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Help - 5150 Signature Head - Red Plating Every Set of Tubes

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  • Help - 5150 Signature Head - Red Plating Every Set of Tubes

    Hey guys,

    something failed here.

    I haven't turned it on in about a year, so something changed in that time, but was not moved, slammed, or anything otherwise changed, just literally not been turned on in 365+ days.

    Automatically starts redplating v1 and v2, even at half power v3 and v4 redplate, any set of tubes.

    Fine in standby, happens when voltage is applied by flicking off standby.

    Likely this points to a problem in my bias circuit, correct?

    This probably would mean the resistor or a cap failed, something that regulates the voltage is giving way more than needed and needs to be brought down, correct?

    I understand a real answer can't be given without readings and further information, I would just like to know what this commonly points to and if a common flaw in the 5150's cause this.

    Where do I start troubleshooting a redplating issue? (Since most work i've done have always involved reading and working w/ the power tubes in)

    Thanks for any insight!

  • #2
    Can you post a schematic so we can see how tour power amp and standby circuits work? Then everyone can follow along and learn together!

    But yes, a failure of some part in your bias supply would be my first guess. What's your bias voltage with no tubes in?

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    • #3
      You only need to remove the power tubes, check at pin5 of power tube sockets for the bias, should probably be somewhere around -45 to -50VDC.
      I think this should be the schematic:
      Attached Files
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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      • #4
        Originally posted by g1 View Post
        You only need to remove the power tubes, check at pin5 of power tube sockets for the bias, should probably be somewhere around -45 to -50VDC.
        I think this should be the schematic:
        Thanks for the response guys.

        I'll check pin 5 to ground and report back. I've always heard mixed thoughts on running without power tubes depending on the head, I know for builds you should troubleshoot without tubes first, not sure why it'd be any different for a 5150 but it still makes me hesitant as to not cause more damage lol

        Will report back when possible, thanks!

        Comment


        • #5
          The only thing that happens if you power up an amp without output tubes is the high voltage will rise a little bit but will still be under the voltage rating of the filter caps.

          Never run a tube amp without a speaker or load connected when output tubes are installed.
          Without output tubes it's okay.

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