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PT hum in Orange OR 80

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  • #1

    PT hum in Orange OR 80

    Picked up a 1974 OR 80 on the cheap. It sounds great, only there is a loud hum coming from the PT, which is amplified when the amp is in play mode.

    In addition, the new powers tube have a blue glow inside the plate structure. I'm not sure what this indicates. At high levels one tubes glow orange on the inside, but not on the plate. The bias is set correctly.

    I'm not sure if the problems are interrelated, and I've never encounted any of them before. Any advise on diagnosing the amp would be awesome.

    Thanks.
    Tags: None
  • #2
    This in not a picture of the tubes in the Orange, however this glow is nearly identical to what I'm seeing. The person who posted this picture noted that it looked like it was coming from the screen...

    http://www.knizefamily.net/russ/blog...ow-249x300.jpg

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    • #3
      This is normal, the blue glow is caused by residual gases in the tube which get excited by the electrons. Only to worry is if the whole tube shines bright blueish or pink, like a Neon bulb as then it has lost its vacuum and will fail shortly.
      For the Hum : Could be caused by tired Caps in the power supply filter section. depending on the age of this amp it might time for a cap job, and if its a 74 then it is time!
      I can fix everything, where is the duct tape?

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      • #4
        I have read much about blue glow, but can't find info about the particular glow in that picture, which is inside the plate structure. Still, I understand, that it may be normal.

        The amp had a cap job before I picked it up. I am concerned about the mechanical buzzing coming from the PT, which is in turn amplified.

        Thanks.

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        • #5
          If the buzz is mechanical, have you tried tightening all of the mounting and assembly hardware?

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          • #6
            Yes, and it all seems really tight. I read on a hi-fi forum that sometimes the windings come loose from the core on old transformers, and a wooden shim can be placed in between to re-tension them. But wow, that sounds crazy!

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            • #7
              On some amps I had the PT interferencing into the OT so a slight hum could be heard out of the speakers even with Stdby on. Fix for that was grounding the PT properly, e.g. removing the laquer from the mountings and the chassis to ensure the iron has no resistance to GND. If still annoyed by it you can put a small Mµ metal shield between the trannies...
              I can fix everything, where is the duct tape?

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              • #8
                Would you happen to have a schematic for this amp that you can post?

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                • #9
                  Sure:

                  http://www.schematicheaven.com/newam...e_mkii1974.pdf

                  Thanks.

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