Calculating bias shift in cathode biased amps?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by RobBozic, Sep 29, 2008.

  1. RobBozic

    RobBozic Member

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    This is taken from Randall Aiken's website where he talks about biasing. I built a cathode biased AB763 type amp with 6L6GC's, and I want to calculate what the bias shift will be at the maximum signal flow.

    "Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.
    If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.
    "

    My amps readings are as follows: Plate Voltage = 404V, Pin 4 = 404V, Pin 8 = 28V, Cathode resistor = 250R. My dissipation at idle is 21W per tube, now how do I calculate what the max dissipation will be at maximum signal flow?

    thanks in advance
    Rob
     
  2. WesKuhnley

    WesKuhnley Member

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    You have to run signal through the amplifier. Your bias seems pretty hot as it is, a 6L6 can really only do around 24 or 25 watts max, and you quote 21 as your idle dissipation, 84% of max. I would try to cool that down a bit, maybe find a 300R 10W?
     
  3. donnyjaguar

    donnyjaguar Member

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    What Wes says. Standard cathode resistor for a pair of 6L6 tubes is 300Ω/10W. If you want to bypass it the standard capacitor value is 330uF/63V. Make sure the resistor is a good one. If it blows, the failure mode is open circuit thus causing the voltage on the capacitor to go w-a-y over 63V and it will vent.

    Don't use separate resistors on each cathode, this doesn't work as well as a shared one under dynamic conditions.
     
  4. RobBozic

    RobBozic Member

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    Thanks for the tip guys regarding the shared resistor, I'll get one of those 50W metal cased resistors. Also where did you get the info on the 330uf cap being the standard value?

    Thanks
    Rob
     
  5. WesKuhnley

    WesKuhnley Member

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    50 watts is a bit overkill, and you'll probably find it hard to mount. The 10 watter should work just fine.
     
  6. RobBozic

    RobBozic Member

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    'Redman', your right about the 30W rating.

    So I should play the amp for a while & see if the voltage goes down (-20V) from 400V to 380V? (Which is actually 370V to 350V if you deduct the cathode voltage).

    How do you know how much the bias shifts? And how does the feedback come into it? (Still learning the theory).

    Thanks
    Rob
     
  7. donnyjaguar

    donnyjaguar Member

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    That 330uF is best for hi-fi. You can go with a lower value for electric guitar. To calculate the value the formula is, IIRC: 1 / 2PI (F*2) R/10 where F is the lowest frequency in Hz you wish to amplify. This is irrespective of feedback.
     

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