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Ampeg SVT VR goes BOOM, now won't come out of standby

Discussion in 'Amps and Cabs [BG]' started by sonic sweets, Aug 28, 2012.

  1. sonic sweets

    sonic sweets

    Aug 28, 2012
    Having problems with an amp I borrowed from a friend and I figure I'd better fix it before returning it to him :)

    Yes, I know there are a lot of threads about this that talk about the circuit that won't allow it to come out of standby if the main voltage is too low, but I think I may have a different problem. I haven't checked the main voltage in the place yet, but I don't think this is the problem because I've used the amp in this room plenty of times.

    The reason I say this is because this amp was working perfectly, then I went to connect it in the studio with an unfamiliar speaker cable, and into the "top 4" input on my ampeg 8x10 instead of the full 8 that I usually use. BOOM! a little smoke out of the back of the amp and blew the main fuse.

    I replaced the fuse and it now turns on, but won't come out of standby. Took the back panel off and all the tubes look fine.

    Any suggestions? Really need to take care of this before returning the amp and I'm hoping I won't need to take it in! I'm pretty handy, but not sure how to diagnose this. THANKS!!
  2. jastacey

    jastacey Supporting Member

    Feb 8, 2009
    My advice .... spend the money on a tech and get it fixed .... that amp is not a amp to learn on, for repairs, plus it's your friends amp
  3. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    If you used the top 4 input of your 8x10 you connected it to an improper speaker load - 8 ohms instead of 4 ohms. SVTs are known for their robust transformers and many people do use them under this kind mismatch but it is not advisable as it can cause issues with the output section of the amp. It's possible you ran into this kind of issue or tripped some kind of protection circuitry in the amp.

    You need to bring this amp to a technician.
  4. craig.p


    Sep 28, 2008
    New Hampshire
    This is unclear. The thing smoked immediately after you made the connection (with no signal applied to the amp), or it smoked after you played a few notes through it?
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Make sure that the 2/4 ohm impedance switch is set to 4 ohms and not half way between the two. Reset the switch just to be sure.

    There is a second, slo blo fuse inside the power amp. You need to remove the power amp to get at it.

    Smoke is a bad sign. Something more than a fuse has blown. Could be a tube, could be a plate resistor, or some other component. In any case, the power amp will need to be removed and inspected.
  6. Sid Fang

    Sid Fang Reformed Fusion Player Supporting Member

    Jun 12, 2008
    Dead right on this last point - as someone else wrote, smoke means something got dangerously hot before the fuse blew. But I'm dubious about the idea that it's purely due to having run it into an 8 ohm cabinet. I've got a vintage SVT that I've been using off-and-on with 8 ohm cabs for over 20 years. As a general rule, it's safe to run amps into cabinets with a *higher* impedance than specified - you just lose some efficiency. It's running with a *lower* than specified impedence, e.g. running an SVT into a 2 ohm cab, that can fry the output stage.

    Now, it would be possible to construct a 4 ohm, 8-way cabinet so that the "top 4" speakers present a 2 ohm load. It would be poor design, not least because there are a lot of bass amps out there that are only rated down to 4 ohms, but *some* amps (e.g. my EBS HD350) are built to be able to run two 4 ohm cabs in parallel, and can thus tolerate a 2 ohm load. While it's hard for me to imagine Ampeg having a 2 ohm wiring of the top 4 speakers coming out of the factory, I could imagine someone with a 2 ohm tolerant amp rewiring the thing in hopes of getting another db out of the top 4. MAKE SURE YOU CHECK THAT BEFORE YOU PLUG ANOTHER AMP INTO THAT JACK. Especially if anyone is going to run a solid-state head through it.
  7. craig.p


    Sep 28, 2008
    New Hampshire
    For tube amps it's the reverse.
  8. sonic sweets

    sonic sweets

    Aug 28, 2012
    @craig - the amp smoked and blew the fuse when I switched the power on, with the amp in standby and all connections in place. What do you mean "For tube amps its the reverse"? That the amp can't handle an 8 ohm load if its looking for 4?

    The top 4 speakers in "dual mode" are an 8 ohm load.

