Mesa F-50 hum problem on stand-by

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by msk011, Oct 6, 2006.

  1. msk011

    msk011 Member

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    Mar 12, 2006
    Location:
    Waterford, MI
    Hey Folks,

    I have tried several searches on this issue with no luck.

    I’ll try and be as short as possible.
    I have a Mesa F-50 combo amp that I purchased new about 5 months ago. Everything was great for a while, and then one practice the thing started breaking up on me. I found that when I attempted to play, a small blue flame of electricity would arc from the center of one of the power tubes to the glass exterior. Called mesa, they shipped me some new power tubes which I installed. This was about 2 weeks ago, and I’ve gone to one practice which went fine but I don’t mic the amp at practice. Two nights ago I have the amp mic’ed up in my basement just messing around when I notice a buzz coming through the small PA I own. Trace the sound to the amp. With the amp turned on, in stand-by mode, all knobs at zero, nothing plugged into the amp (no guitar/efx loop) there is a low buzz coming from the back. The PA mic was just picking it up through the speaker opening I guess. It's not really that noticable when I'm not running it through a PA.

    Unfortunatley, this basically renders the amp un micable (Imagine micing a noisy transformer from a florescent light). Anything more than about 4 on the PA master and it is unbearable. Any of you run into this. It sounds like a transformer humming to me but I’m no tech. VERY annoying.
    I was wondering if the original tubes may have damaged something when they went bad?

    Any input or recommendations would be great.
    P.S. I’m in the Detroit area of Michigan, can anyone recommend a Mesa Tech around these parts. The mesa web only lists 3 in the state and they are all a ways from me.

    Thanks,

    Mike.
     
  2. brad347

    brad347 Member

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    Location:
    Brooklyn, NY
    can you feel the power transformer vibrating or otherwise acting up when you put your hand on it?

    It's possible that the arcing power tube might've put a little stress on the tranny, and if it was marginal anyway, might have damaged it. Should've been protected by the fuse but sometimes these things do happen.
     
  3. msk011

    msk011 Member

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    Location:
    Waterford, MI
    Hi Brad,

    Thanks for the response.

    I did try feeling the transformers under there with no noticeable change in the sound.

    So it's possible that a bad powertube could damage the tansformer. bummer, I wonder if that is the problem. I have an appointment to drop it off this weekend with a mesa tech, hopfully this is a quick fix.
     
  4. donnyjaguar

    donnyjaguar Member

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    Hmmm...are you sure its broken? All power transformers hum. If the laminations are not well glued (common in older trannies, btw) then they can be pretty noisy. I use a 50 year old model in my home-built combo of last year and it buzzes a little. Short of re-dipping it in goo, there's not much you can do.

    As for the hum being picked up by the mic. I'm guessing the coil in the mic is picking up the magnetic field of the transformer. There's an easy way around this and that is to use a mic with no coil. (ie condensor, carbon, crystal mic - listed in descending order of fidelity :) )
     
  5. msk011

    msk011 Member

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    72
    Joined:
    Mar 12, 2006
    Location:
    Waterford, MI
    donnyjaguar,
    Thanks for the reply! :)

    Well my thing is that this is a new problem. Prior to a few days ago I could use my sm57 and PA with no problem. As it stands now, I really can't use my PA with this amp, total bummer :( . Also, I stopped at a local GC yesterday and looked at another F-50, no hum at all and I'm sure it had been sitting there in stand by for hours.

    Thanks,

    Mike.
     

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