Diagnose my problem (Ampeg SVT-3Pro)

Discussion in 'Amps [BG]' started by ih8law, Dec 21, 2013.


  1. ih8law

    ih8law Supporting Member

    Joined:
    Aug 8, 2009
    Right up front I must apologize for my ignorance, as I know next to nothing about tube amps except for how to turn them on and off.

    I have an Ampeg SVT-3Pro that I almost never use. It's remained in my closet for almost a year. It worked fine when I put it away. I pulled it out today, and when I turned it on I got a loud squeal that escalated in pitch and volume until it disappeared. Now there is only the faintest volume coming out of the speakers when my bass is plugged into the front input jack.

    If I plug my bass into a Sansamp and run the signal into the Effects return jack the sound comes out of the speakers just fine.

    Does this sound like a tube failure? A faulty input jack? Something more nefarious?

    Any help you can provide would be so appreciated! Also, anyone know of a good Ampeg tech in the Houston area?

    Thanks!
  2. B-string

    B-string Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Use an instrument cable to "loop" from the effects send jack to effects return jack. If the amp plays normally the internal contacts on the effects return jack need cleaning. Otherwise could be a tube or corroded tube pins (pull and re-seat them a couple times).
  3. ih8law

    ih8law Supporting Member

    Joined:
    Aug 8, 2009
    So if I plug my bass in the input jack, then run an instrument cable from the effects send to effects return jacks I get nothing. If I plug my bass straight into the effects return I get a signal. I pulled each tube (none of them looked burned out, but admittedly I don't know enough to tell what I'm looking at!) and reseated it, but none of this fixed the problem :(
  4. B-string

    B-string Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    You may have had a tube die then. The poweramp is fine as you went direct to it from the effects return. The problem is in the preamp. Buy a spare tube or two and replace one at a time. You can tell very little by looking at a tube.
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  6. ih8law

    ih8law Supporting Member

    Joined:
    Aug 8, 2009
    Thanks, B-string. I just ordered a couple of new tubes. I'll replace them and hopefully sort this out. If this works I'll buy you a beer if you're ever in Houston.
  7. B-string

    B-string Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
  8. ih8law

    ih8law Supporting Member

    Joined:
    Aug 8, 2009
    Rats. Replacing the tubes did not fix the problem. I bought a complete set and changed them all just for the sake of completeness. Any other idea what the issue might be? I'm obviously trying to avoid a costly trip to the repairman, as I don't know that this amp is worth the repair bill.

    Thanks!
  9. B-string

    B-string Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Well they sell between $350 to $450 from what I have seen on average. Usually a tube will be the issue but you have now spent more than a bench fee would have been with a tech. From here it could just be a bad solder joint? User replaceable parts now replaced, time to decide if a professional hands on diagnosis is worth while.
  10. RickenBoogie

    RickenBoogie

    Joined:
    Jul 22, 2007
    Location:
    Dallas, TX
    Your only option is a good amp tech at this point. My advice, even to someone like myself, who's trained and experienced with electronics, is to pay the bench fee to the guy who is qualified to diagnose and repair the amp properly, every time. Good luck.
  11. Jim C

    Jim C Supporting Member

    Joined:
    Nov 29, 2008
    These amps suffer from bad solder joints.
    Sometimes multiple repairs on the same amp (but at different places in the circuit board).
    Don't ask me how I know.
  12. superfly8564

    superfly8564

    Joined:
    Sep 23, 2013
    I second this.
  13. ih8law

    ih8law Supporting Member

    Joined:
    Aug 8, 2009
    I think I figured out the problem: I am a moron.

    More specifically, I am the cause of the issue. As I noted, I had this thing in storage for about a year, and it worked fine then. When I pulled it out of the closet I noticed that I had previously removed the hard rubber feet that attach to the rear face of the amp. I grabbed the feet out of the box, along with a set of screws that I just assumed were appropriate and screwed them in (I did this prior to turning on the amp to see if it still worked). As it turns out, these screws were too long and not the originals. The bottom screw by the tuner out jack made contact with the internal circuit board and broke a connection to a small resistor of some sort.

    Is it a big deal to replace a resistor on an amp circuit board? In other words, does this sound like an easy fix for an amp tech?
  14. JimmyM

    JimmyM Supporting Member

    Joined:
    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps, EMG Pickups
    A tech should be able to fix that quite easily.

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