Unsteady bias in Rivera . . .

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by justonwo, May 24, 2005.


  1. justonwo

    justonwo Member

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    I experienced some weirdness while biasing my Rivera Quiana with my Bias Rite that I wanted to run by the resident tube amp experts. First of all, after warming the amp up and flipping the standby, I noticed that the current would swing wildly even after the amp was running for 10 minutes. In both of the tubes it would jump around, sometimes drifting as much a 2 or 3 mA over the period of 5-10 seconds. Switching over to plate voltage, I noticed the voltage there (which is typically a rock solid 430 Volts) had dropped to 420 and was bobbing up and down by 4 or 5 Volts, just like the current.

    I assume the plate voltage IS NOT tied to the tube? In other words, it's not moving up and down because of something the tube's doing, right? Is this variation coming from the power source in the wall? Is that plate voltage variation causing the bias current to move up and down?

    One last thing: I set the bias to about 48 mA to get close to 70% plate dissipation. While playing the amp, I noticed a very, very faint orange glow in the corner of one of the plates (right where the plate bends closest to the filament, SED 6L6s). I assume with the bias adjusted reasonably, like I have it, that this faint glow is nothing to worry about. This glow is barely noticeable, but I've always heard stern warnings about any glow on the plates themselves
     
  2. John Phillips

    John Phillips Member

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    Try it with the old tubes. If the bias and voltage remain stable, it's the new tubes that are doing it.

    Try biasing it a bit cooler too - you shouldn't be aiming for 70%, that's an upper limit, and red glow on the plate is a bad sign. It may be that particular tube which is causing the trouble too, even if it's not actually running too hot in current terms. If it is, I'd replace it before it shorts out completely.
     
  3. justonwo

    justonwo Member

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    Actually, the original pair of SEDs shorted out a few months ago. The replacement Groove Tubes had the instability problem as well, just like the SEDs I just bought.

    Do you think the short from the first pair might have damaged something that is now causing instability?
     
  4. Swarty

    Swarty Member

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    The bias supply filters would be suspect. Also, does the amp have trem? If so, is it on?
     
  5. Greggy

    Greggy Member

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    Or could the problem reside in the bias rite itself? That's been a constant variable in your experiments.
     
  6. justonwo

    justonwo Member

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    Good point. The only thing I've done to try to rule that out is to replace the batteries in the Bias Rite. I guess it's possible there might be problems.

    Technical support at Rivera suggested I:

    1) check the supply voltage
    2) check for any AC in the plate and screen supply voltages. This could be caused by a bad cap.

    He also said if I liked the sound that there wouldn't be much problem playing with the amp that way. And I do like the sound. I just don't want to blow tubes.
     
  7. mbruffey

    mbruffey Member

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    I had the same measurement variations when biasing a Rivera BM-100, but only when I left the meter hooked up to the chassis and contacts too long. I think some kind of cycling got going between the meter and the amp. It's not the most expensive meter. Anyway, nothing sounded wrong in the amp, so I just let it alone. Mark
     
  8. teefus

    teefus Supporting Member

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    did you try biasing "the rivera way". i think this works on the quiana the same as it does on my bm60 head. it is in the instruction manual. be careful, hi voltages, lots of currrent, etc in there.
     
  9. leoman57

    leoman57 Guest

    I had the same problem, I cleaned and retensioned the tube socket. I guess the biasrite was not making good contact with the pin holders. Problem solved....
     

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