Q's about tube Rectification: GZ34

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JlMMY, Dec 22, 2009.

  1. JlMMY

    JlMMY Member

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    I have a few GZ34 tubes. The amp has 2 x 6L6GC power tubes (Vibrolux). I monitor the power tubes with a Weber Bias Rite and notice that, when using one particular rectifier (Japan made RCA), that my output -ma fluctuates, i.e., when the amp is idling the power tube measures -42ma but when I hit a strong chord the output goes up to as much as -46ma. When I tried a used Mullard GZ34 the output tube doesn't move much from it's idle current during attack. Both tubes test very strong. I am just wondering if there is anyone who can explain which scenario is better or if either scenario are ok? The fluctuation is not very much, maybe 2-3ma with one tube and 1 ma with the other. I am guessing the tube that moves the least is not being taxed and would be the better tube? Also, I run 2 x RCA 6L6GC tubes. Is there a general "best" setting for bias? Right now I am running 2-3ma apart. One tube is 42ma and the other is 44.7ma. thanks
     
  2. pdf64

    pdf64 Member

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    When you hit a class AB output stage with a signal, the current in the tubes goes up - that's the signal increasing the average current level. Hit it with a big enough signal and you'll maybe measure over 200mA.
    You have just been measuring that your amp is working, ie it draws more current from the B+ when it's putting out a signal. The difference between the 2 rectifiers is likely due to different test conditions, ie you didn't pick the string as hard with the Mullard.
    Those idle currents are on the high side.
    It's a balance between tone and tube life - the cathodes will degrade with every ampere/hour under their belt. But can you honestly tell the difference in tone between 35mA and 45mA? If not, turn it down and enjoy those v expensive tubes for longer.
     
  3. TweeDLX

    TweeDLX Member

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    You want your bias at idle somewhere in the 60-70% range. With a 30 watt 6L6GC tube you want between 18-21 watts at idle.
    Calculate: 19 watts/plate voltage = bias setting. Really, go with what sounds good to you. Tube life is a factor for higher bias settings.

    Mike
     
  4. donnyjaguar

    donnyjaguar Member

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    On first guess it sounds like the Mullard's output is sagging under load. Your measurements could however be in error.
     
  5. JlMMY

    JlMMY Member

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    I guess I could lower the bias a bit. Using the formula above 19/435 = 43ma

    I ordered a couple more GZ34 tubes. I will test things again when the new tubes arrive.
     

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