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SVT Bias problem? need help asap

Discussion in 'Amps and Cabs [BG]' started by akaTRENT, Mar 12, 2009.


  1. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    So I just got my svt cl.
    I opened up the manual it led me to check the bias LEDs.

    So I check the back of the amp after fiddling with the buttons, and this is what i saw. Whats going on with this thing?
    l_53338271a56a4500bf1a67bc96030531.

    Does this mean I need a tube change? It is a 2004 and still has the original tubes. Or can i just tweak the bias a bit? When I play the other bias blinks reds according to what i play.

    When i turned the the right bias clockwise a bit the amp started humming like it was drawing more power and when i pulled it back the hum went away and the light stays red either wat.

    Let me know whats up please if you know
    Settings are flat. with gain and volume at 3/10
     
  2. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    PLease someone! I have an important gig tomorrow!
     
  3. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    as quoted from your SVT-CL owner's manual:

    Setting Tube Bias
    Allow the unit to warm up at proper AC line voltage for at least 20 minutes. With no input signal present, adjust each
    control so that only the associated green LED is lit. The controls may be slightly interactive. If neither LED is lit, the
    amp is over biased.This will result in some distortion in the power amp and a generally thin sound. If the green and
    red LED are lit, the amp is under biased and too much current is flowing to the power tubes. This will give a big, full
    sound but will also reduce the life of the power tubes. Once set, the controls should not have to be changed except as
    needed for tube replacement, or to compensate for tube aging. Note that the AC line voltage may vary from place to
    place and the LEDs will read slightly different. There is no need to fiddle with this every other day. Note that it is normal
    for the red LEDs to light when there is a signal present. Bias 1 Control adjusts the three left (as seen from the rear)
    power tubes. Bias 2 Control adjusts the three right power tubes.

    By observing the LEDs as the Bias Controls are slowly rotated clockwise, a number of tube problems can be diagnosed by the user:
    Condition - Problem - Solution
    Green comes on, then red = No problem The longer the green LED is on before the red LED comes on – the better matched the tubes are.
    Red comes on, then green= Tubes not properly matched Set slighty before green comes on, obtain matched tubes when possible.
    Red comes on, no green= One or more tubes are non-functioning Check to make sure tubes are all seated
    properly; if so, find and replace bad tube(s).
    None on= Possibly no high voltage or bad Have unit checked by a service technician.
    Bias Control or bad tube(s)=Both on all the time Possible bad Bias Control or bad tubes Have unit checked by a service technician.

    If the tubes are bad enough to cause damage to the unit, the Fault Indicator (#11, Front Panel) will signal and the unit
    will shut down.
     
  4. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    i think that its either:

    one or more of your output tubes on one side of your amp's push -pull circuit is drawing too much current,

    or,

    (IMO, less likely) a defective bias control component.
     
  5. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    I read the manual. Thats where I am stuck. Like I understand whats its saying but its not clicking in my head.
     
  6. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    one or more of your output tubes is pulling too much current. which means that its time to have them checked and/or replaced.
     
  7. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    What do you mean by push pull? I need new tubes on that side?

    The amp sounds fine.

    So lets say the bias control is messed up whats the next step in repair. I want to know what repairs im possibly looking at. The manual doesnt state that. Ya dig?

    One last thing the bias procedure is assumimg you are or arent in standy?
     
  8. GC4

    GC4

    Apr 6, 2008
    Toronto, Ontario
    I had an issue like this with my svt-cl where the bias lights were acting like yours. The sound on my amp had become very weak and distorted very early. I took it in thinking it would most likely be a tube but found out a resistor had been fried which had to do with the output signal. Nothing to do with the tubes.

    I would have to say that the only way you can figure this out, without possibly wasting money on a reckless retube when it may not even be the issue, is to take it into a qualified tech. It could be anything causing these issues.
     
  9. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    Oh okay! Thanks! As you can tell I am a noob.
    So the red light blinkning to what im playing is normal though?
     
  10. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    Yeah im bringing it to a tech next week. I just want to get an idea of whats going down. I have a few shows this week. So as soon as those are over it can get checked.
     
  11. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    Good thing this pup is under warranty.
     
  12. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    That's normal. When everything is going good they should be green and the red will blink on and off.
     
  13. GC4

    GC4

    Apr 6, 2008
    Toronto, Ontario
    Yeah, probably the safest bet.

    Hope the gigs go well!
     
  14. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    Hey guys. Lets wish that it just needs a resistor.
    The guy i bought this from babied the amp so i know it wasnt abused.
    Its just 4 years old and hasnt been serviced at all in that time. Probably just time for a check up.
     
  15. GC4

    GC4

    Apr 6, 2008
    Toronto, Ontario
    The resistor that blew on mine cost 5 dollars. I was much happier with that being the issue and not output tubes which get pretty pricey.

    Either way I hope its nothing major and you get back to perfect working order soon.
     
  16. GeoffB

    GeoffB

    Jun 1, 2008
    Buffalo, NY
    When a tube goes bad, it can pull too much screen grid current. This will cause the resistor to overheat and fail.

    If you only replace the resistor then you haven't really fixed the problem.
     
  17. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    This is the kind of info i was aiming at. Care to link me to details? :)
     
  18. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    The amp is getting used for two gigs so it will be on for total of like and hour and half more. then its going into the shop first thing on monday.
     
  19. GeoffB

    GeoffB

    Jun 1, 2008
    Buffalo, NY
    I don't have a link. Just an Electrical Engineering degree, Extra Class ham radio license, 33 years of bass playing and 2 SVT's!

    As Power Pentodes age, their screen grids can draw more current. The SVT has a 3 or 5 watt screen grid resistor on each of the 6550's. If the current goes up, the power dissipated by the resistors goes up (in a squared relationship) and they can overheat and "open up". It's a classic failure.

    If you play loudly, then the amp is unbalanced and the other two power tubes on one side of the "push/pull" output section will be working vey hard. They might open up too.

    You have two choices. Do the gig with the amp because you probably need to buy 6 new output tubes anyway. And a couple of resistors. The good thing about tube amps is that they take this kind of abuse pretty well. They may not sound the best when abused but they are pretty forgiving.

    Other option is to borrow another amp. There are plenty of good amps out there. I have not gigged with my SVT's in 20 years. Way too much overkill for the Buffalo bar scene and my back!

    I actually have used a Hartke 7000 with Sansamp RBI combination into the Ampeg 810 and sounded fine.

    Good luck.
     
  20. akaTRENT

    akaTRENT

    Jan 10, 2009
    New Jersey
    Great info! I just got the amp on thursday. So I need to feel it at the shows so to speak. But it will it the shop monday as stated.
     

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