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Uh-Oh! SVT III-Pro help needed

Discussion in 'Amps and Cabs [BG]' started by EricF, Apr 27, 2006.

  1. EricF

    EricF Habitual User

    Sep 26, 2005
    Pasadena, CA
    Electronic and plastic burning smells are bad, right?

    About 45 minutes into band practice last night we started noticing a burning smell, and identified it as coming from my amp - no smoke, just smell, and the top of the amp didn't feel any more than slightly warm. I also noticed that my tone had changed from my usual, moderately clean, warm tone to a mild distortion (fuzzy) in the higher frequencies. Everything sounded normal until the burning smell started.

    This has happened twice now (last time we thought it was the 65 Fender Twin used by one of the guitar players).

    My initial thought was a pre-amp tube going bad. Is there anything I need to know about doing a tube swap myself?

    I haven't opened up the case yet to take a look for any apparent damage. Any other ideas what might be going on?
  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    That's a wide open question.........but......

    Bad tubes don't smell, as a rule.

    Smells normally mean that some component is too hot, and may be in the process of being damaged, if not burnt. Even if a bad tube is the cause, a good one to replace it may not be the cure.....

    Rather than assuming it is a tube, as might be reasonable if you just had noises, I think it might need to be looked at by a tech.

    You might shake it first and make sure that nothing is in there that may have gone in through a vent hole, etc....
  3. EricF

    EricF Habitual User

    Sep 26, 2005
    Pasadena, CA
    Thanks, Jerrold. Do you have a recommendation for a tech in Pasadena, CA?

    As for the tubes...
    Is there anything special I need to know about re-tubing the SVT III Pro? I've done tube swaps in Fender guitar amps before, but those are easy because the tubes are externally accessible.

    A recent thread about a similar problem mentioned insufficient power supply to the amp causing problems. Is this a possibility for what's going on in my case? It would be logical to me that if the amp isn't getting enough power from the outlet, things might sound okay until the parts started over-heating after being used for a while.
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Taking the last first, I don't think low voltage would cause the problem.... At least not the burning smell..... Low enough voltage can surely result in distortion and/or shutdown.

    Replacing tubes is easy enough, unplug old, plug in new, be sure to put the right ones in the right holes. So don't let your tech charge you very much for it!

    Gotta be careful, since to replace the tubes you have to "open the case" and I cannot recommend that for unqualified folks.......

    A burning smell isn't good, usually means some issue with the unit. That combined with distortion could be from any of dozens of causes, I have zero way to determine what it might be from the data so far.

    It needs to be looked at. The fact that it seems to be intermittent isn't helpful, that just makes it hard to find. But I have every expectation that it could happen again, and if it does, the damage will eventually be worse than it is now. If nipped now, probably much cheaper, easier to fix. (Warranty still active?)

    Tell the tech everything you know about what was going on when it happened, and find a good tech. Sorry I have no guidance for you in Pasadena for specific higher quality techs.

    I assume we have some authorized service centers around that area. They have the service info, and they should be able to do the job for you.
  5. eots


    Dec 18, 2004
    Morris, IL.
    My initial thought is that a tube is shorted, causing excessive current to heat up a wire and melting the insulation. Sometimes circuit bd. traces can get burnt open and some point to point wiring will have to be done. Be sure to remove any black carbon deposits on the boards as well cause it will add noise.
    Replace the tubes as a set cause they're all probably the same age. Also replace obviously damaged components such as the wires or what ever melted. Do some static checks on other components checking for shorted caps,chips etc., open resistors, and perhaps solder that has melted and may have been blown over other areas. If a VOM meter and soldering skills aren't your thing , then take it to an amp doctor.
  6. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The tubes in 3 PRO are preamp only. So there are plate resistors in all of them.

    Shorts should at worst stop the sound totally, not what was described. Anything less than that is lower power dissipation, less problem.

    This is speculation. We dunno what happened.

    There are different smells, and they mean different things..... At least there aren't any selenium rectifiers (that's for PBG to smile at) in the unit.....

    No foolin, it should probably be looked at, because from what was said, it isn't going to get better, and might get worse.

    I've been wrong before.....but.......
  7. EricF

    EricF Habitual User

    Sep 26, 2005
    Pasadena, CA
    UPDATE: I took it to a tech today. There are a couple of caps that have burned and have been replaced, but the cause hasn't been identified yet. They are currently running a test on it and will let me know what they come up with.

    I'll do the tube-swap myself once I get it back. new tubes couldn't hurt in an amp that's 10+ years old and still probably has the originals.

    Thanks for everyone's input. I'll let you know what the culprit turns out to be.
  8. tadawson


    Aug 24, 2005
    Lewisville, TX
    How old is the amp? Caps can fail merely due to age, and if one got really leaky (more like a short) what you have described could happen . . .

    - Tim

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