Bias Test points on a stock DRRI?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Denny, Jun 14, 2006.

  1. Denny

    Denny Member

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    where are the bias test opints on a stock DRRI? Is there a link I can visit that has the "destructions" on how to perform the task? I am 100% aware of the HIGH VOLTAGE and am 100% capable to perform the task Safely - I've done it on a Rivera but not a DRRI. I have a good, fused Digital meter. I'm also thinking of installing 6L6's and need to know what to re-bias-it for those tubes as well. Thanks everyone.

    Denny
     
  2. tmac

    tmac Goldmember Gold Supporting Member

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    Let me look at my schematic when I get home and I'll let you know.
     
  3. tmac

    tmac Goldmember Gold Supporting Member

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    Found a schem online: "bias set at -37VDC at test point 36"

    That's for 6V6's of course. They don't give a setting for 6L6's.
     
  4. VaughnC

    VaughnC Supporting Member

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    Setting the bias voltage doesn't tell you what the idle current is....which is the most important thing to know....especially with the inconsistency of modern tubes. In the good old days, tubes were consistent enough that measuring the bias voltage would be good enough....but not today. I've seen modern 6L6's vary as much as 60ma from tube to tube.

    So, you'd probably be better off buying a bias meter (which actually measures the current of each individual output tube.).
     
  5. Blue Strat

    Blue Strat Member

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    Absolutely! That -37V thing is virtually useless and gives only an EXTREMELY rough idea of where you are.

    Get a Bias Rite from www.tedweber.com (or similar) and you won't even have to pull your chassis. Highly recommended.
     
  6. tmac

    tmac Goldmember Gold Supporting Member

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    I agree with that recommendation - I use a Weber Bias Rite too and use this formula: For 6V6's (12.5W max plate disipation per tube/plate voltage) x 1000 x .70 = bias in ma. The 70% rule is a maximum and as others have pointed out 60% is probably closer to optimum, still rough but a better than just setting -37V. But if all you've got is a digital VOM the setting on the Fender schematic is about the only way to do it - rough yes, but add in your ears to hear to best setting and your eyes to make sure you're not red plating the tubes you're going to be close. Run it up 'till the plates start to glow a bit and then back it down to where they'er not glowing and sound good. If you get a signal generator and a good scope you can nail it of course. Still gotta use your ears a bit though.

    FWIW I've got mine set about 20 to 21 ma per tube and that's between 60 - 70% (I'm ass-uming 12.5W is correct for JJ 6V6's - I don't know that for sure as i understand they're kind of an odd 6V6 and I think they actually put out more power than the 6V's we're used to). In another post someone said they like theirs set around 25 ma. If they like the tone and response set at 25 ma then that's great. Anything below glowing the plates red should be fine.
     
  7. Blue Strat

    Blue Strat Member

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    Note on redplating: Redplating doesn't occur until 150 to 200% of maximum dissipation. Try it and see. This is the worst, and least safe, biasing method ever conceived.
     
  8. Guitarfever

    Guitarfever Supporting Member

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    Do yourself a favor and buy a Weber Bias Rite with the V/I option.
    From there it's really easy to calculate your dissipation.
    By trying different percentages of dissipation, your ears will tell you what you like and it will keep you within the design spec's of the power tubes.
    If you violate the max dissipation, you probably won't be looking for just new tubes, it can cause much more circuitry trouble. I run my amp at about 70% of max dissipation. Remember, the hotter you run 'em the sooner you'll be replacing them.
    Hey Mike (KCA), I've had the old 1940's 6V6 Westinghouse tubes you sold me in my DRRI for at least 3 years now and they still sound GREAT! The amps seen at least 200 gigs now, I highly recommend these tubes for all the DRRI owners.
     
  9. Denny

    Denny Member

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    Ok - here's what I did:

    I'm sure someone will find this procedure wrong - but there's a amp builder up here in VT who know his stuff so I just went with what he sadi and it's fine.

    I located the test pins on the tube sockets (#3 - blue on one and brown on the other) - I attached the black test lead from the DMM to one of them. I then found the red wire from the same transformer that the other 2 wires came from (the blue and brown) and connected the red test lead to that one (it terminated on the board). I adjusted the bias trim pot so the meter read 25.0ma (DCA). This is on the high limit for the tubes I'm using because I wanted a thicker sound and sooner breakup. It sounds great - the tubes are NOT glowing or burning up. Easy.

    Denny
     
  10. Dave C

    Dave C Gold Supporting Member

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    That's called the transformer shunt method and works well enough, a little on the dangerous side since you had your meter connected to 400+v. You're actually set a little lower due to screen current but not much.
    Dave C
     
  11. Denny

    Denny Member

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    thanks - I was VERY, VERY CAREFUL LOL!! I'm using the EH 6V6 tubes. It's definately cleaner. Can i go up more than the 25.0 ma? what do you suggest. My tech guy wasn't sure about that brand tube and what thier high limit range was. I want early break up but don't want to cook anything.

    d.
     
  12. Blue Strat

    Blue Strat Member

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    You really need to try a lower rated rectifier (5R4) that I told you about if you're serious about decreasing headroom.

    Also try pulling V1, and/or replacing the 12AT7 in V6 with a 12AX7.
     
  13. Denny

    Denny Member

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    will do - tomorrow. do you mean a 12ax7 in both those loctions (V1 & V6)? I'll call you tomorrow and order that rectifier (if my better half isn't around). she's got the "pocketbook". ;o)

    d.
     

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