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Ampeg SVT-CL (USA) weird squealing noise

Discussion in 'Amps and Cabs [BG]' started by seth4texas05, Apr 2, 2013.


  1. Hey everyone,

    This Is my first post on TB. I bought an Ampeg SVT-CL around two weeks ago, used for 850. Sounded great when I bought it. I gig almost every night and I have played 12 4-hour gigs with it in the past 2 weeks. Last night I was hosting an open jam and as we were playing the opening set, my bass cut out and all that came from the amp was a loud squeal. I muted my bass, turned the gain and master down and it still was squealing. I looked at the back to check the bias and it would not bias at all (either no lights or both red and green when adjusting). The squeal would stop on stand-by. The tubes were glowing bright blue when it was not on stand-by. I am not very familiar with tube amps so any ideas would be greatly appreciated. Also I can return the head but if the problem is not major I would prefer to keep it and fix it since the tone is incredible with my precision basses.

    Thank you
     
  2. Sounds like one of the small tubes in the lower "power amp" has gone microphonic. IIRC one 12AX7 and two 12AU7's? Don't remember for sure, the original design used a 12DW7 and two 12BH7's.
     
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Yes I think it sound like a tube.

    It's always a good idea when you buy a used amp to take it to a tech and have it checked out. Consider it part of the purchase price. A tech will inspect the amp, clean it up a bit inside, clean the contacts, lube the pots, check the tubes, and set the bias. Basically bring the amp up to spec. Sometimes preventative measures can save you more costly repairs down the road.
     
  4. Thanks for the insight. I am new to the tube heads so I wasn't sure what to watch out for. All of the service techs were not working today, so I posted on here. Cannot seem to find any other threads that explain this problem. Hopefully it is just one of the driver tubes, as they are cheap and easy to replace.
     
  5. By the way with the squealing going into the output tubes the bias lights will NOT work. To use the bias lights you need zero signal. They will work again once you get the noise sorted out.
     
  6. It is not a driver or power tube problem. Took it to one of my amp techs and he thinks there might be something wrong with the bias control. It is strange that no one else has mentioned anything similar about these amps.
     
  7. Squeal from the output tube bias control? Not likely. Have him pull the phase inverter, if it stops squealing he is looking in the wrong place.
     
  8. RickenBoogie

    RickenBoogie

    Jul 22, 2007
    Dallas, TX
    AGREED WITH mR bEANSONtOAST, (A DELIGHTFUL MEAL, AS WELL), ANYTIME YOU BUY A USED TUBE AMP, IT'S A GOOD IDEA TO TAKE IT IN TO A QUALIFIED AMO TECH FOR A GOOD GOING OVER. tHIS WILL ENSURE YOU GET OFF ON THE RIGHT FOOT WITH THE AMP, AND WILL PREVENT ISSUES AS DESCRIBED IN THE OP. gOOD LUCK, AND FIND A BETTER TECH. (please excuse the unintended caps lock error, I'm not shouting.)
     
  9. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    I was miffed for a little while there Rick! :D

    Agreed on the comment on David's user name, especially if you stick a nice fried egg on top of the pile! :hyper:
     
  10. DethByDoom

    DethByDoom

    Sep 23, 2012
    Houston,Tx
    Sayeth dunwichamps, an awesome custom amp builder.
     
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Connoisseurs prefer Heinz. Makes bass players even better on the low end. :p


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  12. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Tech needs to check the feedback signal from the output transformer..... if loose or otherwise messed up, it can do that, although it is more common for the amp to go quiet and have the output tubes turn bright red......
     
  13. Not sure if that "tech" can identify the NFB loop? I know where you are headed though.
     
  14. Thanks for all the great "advice." My amp tech isn't finished looking at it. We just checked the tubes while I was there and made sure that that wasn't the problem. Have any of you actually had experience/heard of this issue? Or are y'all just shooting suggestions in the dark. The manual says it could be the driver tube, but it isnt a tube issue.
     
  15. The best tube tester is the circuit it is used in. Has he swapped with a known good tube?
    Jerrold Tiers was a design engineer for SLM/Ampeg and I spent close to 40 years on a bench while also designing broadcasting equipment. Yes I have seen it.
     
  16. They are all functioning winged c's (power) and jj (preamp/driver)
     
  17. You didn't answer the important question, did he try swapping a known good tube in the driver and PI sockets. Tube "testers" are not the end all and no one method used by testers are infallible. Even when all three popular methods are available in a single tester they will not be at full operating voltages. Not being a jerk here just providing some facts.
    If he has a scope and knows how to use it, it will provide much more info than a bench tube tester. Swapping is fast and efficient.

    It also sounds like Jerrold has seen a connection problem before in the NFB loop at the OT in his time with Ampeg.
     
  18. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The reason why you need to replace the tubes in the amp like Jon said is that the tube tester doesn't test the tubes under the same conditions that they are subjected to in the amp. Tube testers have their uses but are also limited in what they can do.

    When an amp squeals, most often it is a tube but as Jerrold pointed out, a problem with the feedback circuit can cause an oscillation in the amp resulting in a squeal.

    As was said, the easiest and fastest thing to do is to swap out the tubes, one at a time. You could then test the pre amp and power amp separately buy running the pre-amp out into another amp; likewise you can run a signal into the power amp. This can help isolate where the problem is.

    This is a quote from the SVT-CL user's manual: "When a preamp tube wears out, the amplifier may squeal, get noisy, lose gain and sensitivity, or just quit working.
     
  19. Listen to what these guys are telling you, they ALL know what they are talking about.
     
  20. I guess I am just getting a little frustrated. I would like to know if there would be a way to figure out if it is or is not a tube issue before spending $500 on all new tubes for a head that still might not work after all tubes are replaced. I am waiting to hear back from another guy who works on amps here in town. The first guy I took it to couldnt figure it out. There arent many options here so I may try to isolate the problem myself before letting him look at it. Does the power need to be on in the pre amp if I run it into a working power amp? I have never tried this. Thanks in advance.
     

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