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Help! Unidentified buzzing in Ampeg SVT 3 Pro

Discussion in 'Amps [BG]' started by Wgtroutman, Feb 20, 2014.


  1. Wgtroutman

    Wgtroutman

    Joined:
    Sep 9, 2011
    hello all,

    I've been charged with the task of figuring out what's wrong with this amplifier and determining what needs to be done to repair it. There is some weird buzzing that apparently happens in the higher register of the amp at random times (I haven't played through it yet). I've perused the forum and the possibilities seem to be bad biasing or failing components inside of the amp. Below is a low quality sound clip that the guy who asked me to figure it out made for our local music shop. They couldn't find the problem.

    One tube has been replaced (not sure if it was the 12ax7 or the 12au7) since the problem has started and it didn't fix it. As far as I know, the buzzing is heard even from the direct out.

    Thanks a ton in advance. I know if anybody can figure this out it's the experts here on the forum. I'm hoping to use this as a learning experience as well as do a favor for a friend.
     

    Attached Files:

  2. sincity

    sincity

    Joined:
    Oct 16, 2002
    Location:
    Connecticut
    Subscribed. I get a very faint fuzz from the high register notes as well. Mainly above the 12th fret. I know, we should never be that high up the fret board anyway. :p
     
  3. Wgtroutman

    Wgtroutman

    Joined:
    Sep 9, 2011
  4. frankenp

    frankenp

    Joined:
    Jul 17, 2008
    Location:
    Halifax, NS
    Can you isolate the problem at all?

    - Do you still have a problem with the EQ on vs. off?
    - Is the noise heard on the preamp out?
    - Is the noise heard on the DI... pre vs post settings?
    - Is the noise present if you use another preamp into the power amp in?
    - Noise present on all frequency knob settings?
    - Is the peak LED lighting up?
    - How is the sound from the tuner output? This is buffered directly from the input jack it should be nice and clean.

    If it seems like a tube, try swapping one at a time.

    Use the block diagram on page 13 to help you isolate the problem: http://www.ampeg.com/pdf/SVT-3PRO_OM.pdf

    I had a similar problem on a Trace Elliot amp. It turned out to be a cracked solder joint on the input jack and a dirty tube socket. The cabinet vibrating the amp caused the amp to fail intermittently with a slight loss of volume and a fuzzy tone.

    Best of Luck.
     
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  6. Jim C

    Jim C

    Joined:
    Nov 29, 2008
    I had a 3 Pro that went to the shop twice for bad solder traces.
    It worked for months after the first repair and then had another noise problem a few months later; it was in a different spot.
    Not sure that is your problem but the shop told me this was not uncommon.
     
  7. Jerrold Tiers

    Jerrold Tiers

    Joined:
    Nov 14, 2003
    Location:
    St Louis
    If you have it in the tuner out, it;'s right at the front end. if in the line out when on "post", but not tuner out, it's later, but still in the pre.

    Look around the mute circuit... the mute FET can be bad, possibly from a very strong static zap to the input.

    if it seems to be later in the circuit, check all the points where the clip light has sensing.... if something goes bad there, it will clip the signal.
     

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