About ready to bias - Troubleshooting?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mightypudge, Jul 29, 2006.


  1. mightypudge

    mightypudge Member

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    On the FedEx truck to be delivered today is my new Weber Bias Rite.
    I've read the instructions and checked out the Weber Bias calculator, so I feel like I'm going to be fine adjusting the bias.

    Here is my question. If the amp is operating correctly I'm expecting no problems biasing the amp. I'm going to start with setting the bias to about 38 mA and then checking the plate voltage, which should be between 430-450V.

    Hypothetically speaking, what might I see if the amp isn't operating correctly? I know I'm putting the cart before the horse here since I should probably try biasing the amp first, then report back if I see anything goofy. But I'm trying to prepare myself just in case.
     
  2. Blue Strat

    Blue Strat Member

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    Don't worry about it. First check the bias of the tubes that are in there and adjust it, watching what happens to your meter reading.

    What amp and power tubes?
     
  3. mightypudge

    mightypudge Member

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    Rivera Knucklehead K100 with E34L's.
     
  4. Blue Strat

    Blue Strat Member

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    So, are you replacing the tubes or just checking/biasing the existing tubes?

    BTW, 33 mA would put you at 60% of max dissipation if you assume 450 Vplate. Anything higher will likely just wear out your tubes faster with no particular sonic benefit.
     
  5. mightypudge

    mightypudge Member

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    Not installing new tubes, these are fairly fresh - about two weeks old. Just checking the bias since when I bought the amp used I had no way of knowing it was set properly by the previous owner.

    OK, measurements taken. I did my R55-12 first since there are only two power tubes and I wanted to make sure this worked before moving to the K100.

    Tube A is running at 31 mA and 430 plate voltage. Tube B is running at 33 mA and 450 plate voltage.

    First question: This is a matched set of tubes, so should the settings vary like this? If not, is this a tube issue or an amp issue?

    Secondly: When setting bias and plate voltage, do you add the numbers of the two tubes and then the average is where you set it? Or do you base it on the higher of the two numbers? In other words, at my current setting is the amp at 32 mA, or is it at 33 mA?
     
  6. Blue Strat

    Blue Strat Member

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    The average OR high number is ok to use. Biasing is not an exact science. If you calculate using both numbers, you'll notice that the result changes by only a few percentage points. Tubes don't care about a few percentage points.

    31 and 33 are definitely matched. Anything within 5mA is more than close enough.

    It's wierd that the voltage measurements are off by 20 volts. Try that again, and try swapping bias probe heads/sockets around to see if the difference continues and/or follows the bias heads.
     
  7. mightypudge

    mightypudge Member

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    I just biased my K100. The amp has four power tubes so I used the two outer sockets to bias the amp. I set the tubes at 35 and 33 respectively, and the plate voltage was about the same on both tubes: 450V give or take 2V.

    So, does this indicate a problem with the R55-12, or with the tubes? The plate voltage shouldn't be off by 20V I take it?
     
  8. Blue Strat

    Blue Strat Member

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    Yes 20 volts is a lot. It's also virtually impossible unless something VERY unusual has happened to the output transformer. So until you take another set of measurements, I owe the difference to operator or measuring equipment error.
     
  9. scottl

    scottl Member

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    The plates should not vary like that IMHO. Maybe the AC kicked on and the wall voltage changed a bit between you measuring the tubes. Try it again.

    Also, the real bias current will be 2-3ma less than what teh meter shows due to the screens being in the picture.....

    So, 33ma is really 31ma.....

    Scott

     
  10. mightypudge

    mightypudge Member

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    OK, weird. I just tried again and now the plate voltage is equal on both tubes, 450V. Must have been a loose nut behind the amp, meaning me.

    Whew!!
     

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