BF Fender question: does rectifier tube affect bias?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Scott Whigham, Feb 6, 2012.

  1. Scott Whigham

    Scott Whigham Member

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    I thought of this - I swapped out a rectifier tube recently in my 67 Vibrolux and, within a month, I blew an NOS RCA blackplate 6L6-GC. Weird. I wonder if that's related? There was a chinese tube in it and I swapped in an NOS Sylvania without thinking about it... Now my tubes don't bias worth a crap - I'm getting 34/35ma on my power tubes with the bias pot cranked up.

    Does the rectifier tube (quality of, brand, etc) have anything to do with biasing/current to power tubes?
     
  2. Chris Scott

    Chris Scott Silver Supporting Member

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    No, but everything to do w/ the plate voltage, which is your "reference" for setting the idle current in the first place. What does your tube chart (it should be inside the cab) call out for the rectifier- 5U4? GZ34?
     
  3. Blue Strat

    Blue Strat Member

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    It can but not by much as long as it's the same TYPE of rectifier. Since it takes about 20 seconds to check the bias it's recommended to recheck periodically anyway. A bias check can indicate problems or impending problems.

    34-35mA is fine for 6L6GCs in that amp. Where had you previously set it?
     
  4. Scott Whigham

    Scott Whigham Member

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    I put in a Sylvania 5U4 but it says GZ34 on the tube chart.

    I didn't check before - I *just* got a Bias Rite a few weeks ago (partly because of my issues).
     
  5. Blue Strat

    Blue Strat Member

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    So it's likely that nothing has changed.
     
  6. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    A rectifier tube change can definitely affect the supplied voltage level, and you should change the power tube bias accordingly. I've measured 15-20 v. difference between GZ34 rectifiers, for example.

    Always re-bias after changing a rectifier tube in a fixed/adjustable-bias amp.

    - Thom
     
  7. LarryN

    LarryN Member

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    The power transformer may not handle the current draw of the 5U4, from what I remember. I'd hate to see you lose that PT.
     
  8. Scott Whigham

    Scott Whigham Member

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    Thank you to everyone for the advice. I swapped back in a GZ34 so that there isn't any issue. I'm having the amp checked out next week too - just to be safe!
     
  9. TweeDLX

    TweeDLX Member

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  10. Zerksies

    Zerksies Member

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    In theory they say no. But a new rectifier will change the plate voltage which can change the bias voltage slightly. So i would say yes. In my Deluxe reverb a new rectifier tube changed the plate voltage +20 volts
     
  11. donnyjaguar

    donnyjaguar Member

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    Just to add to this that a small change in HT voltage can make a surprising change in bias current depending on the circuits. So be careful here gents.
     
  12. DT7

    DT7 Member

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    This.
     
  13. vibrostrat43

    vibrostrat43 Supporting Member

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    If the power tube blew after changing from a rectifier with less sag to one with more (like you did here) then I suspect that it was old and worn out, and would have eventually with either rectifier.

    Plate voltage plays a big role in how much plate current is safe for the power tubes. The higher the plate voltage the less current you should be drawing in order to reach the same amount of idle dissipation. So if you switched for a lower voltage rectifier without rebiasing then you likely are biased too cold, which would expectedly lead to extended tube life. That's why I doubt the new rectifier had anything to do with the old rca giving out.
     

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