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  • Mesa F-50 Clean Channel Hum

    Hi,

    A friend of mine bought an used F-50 and asked for retubing. After changing all tubes besides the well known hiss issue in these amps I also noticed that the clean channel has a pronounced hum (the more you turn the gain up the more hum you get). I narrowed down the problem to the input stage V2B. When grid (pin 7) is grounded the problem disappears. When the 1M5/180p combo is shorted the hum is almost completely gone. Tried different tubes in V2 position without any improvement.
    I was also suspecting bad filtering of the relay supply but adding more filtering didn't help. Drive channel doesn't have the hum.
    So is this 1M5/180p hum a "built in" issue in these amps or I'm missing something? Any ideas would be appreciated.

    F 50.pdf

  • #2
    Clean channel does not have hum originally but it does have more hiss than the lead channel with medium gain.
    Sometimes some Russian 12AX7 generate an abnormal hum. If you have equipped it with 12ax7 LPS, EH, Tung Sol... in the second socket, try a Chinese or JJ standard. Just to discard it.

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    • #3
      V1 and V2 have rectified heaters so I guess tube type shouldn't be a problem but will try.

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      • #4
        Tried different tubes without improvement. Took a scope from DC heaters' rectifier and got this (all diodes check OK):

        Click image for larger version

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        • #5
          Originally posted by GainFreak View Post
          Hi,

          A friend of mine bought an used F-50 and asked for retubing. After changing all tubes besides the well known hiss issue in these amps I also noticed that the clean channel has a pronounced hum (the more you turn the gain up the more hum you get). I narrowed down the problem to the input stage V2B. When grid (pin 7) is grounded the problem disappears. When the 1M5/180p combo is shorted the hum is almost completely gone. Tried different tubes in V2 position without any improvement.
          I was also suspecting bad filtering of the relay supply but adding more filtering didn't help. Drive channel doesn't have the hum.
          So is this 1M5/180p hum a "built in" issue in these amps or I'm missing something? Any ideas would be appreciated.

          [ATTACH]55966[/ATTACH]
          1meg in series with a grid....bad idea
          i guess in boost mode it becomes a feedback,so less noise.
          sometimes there is stray capacitance that couples rectifier buzz through the filament in RF,sometimes its a microphonic cap,that 180p could be the culprit.
          what if you play through a good buffer in front?

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          • #6
            The 180p cap is a blue kV type of cap. Drive channel doesn't have his problem - only hiss there but that;s normal. Clean channel is bearable with all controls at mid position (max).

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