Need help on correct bias settings

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by cjr020, Jun 21, 2008.

  1. cjr020

    cjr020 Member

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    I hope someone can help me on this one. I just bought a bias probe (Weber Bias Rite, BR2) so I could check and adjust the bias on my Fender Deluxe Reverb Reissue (DRRI). I'm using TungSol RI 6V6GT power tubes. All the information that I've read suggests that my bias should be set at around 20mA (i.e. 70% of maximum).

    Here's the the thing that puzzles me: I conected my bias probe and warmed the amp up for several minutes. I checked the voltage and it reads around 415V. I checked the current and it's reading about 33mA. This is nowhere near the 20mA I was expecting, plus that equals 13+ watts. I thought the tubes were only rated at 12 watts. Am I missing something here?

    Not that I think it should make any difference, but my mains supply is 220VAC. I pulled out the boxes the tubes came in and noticed that someone had written 33mA on both boxes.

    Please help me understand the correct bias settings and the calculations for these tubes.

    Thanks!
    Jim
     
  2. phsyconoodler

    phsyconoodler Member

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    We bias for a couple of reasons.One,to save tubes from melting down prematurely and two,for tone.
    If you go by 6v6 data sheets you end up using a rather cold bias setting.20ma for a DRRI is cold in my book.I like up around 25-27ma.
    At 33ma the tube life is shortened but it may sound great.For the price of a couple of 6v6's why not experiment a little?
    I have had Deluxe's sound great at 25ma and some that liked 35ma.Each amp is a little different.Different speaker,different transformers,preamp tubes etc.
    You may like the sound at the present bias,but try it lower and see what happens.You have a bias pot so you can adjust it to whatever you want.Let your ears be the guide here.
    To drastically shorten tube life at 35ma you would need to be playing at '10' all the time.At the levels most guys plsy at,the tubes may last a long,long time.
     
  3. cjr020

    cjr020 Member

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    Psychonoodler,

    Thanks for your advice. The amp sounds really good so I guess I'll leave it where it is.

    One thing that still puzzles me is if I'm running at 33ma at 415 volts, isn't that higher than the 12 watts that the tubes are rated at? Am I missing something from the formula I x V = W ???
     
  4. booj

    booj Member

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    13.695 watts disipation. According to my Groove Tubes datasheets, for 6V6's, 7.2 watts is the in between setting. That's per tube of course.
     
  5. Blue Strat

    Blue Strat Member

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    You're right about dissipation and your calculations. The reality is that tubes will work for quite a while even when biased beyond their maximum spec.

    Try them where they should be, 20mA, and see if the difference is noticeable. You might be surprised. Set them as low as you can stand them. They'll last longer at 20ma than they will at 30+ma.
     
  6. cjr020

    cjr020 Member

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    Thanks, Mike.

    I'll try some lower settings and see if there is any noticeable difference.
     

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