Set the Bias in MA, what about voltage?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by NOVA70, Apr 9, 2005.

  1. NOVA70

    NOVA70 Supporting Member

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    Hi,
    I always set the bias using a volt meter to milliamps but I have read about setting plate voltage. How do you do that? Is it effected when you set the bias?
    Thanks,
    Pat
     
  2. VaughnC

    VaughnC Supporting Member

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    Typically you can't "set" the plate voltage in an amplifier. Bias ma readings are worthless unless you know the plate voltage because you need to keep the tube operating within its power ratings. Power(watts)=ampsXvolts and you typically want to bias your tubes to less than 70% of their maximum power rating at idle. As long as you don't exceed 70%, use your ear for the best sound to fine tune things from there.
     
  3. NOVA70

    NOVA70 Supporting Member

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    OK. Say on our Komets, I set the bias on EL34's at 30ma which sounds best to my ears. So what does that convert to in voltage?
    Pat
     
  4. RussB

    RussB low rent hobbyist Silver Supporting Member

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    You don't "convert" anything...the plate voltage exists...measure it, then calculate the plate dissipation

    Read about biasing here
     
  5. Blue Strat

    Blue Strat Member

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    If memory serves, Komets run approximately 470V on the plates.

    General rule: If you assume 450V in amps with 2 6L6GCs or EL34s, you'll never be off by more than 10%. In biasing, 10% is negligible...biasing is NOT an exact science.

    30ma = .03Amps

    .03 X 470 = 14 Watts

    Max dissipation (published spec) for EL34s is 25 watts.

    14/25 = .56 = 56 %

    You can go up to 60% (some say 70%).
     
  6. Latif

    Latif Member

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    Thanks Mike, that was very clear and helpful. I'm slow on the draw in understanding the whole biasing thing.I'll have to ask Joe at TR what my Customs plate voltage is so I can understand more of what I'm doing when I adjust the bias.
    One question. How long should the tubes be on and warmed up when checking? I've noticed a difference if I leave my meter plugged in for a while as the readings always go up as they warm up.

    Latif
     
  7. sws1

    sws1 Member

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    In the instructions that came with my Aiken Invader, it says to bias at 30 mA per tube. However, there is no ability to measure plate voltage. I know that Randall certainly knows that you have to measure output. I'm guess he knows the range of voltages that exist on the plates.
     
  8. RussB

    RussB low rent hobbyist Silver Supporting Member

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    There most certainly is a way to check the plate voltage, measure it with a voltmeter! Ground to the chassis, and touch the red lead to PIN 3 of the power tube...

    Aiken suggest 30mA, because he is intimately familiar with the operating parameters of his amp.
     
  9. Blue Strat

    Blue Strat Member

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    Check them at powerup and a few minutes later. By about 10 minutes of operation or less, they should be settled in unless there's a problem with the amp or tubes.
     
  10. NOVA70

    NOVA70 Supporting Member

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    Thanks to Mike and all for the information.
    Regards,
    Pat
     
  11. Latif

    Latif Member

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    Thanks again Mike, just what I needed to know.We are so lucky to have the quality of advice happening here!

    Latif
     
  12. sws1

    sws1 Member

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    I meant to say, no way to check the voltage externally. Given that you can check current externally, seemed strange not to be able to check voltage externally. That's the other half of the plate dissipation formula.

    Plus, I tried to get the Aiken chassis out of the cab and it seems stuck with some thick tape or glue. Hmmm
     
  13. Greggy

    Greggy Member

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    You can check plate voltage externally, use a Weber bias rite or similar measuring device. With the VI option, you can measure both plate voltage and current with one tool. Then use the formula above for estimating power dissipation (wattage) based on the known values for current and voltage.
     
  14. hogy

    hogy Member

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    Pat, I just now saw this thread. If you're talking about your Komet, call me and I'll explain everything to you.

    One thing worries me, are you setting your Voltmeter to mA when checking your Komet's bias? If so, that would be wrong.
    You MUST set your Voltmeter to mV (millivolts). Your reading
    will convert 1:1 to mA, but you have to read it in mV.

    In other words, if you read 30mV on your meter, your Komet will be biased at 30mA. You want it to run at 40mA, you set the bias to read 40mV on the meter.

    If you have your meter set to mA, you will get a meaningless reading.

    This is covered in the Komet manual, e-mail me if you need a copy.

    The best way to bias a Komet is to do it by ear. Use the meter to check and make sure you are in a safe range as described in the manual. Other than that, what sounds good, is good.

    Don't worry about the plate voltage. There is nothing you can adjust. The bias is the only parameter you can control.

    Hogy, Komet Amplification
    www.kometamps.com
     
  15. reaiken

    reaiken Member

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    The reason you don't check plate voltage externally, is because we manufacturers don't want high voltage accessible on the rear panel where you can electrocute yourself.

    That's why I put the bias jacks on the back, and specified the max idle current for the amp in the manual. That max value stated in the manual takes into account the plate voltage, and is designed to protect you from yourself. :) Note that it is a *max* setting, not necessarily a "must-set-it-to-this-value" setting - I suggest you bias by ear, but not exceed the max value stated in the manual. Lower settings will result in a crisper tone, with more headroom and longer tube life. Higher settings will result in a squishier tone, with more distortion and shorter tube life.

    Oh, and you are measuring mV because there is a 1 ohm resistor in there that you are measuring the voltage across - since voltage is equal to current mulitiplied by resistance, the voltage across one ohm in mV is equal to the current in mA. Be *sure* to set your meter to the mV range as stated in the manual.

    By the way, the chassis has a strip of thin double-sided adhesive tape along the edge of the front to keep the front panel from vibrating and causing buzzing noises. You can carefully separate it if you try. However...I went to all the trouble of putting those jacks and that expensive locking bias pot on the back to keep you guys out of my amps so you don't kill yerself!!!! Don't make me come up there, now, y'hear?!!!! :D

    Randall Aiken
     
  16. reaiken

    reaiken Member

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    Thanks!

    RA
     

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