Cathode bias help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by amstrtatnut, Apr 27, 2015.

  1. amstrtatnut

    amstrtatnut Member

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    Caththode bias help
    I bought a set of 6l6 tubes for my vht 12/20. One tube failed suddenly. The dealer asked me what it was biased at. I thought cathode bias was kind of a set it and forget it thing.

    Do I need to check bias on cathode bias amp? Ive had it two years and never did before.
     
  2. Silent Sound

    Silent Sound Member

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    Yes, you should always check the bias whenever you switch tubes. Cathode biased amps are self biasing to a point, but there are a few situations where new tubes won't bias properly in a cathode biased amp without swapping out some resistors. So it's always a good idea to check the bias when you switch tubes, but it's not often that you'll actually have to adjust anything.

    It's like wearing a seat belt in a car. 99% of the time you won't need it. But when you do, you'll be glad you did.
     
  3. amstrtatnut

    amstrtatnut Member

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  4. Dan40

    Dan40 Supporting Member

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    A new tube can also fail at any time. I would agree that the bias should always be checked, but like any electrical component, a tube can fail even if it was brand new.
     
  5. amstrtatnut

    amstrtatnut Member

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    This was the assumption I was operating under. I did not know to check bias.
     
  6. Cirrus

    Cirrus Member

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    The thing with cathode bias is that it's kind of sold as a thing that means you don't need to worry about biasing when putting new power tubes in... but often the tubes will be biased far beyond their technical ratings, so if they do break early and you go back to the tube vendor, it's not unreasonable for them to say "well what was it biased at?" when the answer might well be "I don't know, it's cathode bias. let me check. Ah, 150%".

    And they'll still usually give you free replacement tubes, which is pretty nice of them.

    Just to give you an example, when I got my AC30hw it was biasing quads of EL84s to ~19 watts/ tube, when their max rating is supposed to be 12. It had gobs of headroom but it was blowing tubes every few weeks.
     
  7. donnyjaguar

    donnyjaguar Member

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    In cathode biased amps you really need to run the tubes at the high-end of their current tolerance to get maximum power. So I agree that if you put a pair of new, presumably more lively, tubes in there is a danger of having too much current passing through them.

    Its easy to test for bias current on cathode biased amps. Measure the voltage across the resistor that is shared between the cathode pins and divide by the resistor's value.
     
  8. amstrtatnut

    amstrtatnut Member

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    Amp manufacturer responded that I should not have to adjust or test anything when swapping tubes. Im going with that. I dont know a resistor from anything else inside my amp.

    Tube dealer has agreed to replace defective tube.
     
  9. Carol-AnnAmps

    Carol-AnnAmps Gold Supporting Member

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    If the amp is designed right, this is the correct advice. You can bias up to 90% dissipation typically, but I tend to never go that far just to allow for the differences in modern tubes.

    With cathode bias the tubes should find their own point of equilibrium. There are some instances, especially with older Fender and Vox amps where that point of equilibrium can be over 100% dissipation. This is due to a)Wall voltages being higher these days b)The fact that the US isn't 110V (Vox) and c)The tubes back when these amps were designed were much stronger. In these cases, it is wise to increase the cathode resistor(s) valueand replace the cathode bypass caps.

    While cathode biased amps do protect from most cases of current runaway, they do idle the tubes harder and may find a weak tube much quicker.
     

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