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How hot do you like your tubes?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by War Admiral, Jul 31, 2006.

  1. War Admiral

    War Admiral Member

    Dec 4, 2005
    I have a '79 JMP MV with EL34's and I just did the mod of putting 1 ohm resistors from pin 1 to ground on each output tube. Cathode follower method I think. I know its not the most exact method for bias but I think its the easiest. I have about 390 VDC on the plates so about 36ma's for 70 percent. With this method you get the screens I think too in the measurement so 36 would be a little conservative(lower than 70 percent)
    Anybody Have this kind of amp and can tell me where a good ma reading would be?
    With the screens in the measurement, the voltage drop across the resistor to ground, the percentage will be lower than 70 @ 36ma's. I heard that these kind of amps like to be biased a little hotter to sound good. Does anybody know how I can subtract the screens from the measurement and get a more precise measurement? Its probably just a few ma's but it would be nice to know.
  2. Blue Strat

    Blue Strat Member

    Aug 2, 2002
    Sterling, VA (not far from Washington DC)
    70% is considered the max, 60% is more reasonable. Since there's virtually no screen current at idle (has anyone ever seen a voltage drop across screen resistors? I haven't) you can ignore that detail.
  3. aleclee

    aleclee TGP Tech Wrangler Staff Member

    Jan 4, 2002
    5868 ft above sea level
    I like 'em hot enough to sound good and no hotter.

    The best advice I ever got on the subject was to adjust the bias 'til you like the tone and then just check to make sure it's not too hot. Your ears are a much better judge of tone than a multimeter. :)
  4. bob-i

    bob-i Member

    Oct 16, 2005
    Central NJ
    It depends on the tubes. Most EL-34's don't need to be run hot to sound good IMO, neither do most 6L6's but for some reason the chinese KT66's seem to need to run at about 75% to get a great tone.

    Personally I like to bias about about 65%, listen for awhile, a few days. Heat 'em up to 70% and listen for awhile. If I hear a noticable difference I heat 'em up a little more and test again. Above 75% and you're just asking for trouble.
  5. Hacksaw

    Hacksaw Time Warped Gold Supporting Member

    Jul 16, 2006
    T-Town, OK
    60% usually... not many of my amps at 70%, bogner shiva is close to 70%..
  6. Tuberattler

    Tuberattler Member

    Jan 7, 2006
    Eastern Washington in the Desert
    I agree bias 'em to taste then make sure they're not runnin' too hot and you're done!
  7. kaseri

    kaseri Member

    May 23, 2005
    60 - 65% has always worked for me.

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