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Red Bear MKX50

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  • #1

    Red Bear MKX50

    I have one of these combos in front of me. It has had previous repairs. Notably, the PT was replaced with a Weber W022798, specs attached, as it started it's life w a 220V PT.
    The Weber is 720V centertapped.

    Apparently it blew some diodes upon installation(a year ago), and the tech then replaced some burnt diodes. This is what the customer tells me. I assume he grounded the neg of the bridge AND the PT CT, but the fact that it didn't happen immediately is troublesome.
    So as it's been now for a year, it's wired as a bridge rectifier, but with the neg of the bridge NOT grounded, and the CT of the PT grounded.

    That should be fine from my understanding, but my question is, is that the best way to wire the transformer? How does that differ from the same grounded CT arrangement, but with just diodes in series?

    I'm seeing the amp for the first time, and while it's not happening to me, apparently it'll sound great then "sound thin and half power". He's had every power tube known to work in here, and the amp has been rewired from the original russian 6n2pb to 12ax7 and now back again! He's swapped tubes to death as well, he tells me.

    He said the amp was always fine and never had this issue before the new PT install.....Currently there's JJ 5881s in here with 510VDC on the plates and screens. I'm starting to think it's a choke issue as a best guess at the moment. All wiring is solid...

    Click image for larger version Name: w022798sch.jpg Views: 167 Size: 131.5 KB ID: 985903
    redbear_cubcombo_mkx50.pdf
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  • #2
    What are actual voltages at output tubes (all pins). Which transformer taps are you using (primary and secondary)? I'm not sure why the previous tech wouldn't have grounded the bridge -?
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Well, I just rewired it to use the lower voltage taps, and now have a reasonable 470V on the plates with redwhite/redwhite secondaries.
      Previously plates were 510 with red/red, screens 510, -55V or so on the grid(there are no grid resistors on this amp. I think I will install some.
      Brown and black 120V are connected on the primary


      Well, the tech grounded the centertap(as it is now) so therefore the neg of the bridge shouldn't be grounded, if I'm understanding things correctly, otherwise things would go poof? If he did ground the bridge neg but not the CT, would it then be a voltage doubler?

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      • #4
        I found a link to the Red Bear MKX50 schematic - https://schematicheaven.net/newamps/...ombo_mkx50.pdf

        LarBal.... does that look right?
        It's not just an amp, it's an adventure!
        • Likes 1

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        • #5
          Originally posted by TomCarlos View Post
          I found a link to the Red Bear MKX50 schematic - https://schematicheaven.net/newamps/...ombo_mkx50.pdf

          LarBal.... does that look right?
          Yessir, that's the one

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          • #6
            The original circuit uses a bridge rectifier across the full HT winding.
            The CT is only used to balance the reservoir caps and sits on B+/2.
            Full HT winding voltage should be around 330V (meaning a 165V - CT - 165V secondary).

            The replacement PT has twice the HT voltage of the original, so it is wired for a full-wave/2-phase rectifier using only 2 diodes of the bridge (bridge negative not connected so the left side diodes are disengaged)
            Here the CT must be grounded.

            A major difference is that with the new PT wiring the 2 active diodes have to withstand twice the peak inverse voltage (PIV) compared to original.
            The 1000V PIV of a 1N4007 might no longer be sufficient.
            A solution could be rewiring the 4 diodes so that 2 diodes are wired in series for each side..
            - Own Opinions Only -
            • Likes 4

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            • #7
              With a 2x330V secondary the PIV is 933V. Allow for some transformer regulation and mains voltage fluctuation and the 1000V limit is easily exceeded.
              - Own Opinions Only -

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              • #8
                Originally posted by Helmholtz View Post
                The original circuit uses a bridge rectifier across the full HT winding.
                The CT is only used to balance the reservoir caps and sits on B+/2.
                Full HT winding voltage should be around 330V (meaning a 165V - CT - 165V secondary).

                The replacement PT has twice the HT voltage of the original, so it is wired for a full-wave/2-phase rectifier using only 2 diodes of the bridge (bridge negative not connected so the left side diodes are disengaged)
                Here the CT must be grounded.

                A major difference is that with the new PT wiring the 2 active diodes have to withstand twice the peak inverse voltage (PIV) compared to original.
                The 1000V PIV of a 1N4007 might no longer be sufficient.
                A solution could be rewiring the 4 diodes so that 2 diodes are wired in series for each side..
                Thank you! This is exactly the clarification I needed.

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