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Tube Issue with SVT II pro

Discussion in 'Amps and Cabs [BG]' started by Jonyak, May 11, 2011.


  1. Jonyak

    Jonyak

    Oct 2, 2007
    Ottawa, Ont
    Hey I am hoping maybe someone else has had this issue and knows whats up.

    One of my 12au7 driver tubes went recently in my SVT II pro, and I replaced it. I had trouble biasing the amp properly as the side that was replaced seemed to be overbiased no matter what I did. I looked up in the manual and it claims that it means the tubes are not matched... makes sense.

    I replaced both the driver tubes with a matched set of 12au7's and I still get the same conditions.

    I am hoping this does not mean the power tunes are going on the one side but I fear that is the case.

    Any other causes to this?

    I have done some studying and come to the conclusion it is ok to run the amp like this but that I really should find the cause and correct it ASAP.

    I am an engineer and have alot of experience repairing and maintaining equipment, not tubes, but I understand all of the safety precautions and theory behind them, so I will be doing all the servicing myself.

    Could it have to do with the actual circuit perhaps?

    any help or hints would be appreciated.

    PS: As an addition, the tube was not overbiasing before it went on me, I am kind of a nut and regularily check my biases.
     
  2. jackcheez

    jackcheez

    Sep 13, 2010
    Long Beach, CA
    My understanding is that only the power tubes have anything to do with bias. :confused:
     
  3. Jonyak

    Jonyak

    Oct 2, 2007
    Ottawa, Ont
    Yes, in the SVT the signal is split by a 12ax7 and then run through 2 driver tubes (12 au7) which drive the 6550's.

    it is my understanding that this is all part of the power system, and they all effect the bias.

    but I am getting closer to beleiving that the 6550's are my issue now.
     
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The two "sides", or sets of 3 6550 plus driver, are separate, and each have their own bias. Therefore, "matched" drivers, or matching of the two sets of 3 6550, is not required. That model does not have the "balance" control, so some matching will help as far as best output etc.

    Matching is really only needed within the set of 3 6550 on each side.

    That said, it is certainly possible for a tube to have a problem in such a way as to either have too "hot" a bias (can't turn it down enough) or for it to be so weak that the least bias is still too much.

    Or, the bias detector circuit can be messed up and be "claiming" that there is a problem when there isn't.

    If you swap the sets of 3 6550, reversing positions, you can see if the problem "follows the tubes" or if it stays with the same side. if it stays, it may be the 12AU7, in which case you can swap them also to see, or it may be a circuit problem, in which case it will stay regardless.

    If it "follows the tubes", swap them individually and see if it follows a particular tube, which then is likely bad.
     
  5. Jonyak

    Jonyak

    Oct 2, 2007
    Ottawa, Ont
    Great Idea... I didn't think of that for some reason. Thanks. Will do and report back.

    I have swapped the 12au7's and it stays in the same place, so I know its not that for sure.
     
  6. NKBassman

    NKBassman Lvl 10 Nerd Supporting Member

    Jun 16, 2009
    Winnipeg, MB, Canada
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    That's different..... the phase inverter CANNOT affect the "DC" bias, the bias you measure at the test points (unless a capacitor is shorted or leaky).

    The phase inverter CAN AND DOES affect "balance", i.e. how hard the two sets of tubes are driven.... and that can possibly show up in the bias lights and /or protection when playing. However, the lights have little meaning then, so ............


    There could be a bad capacitor between the phase inverter and the driver tube grid.... that will overbias the unit, meaning it will be too "hot" a bias.
     
  8. NKBassman

    NKBassman Lvl 10 Nerd Supporting Member

    Jun 16, 2009
    Winnipeg, MB, Canada
    That could be man. I don't know much about all that stuff, but what i do know is that after replacing the phase inverter tube my problems went away and the amp sounds better than ever.
     
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Amps do work better when the bad parts are replaced.........................
     

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