Do you need to bias an amp with 1 preamp and 1 power tube?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by inscho, Mar 21, 2006.

  1. inscho

    inscho Senior Member

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    I am getting a epiphone valve jr. head that has a single 12ax7 and el84. does this need to be biased? or can I just switch out tubes worry free?

    thanks
     
  2. 6789

    6789 Member

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    Wait for responses from the experts, But I don't think you have to bias, I have a gibson GA5 les paul junior with one 12ax7 and one el84 and I don't need to bias. I used to have an amp with one 6L6 power tube that didn't need biasing.
     
  3. electroid

    electroid Member

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    There is a lot of incorrect information floating around about this issue. You are talking about a single ended class A amp that is probably cathode biased. That means that the amp is self biasing to a degree. You can switch out the 12AX7 with no worries and you can also switch out the EL84 without biasing but some EL84s run extra hot. If you happen to get one of those, it could overheat and short out. When buying replacement EL84s, ask for tubes with low bias current and you will probably be ok. Otherwise, watch for a glowing metal plate in the tube when you play the amp hard. If you see the gray plate in the tube getting a big orange spot on it when you play the amp hard, take the tube out look for a cooler-running replacement.
     
  4. songsmith1950

    songsmith1950 Member

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    Electroid is exactly right all the way around. Just make sure not to put a hot EL84 in the amp and it should be fine. Usually cathode bias amps do not really ever need a rebias at all.

    Tom

    Songsmith
     
  5. VintageJon

    VintageJon Member

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    Addemdum: MODERN cathode-biased amps for an EL84 usally don't need to be be re-biased, unless it's a VOX or Mesa design.

    In any event all you need is a DMM, a calculator and a little time to figure it out...

    -Jon
     
  6. inscho

    inscho Senior Member

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    thanks for the help :AOK
     

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