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SVT Classic - went into protect mode. Maybe it overheated?

Discussion in 'Amps and Cabs [BG]' started by ultra60, Oct 25, 2015.


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  1. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
    SVT Classic through (2) 1x15 cabs, 4 ohm load. The head is USA made (if that matters) from around 2008. I've never had any other issues with it.

    Last night, we split a gig with another band. We did a set, they did a set, then we finished off the night.

    Amp worked great for our first set. I let the other guy use the amp and it was fine. Last set, and I notice the fan is pushing a lot of heat out the front before we start. Maybe it was normal, and I never noticed that before. First song, and the amp cuts out and goes into protect mode... flashing green/red led's. No pop/bang, or smoke. I always carry a backup, so I switched to that.

    This morning, the amp appears to be fine. I was prepared to test tubes due to the lack of any kind of smokey smell, etc. This has never overheated on me before. But I guess it was powered up for over 5 hours. Any thoughts on what may have happened, or what I should be checking?

    Thanks.
     
  2. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
    I should add that I always run the master on 10, then use the pre amp gain to set volume. Would this cause the power amp tubes to run a hotter?
     
  3. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Running like that is fine. No idea what happened, though.
     
  4. As our Jimmy said: No.

    I would suggest that you have the output tubes tested.
     
  5. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
    update: last night, when coming out of standby, there was a pop then protect mode. Tested again this morning. Comes on, no protect mode, but bias 1 is not lit. I can adjust it red, but not green. Some research I've done indicates it could be a power tube. I have a B&K tester, and all tubes tested fine last night. I'll test again this morning.

    I don't have a spare 6550 so I'll have to order some. Can anybody tell me which 3 tubes are biased by bias 1 pot? I would assume if this took out any resistors, I'd smell smoke, which at this point, I don't.

    Thanks for your input.
     
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The power tube triplets are grouped in triangles, left and right. If a tube is operating out of spec, it will not allow the bias to be set. The resistors that typically blow in these amps are the screen resistors, there is one on each power tube. You don't have to smell smoke for one to have blown open. They do act as fuses in the event of a short in the tube. If the tube is just worn down to the point where the bias can mot be set, the screen resistor can be fine.

    To inspect these resistors, you have to get at the tube board. To do this, the plate on top of the chassis with the holes in it for the tubes has to be removed. See this thread SVT-CL - disassembly to replace screen resistor | TalkBass.com

    It is possible that low or high line power at the gig caused the amp to be strained. In severe cases, it could cause the amp to shut down. It could also cause a tube to blow.

    The bias is also affected buy the line voltage. Some people set that bias at the venue when all the amps and stage lighting is up and running to optimize the performance of their amp. If the bias is off, it can affect how the amp sounds. It isn't really necessary though.
     
    vigor&kandor likes this.
  7. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    I went through the same thing recently with my SVT-CL (pop, protect mode, bias red or nothing). In my case, I also saw a bright flash in an output tube when it popped, so I knew where the problem originated.

    And yes, it took out a screen resistor. Replaced that, put in a new set of JJ KT88s, now all is well.

    The only thing wrong now is that I rarely play a gig where I need to be that loud :laugh:
     
    LiquidMidnight likes this.
  8. I didn't think that the SVT HAD a protect mode. Perhaps someone can enlighten my on this feature.
     
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Page 5 and 6, items 11 and 13 touches on the power amp protection: http://www.ampeg.com/pdf/svtcl_om.pdf. The VR also has a low voltage protection circuit which many disable because it is a little overzealous.
     
  10. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Should also mention that if your head is US-made, it had to be made before 2008. Dating Ampeg Amplifiers has instructions for dating it at the bottom of the page.

    As for tubes, I've got JJ 6550's in my Heritage and they've done a fine job so far after 2 years.
     
  11. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
    Thanks everybody for the input. I'll test the tubes again as well as test for open resistors. I have enough skill to replace a burned resistor. Anything past that and I'll have to find a tech in Northern Va.

    Jimmy... the 5th digit is "S". If I counted correctly that would make the amp 2006, not 2008 as I originally thought.
     
    JimmyM likes this.
  12. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Right you are!
     
  13. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Last edited: Oct 26, 2015
  14. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
    Thanks Beans.. tonight, the B&K 707 has identified the shorted 6550. I'll use your link above to get to the tube board. Unfortunately, no more time tonight. May have to wait until the weekend. I'll go ahead and order some new tubes.

    Once again, thanks everybody for the input. It's very calming when such a tragedy has struck, lol.
     
    beans-on-toast likes this.
  15. Your tube tester may not always show a faulty tube. The trouble with tube testers is they test at a much lower plate voltage than the amp operates at. The exception to that rule are very few and very expensive.
     
    Bassmec likes this.
  16. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
    update: I have the power tube board out. Was not too bad. The screen resistor is open. I had ordered new tubes but not the resistor yet. There's an electronics supply store, near me, which is closed on Sat. Will have to get parts next week.
     
    JimmyM likes this.
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Good news. It is important that the resistor meets the specs of the original like the one that I linked to in post #13.
     
  18. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
    Pictures are always fun. Once I got the tube board out, I ended up removing all the tube spring retainers to separate the metal plate from the board to get to the resistor. It's r9 in the second pic (top left) and I've already removed it. Since it was near the edge, I might have been able to get a new one in without removing the plate. What would you experienced guys have done?

    DSC02817.JPG DSC02818.JPG
     
  19. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Taken it to a shop :D
     
    Passinwind, TheBear and ultra60 like this.
  20. I would be taking the board out anyway to check all remaining screen resistors. If there was much drift from the value they would be replaced.
     
    beans-on-toast likes this.

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