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SVT-CL is overdriven at low volumes.

Discussion in 'Amps [BG]' started by VeganThump, Dec 20, 2013.


  1. VeganThump

    VeganThump

    Joined:
    Jun 29, 2012
    Location:
    South Jersey
    Hi all, I'm pretty green when it comes to tube amps and I'm looking for some help. I recently bought an SVT-CL used and had it looked over by a tech and retubed. Now about 2 months later it's starting to sound really dirty at low volumes and even if I back the gain down and crank the master up, it still sounds really dirty with very little volume. Can anyone give me any insight as to why this would happen?
     
  2. coreyfyfe

    coreyfyfe Supporting Member

    Joined:
    Nov 19, 2007
    Location:
    boston, ma
    Most likely is bad tube, probably in the preamp. Try swapping in a known good 12AX7 in each of the two tube slots in the preamp and see if that helps. Double check that the two power tube banks bias properly. If the two tubes out front don't sort it, you can also try swapping in a known good tube for the 12AX7 and 12AU7s in the power amp, but I'd start with the pre. From there it could be worn power tubes or other less likely things.
     
  3. M0ses

    M0ses

    Joined:
    Sep 11, 2009
    Location:
    Eastern Wisconsin
    Take it back to the tech and make him fix it.
     
  4. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    Check the bias of the power tubes. This can change as the tubes wear in. It can also change if you wall voltage changes from when the amp was biased last.

    A bad driver tube in the power amp can cause distortion as well. The tube need to have sufficient current to drive the power tubes. Look at the two 12AU7's in the power amp on the bottom. First, make sure that there are 12AU7's installed in their proper positions. Then try new 12AU7's in there. Some people install 12AX7's here when they want more dirt.
     
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  6. VeganThump

    VeganThump

    Joined:
    Jun 29, 2012
    Location:
    South Jersey
    Do the preamp tubes need to be matched the same way power amp tubes do?
     
  7. JimmyM

    JimmyM Supporting Member

    Joined:
    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps
    He fixed it. Tubes go bad sometimes, and that's no fault of the tech.

    I would suspect a bad preamp tube as well, and V1 is a usual culprit. If you have a good 12ax7 laying around, swap it one at a time with all the other tubes (except of course for the 12au7's), starting with V1 in the preamp. If you don't have one, get one...they're cheap enough.
     
  8. JimmyM

    JimmyM Supporting Member

    Joined:
    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps
    Nope. Some have discussed matching the phase inverter and V1, but I never bother with it and my amps all sound fine anyway.
     
  9. VeganThump

    VeganThump

    Joined:
    Jun 29, 2012
    Location:
    South Jersey

    Ok I don't have any spare tubes so I guess I'll have to buy one. Or should I just replace all the preamp tubes?
     
  10. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Buy a couple 12AX7s and a couple 12AU7s. Small tubes that remain stable after 6 to 12 months will last a long time. Why throw good tubes away just to deal with brand new ones that may fail in that 1 week to 12 month period?

    Swap tubes one at a time till you find the clunker and just trash that one only.
     
  11. JimmyM

    JimmyM Supporting Member

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    Apr 11, 2005
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    Endorsing: Ampeg Amps
  12. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    To add to that. Do what is easiest first. The power amp has one 12AX7 and two 12AU7's. These are easy to swap out first. All you have to do is remove the back cage. Try that. If the problem isn't resolved, then start changing the preamp tubes. To do this you have to remove the chassis on the top of the amp. A lot more work.
     
  13. JimmyM

    JimmyM Supporting Member

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    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps
    The preamp is 4 screws on top, and gently pull it out from the front. That's pretty easy. Now putting it back in is the hard part ;) I've found it's best to take the back off and hold it from the back as you put the screws in so they go in easier.
     
  14. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    Also take care not to scratch anything. I like to put my fingers around the end where the screw head is to stop the screw driver from skidding out and damaging anything.

    Take pics before so you know where everything goes and where each screw came from. I go so far as to put the same screw back in the same hole. Not necessary, I'm just a bit nuts when it comes to detail.
     
  15. beans-on-toast

    beans-on-toast Supporting Member

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  16. VeganThump

    VeganThump

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    Jun 29, 2012
    Location:
    South Jersey
    You guys are awesome, thank you so much. I'm gonna get to work on this ASAP, I'll report back!
     
  17. anderbass

    anderbass

    Joined:
    Dec 20, 2005
    Location:
    Phoenix. Az.
    A couple easy things you could try before tube-swapping are:

    If your using an active bass, try a new battery first.

    Then another easy diagnostic test is to plug your bass into your svt's power amp-in jack,
    and then let us know if the tone is clean, or still sounds really dirty.
     
  18. David Hayes

    David Hayes Guest

    Has the OP checked the user bias lights?
     
  19. VeganThump

    VeganThump

    Joined:
    Jun 29, 2012
    Location:
    South Jersey

    Ok I'll definitely try that. Thank you.
     
  20. VeganThump

    VeganThump

    Joined:
    Jun 29, 2012
    Location:
    South Jersey

    Nah I hadn't looked at them but i will.
     
  21. VeganThump

    VeganThump

    Joined:
    Jun 29, 2012
    Location:
    South Jersey
    So I was finally able to get to the rehearsal studio last night and I did what someone suggested, I plugged my bass directly into the power amp in jack in the back of the head, it was completely clean sounding but it wasn't insanely loud or anything, I'm not sure if it was even loud enough to break up. Anyway, I then plugged it back in the instrument jack in the front and it was again, really dirty at low volumes. Is it safe to assume it's a preamp tube now?
     

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