    Any other thoughts about what may have caused this? I am going to pull the power amp out and check out the second fuse. Like I said, all tubes look fine. I am suspecting a blown resistor or something of that nature, and I'm down to pop in a replacement if I can identify it.
  9. craig.p


    Sep 28, 2008
    New Hampshire
    SS, I meant tube amps will tolerate a lower-than-spec load better than a high-than-spec one.

    For your situation, yes your impedance was mismatched, but since you never played through the amp under that condition, that wasn't what smoked the amp. I'm thinking shorted output tube, and it took one or more plate and/or cathode resistors out in the process (which is what you *want* to happen, absent fused tube circuits, so a general meltdown doesn't occur). It's the type of deal where you accept the lesser of two evils.
  10. heynorm


    Oct 21, 2010
    Omaha, NE
    And, put their bill in your tax writeoffs under repairs.
  11. If it blew up the instant you turned it on or plugged in the speaker cable, something was ready to fail and just happened to do it while in your possession. A few seconds of incorrect impedance will harm nothing. And with a tube amp, even a SHORTED speaker cable will do no harm- it's actually better for the amp than an open circuit. Something was ready to die, and it chose that moment to do it. Not your fault.
    If you want to fix it, take it to a tech. It won't be simple.
  12. craig.p


    Sep 28, 2008
    New Hampshire
    Oh, forgot to mention: If what I think is the problem is what it actually turns out to be, and hasn't spilled over into adjacent circuits, then it's a quick & fairly routine fix. Biggest expense will probably be retubing & rebiasing the amp. Also, careful how you handle the amp when it's being transported, don't knock it about -- especially when the tubes are still warm, wrap it in padding when it's on the truck, blah bah blah, all the usual caveats for tube gear.
  13. Sid Fang

    Sid Fang Reformed Fusion Player Supporting Member

    Jun 12, 2008
    Perhaps, but in any case putting an 8 ohm load on a correctly functioning SVT will not cause it to blow a component on power up! I've got an 8 ohm 2x12 cab that's easier to wrestle into a car than my 810 'fridge, and I've often used it with my SVT with good results and no problems.
  14. craig.p


    Sep 28, 2008
    New Hampshire
    Sid, your two questions are unrelated.

    1. Putting ANY passive load on ANY tube amplifier will not cause problems on power up as long as there's no signal present at the amp's input. You can't blow an output transformer (or cause it reflect power back into the amp's tube output stage) due to an impedance mismatch if there's no current flowing through the transformer's primary winding.

    2. I can't argue with success. But it certainly doesn't qualify as "best practices." And if I were that amp's output transformer, I wouldn't be sore from grinning, that's for sure. There are other people on this board who do things that ask for trouble, and their gear keeps on working, too. But that doesn't make what they're doing correct, or wise, or advisable.
  15. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Supporting Member

    It just hasn't given an issue yet, I assure you with %100 certainty, the amp is suffering it just hasn't failed yet. It will eventually cook at least one tube.

    Sounds like OP has some tube problems, as stated this is not a child's amp, this needs to go to a tech unless you want to do more damage or accidentally kill yourself with a shock.

    Odds are it was going to happen and you just happened to be using it, but you are in a sticky situation with your friend if he refuses to pay for repairs. That is why I don't lend gear and I would not want to use a friends SVT ever...
  16. Alex1984


    Jan 16, 2010
    I'd say pay to have a tech fix it up properly.
  17. rust_preacher

    rust_preacher Supporting Member

    Dec 17, 2009
    "Unfamiliar" speaker cable? Please check that one so it won't cause trouble to someone else down the road.

    Most likely the cable is ok.

    On power-up, with the amp on standby, only the heater filaments are powered up (I don't know if there is some other circuitry in an SVT powered up at this stage). therefore, the tubes ought to be OK. I suspect the heater rectifier diodes (if they are DC heated).
  18. craig.p


    Sep 28, 2008
    New Hampshire
    Ugh, I missed the part about the amp not being brought out of standby. Yeah, I agree, probably not plate or cathode resistors.
  19. jmpiwonka


    Jun 11, 2002
    how old is the amp? still under warranty?
  20. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    If I'm lending someone an SVT (not going to happen in a million years so don't ask), and it breaks while in their possession, and then I find out that they worked on it themselves, they would find out whether an SVT could fit in their ass sideways.

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