Fixed that problem, so why are the plates orange?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Aug 31, 2006.

  1. pfrischmann

    pfrischmann Member

    Messages:
    2,988
    Joined:
    Jan 6, 2002
    Location:
    In the ATL.
    O.K.,
    So I verified that I did indeed have the 4 and 16 ohm taps wired backwards.
    I'm good now and verified.

    BUT... I'm using a bastardized set of tubes as I don't have a coplete factory matched set. I have three winged c's and a svetlana and JJ I swap out. I got a little nervous about the JJ's pins

    the tubes a biased like this with 490 volts on the plates

    v4..... v5...v6. v7
    31ma 30.7 28 32 (JJ)
    ...................27 (svet)

    ....So , I turned out the light so I could really look at what's going on. playing through a 16 ohm THD hotplate (it's 5.a.m.) into a 16ohm cab....


    v4 and v5 seem to be pulsating to the music, v6 not at all v7 is a bit. When I dime the amp and really start playing, the plates on v4,v5 and v7 start to glow a nice red/orange. The more I play the more they glow. When I stop, it fades.

    What is this all about??
     
  2. Guinness Lad

    Guinness Lad Silver Supporting Member

    Messages:
    15,339
    Joined:
    Nov 11, 2004
    Location:
    On top a mountain of Chocolate Chips
    I might be wrong but plates glowing red/orange is a sign of a bad tube. My understanding is when plates start to glow red you better watch out because it's a sign of bad things to come. I don't know why they would stop glowing though, this is odd to me.
     
  3. wilder

    wilder Member

    Messages:
    769
    Joined:
    Feb 20, 2004
    Location:
    Denton, TX
    You've got it biased too hot. It's running well over 70% power dissapation at that plate voltage. Dial it down to 25ma or so.
     
  4. pfrischmann

    pfrischmann Member

    Messages:
    2,988
    Joined:
    Jan 6, 2002
    Location:
    In the ATL.
    I'm going to have to show my ignorance here as there are two things I don't get.

    1. The plates only go red with the amp dimed and me playing something really constant through it...."Bark at the moon" is my test piece. The more I play, the more red the plates get. When I stop, the plates cool down.

    2. The tubes are fine if I put the amp on 8 ohms into a 16 ohm load (hot plate)+ 16 ohm cab.

    3. If I go lower, to say 25 MA don't I get into cut off?...Won't the screens get to high?

    Thanks, Paul
     
  5. rooster

    rooster Member

    Messages:
    2,123
    Joined:
    Mar 9, 2005
    Location:
    Arizona
    Try a 47-100pf cap rated at 1000V or more across the coupling caps from the PI like Marshall did. That will kill ultrasonics, or at least attenuate them.

    rooster.
     
  6. JJman

    JJman Member

    Messages:
    992
    Joined:
    Mar 12, 2006
    Location:
    New Jersey
    This is what a class AB amp will do when biased "too hot."

    Yes, because the increased impedance is "impeding" the tubes from doing their thing. Also increasing risk of OPT damage.

    Class AB amps go into "cutoff" when output goes up (by definition.) Cutoff is occurring while you are watching them glow red. Cutoff on the bottom of the wave and high current on the top of the wave.
     
  7. wixedmords

    wixedmords Member

    Messages:
    409
    Joined:
    Sep 18, 2004
    Location:
    Lakes Region of NH
    PF,

    I am taking it for granted you are talking about an EL84 amp.

    You said you had a plate voltage of 490. Insert it into the equation.

    (0.7 * 12/490) = 17mA

    This would be for the max dissapation of 70%.

    If it is an EL84 amp, you are running very hot. I wouldn't be suprised if it wasn't (near) smokin' cherry-red hot. ;)
    [FONT=&quot]

    [/FONT]
     

Share This Page