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Ampeg SVP-CL preamp is Out of Order

Discussion in 'Amps [BG]' started by eyoyo, Feb 12, 2014.


  1. eyoyo

    eyoyo Supporting Member

    Joined:
    Jan 18, 2014
    Dear Ampeg owner and electronicians,

    My amazing Preamp, an Ampeg SVP-CL, is not working anymore.
    When I switch it on I get some kind of quite high hum (white noise, snow, rain...) that comes and goes even when I don't plug a bass into it. When I play I only get a very saturated and weak sound. When I swith it off and leave the power stage on I get a bigger hum until it is completely off.
    It's weird! I've replaced teh 4 tubes and it doesn't solve anything.
    Any idea or advice ?
    Thanks

    Lionel
     
  2. InsanityAmps

    InsanityAmps Supporting Member

    Joined:
    Jul 9, 2013
    Location:
    Portland, OR
    Time to take it to a tech.
     
  3. eyoyo

    eyoyo Supporting Member

    Joined:
    Jan 18, 2014
    I'm a tech, but I have no idea what to start with :p
     
  4. mbelue

    mbelue

    Joined:
    Dec 11, 2010
    Location:
    Bremerton, WA
    More school.
     
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  6. InsanityAmps

    InsanityAmps Supporting Member

    Joined:
    Jul 9, 2013
    Location:
    Portland, OR
    Well then you should know to check the power supply first! I'm not familiar with that amp but there should be a high voltage supply for the tubes, +/-15 if there are any opamps, and maybe others. Did you try running a sine wave into the input and tracing it through the circuit or seeing if it made it to any of the tube inputs?
     
  7. eyoyo

    eyoyo Supporting Member

    Joined:
    Jan 18, 2014
    I didn't try anything yet.
    I wanted some advice first. I will probably start by the power supply.
    There is a 300V and 12V as far as I know.
     
  8. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    You have to be methodical, look at one sub section or stage at a time.

    In this case, I would start with a basic service. Clean the inside the chassis if necessary, clean the jacks, pots, tube sockets, tube pins, etc. Then look for issues starting with the power supply.

    Remove the tubes. Then take voltage readings in the power supply and compare them to what is listed on the schematic. If the electrolytic caps are old, they may need to be replaced. This can be the source of the hum. A low voltage can also account fot the saturated sound. But there are a lot of possibilities.

    If the power supply checks out, start with the first stage and examine each subsequent stage in tern.
     
  9. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    In case you don't have them, here are some schematics and additional documentation. Note the output upgrade that addresses issues on some models.
     
  10. eyoyo

    eyoyo Supporting Member

    Joined:
    Jan 18, 2014
    Oh thanks a lot for the schematics
     
  11. bgavin

    bgavin

    Joined:
    May 3, 2001
    Location:
    Orangevale, CA 95662
    Hey... that looks like my web site... :D
     
  12. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    ...and a very nice web site it is. :)
     
  13. eyoyo

    eyoyo Supporting Member

    Joined:
    Jan 18, 2014
    It's fixed! Resoldered everything under both boards and it works fine :)
     
  14. Flatbass

    Flatbass

    Joined:
    Mar 13, 2004
    Location:
    Yurop
    Good to hear that it's working again! This gives me hope for my SVT-IIP.

    Sorry for the thread hijack, but could any of you guys suggest something to repair a IIP on which the bright input keeps crapping out? It's mainly sudden persistent drops in volume.

    The schematic shows that the bright input only adds a resistor and capacitor or two, it doesn't have it's own tube or signal path. Would that mean it a simple and easy fix?
     

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