 is critical.

                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            ​
                            • Likes 1

                            Comment

                            • #15
                              I also want to replace C10 and C9 , they are rated 47nF /400v ... Can I use 47nF /630v? How can affect Using a more voltage version ?

                              Comment

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