Cause of Ghost Notes?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by KCblues, Aug 12, 2005.


  1. KCblues

    KCblues Member

    Messages:
    217
    Joined:
    Feb 12, 2005
    I'm working on one of the new AC30CC2, and I'm getting ghost notes, they are pretty severe.

    First I thought one of the preamp tubes had gone microphonic, but I have the chassis sitting on the bench with the cab placed on a chair beside it and get the same problem.

    I've sub'd speaker cabs, that's not it. Also swapped out V1 to no avail, but have yet to try the other positions.

    Whacking around with a chopstick, the tubes don't seem noisy, but there is ceramic cap near V1 that generates a lot of noise when I hit it. Could that be the problem?

    Thanks,
    KC
     
  2. TieDyedDevil

    TieDyedDevil Member

    Messages:
    2,453
    Joined:
    Apr 10, 2003
    Location:
    Portland, OR
    I always thought it was small power-supply filter caps that caused ghost notes. Doesn't the AC30CC2 have a switch for that? What happens if you set it to "modern"?
     
  3. pfrischmann

    pfrischmann Member

    Messages:
    2,984
    Joined:
    Jan 6, 2002
    Location:
    In the ATL.
    I have a Marshall that has some pretty bad ghost notes. In my case it is the filter caps. The ones for the screens specifically.

    Thanks,
    Paul
     
  4. Steve Dallas

    Steve Dallas Supporting Member

    Messages:
    8,203
    Joined:
    Feb 22, 2005
    Location:
    Surrounded by Idiots
    That usually will be the filter caps. It can also be cone cry if it happens when the amp is loud and you are playing the upper registers.

    Also, some cheaper amps are underfiltered which can do that as well. I doubt that is the case here.
     
  5. KCblues

    KCblues Member

    Messages:
    217
    Joined:
    Feb 12, 2005
    Thanks everyone.

    I looked at the B+ with a scope and there's about 40 V-pp of 120 Hz ripple riding on about 350 VDC before the choke. After the choke it's clean as a whistle.

    It does get better by about 40% or so with the "Smoothing" set to Modern (44 uF vs 22 uF).
     

Share This Page