Is biasing really necessary?

Discussion in 'Amps and Cabs' started by coyoteblue, Nov 16, 2005.


  1. coyoteblue

    coyoteblue Member

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    I'm wondering how important biasing an amp is? I know it's not a big deal to pay to get it done, but if you have a matched pair of power tubes, say JJ's, what does it matter? Is the difference in tone noticeable? Do unbiased tubes damage the amp over the long haul? Does a Princeton Reverb need it? Some enlightenment would help. Thanks!
     
  2. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Yes.
    No.
    Maybe?!?

    Not necessary for single-ended amps, nor for cathode-biased amps (to a degree). It is necessary for all other power amps, with the "sort of" exception of fixed-bias amps, such as Mesa/Boogie (where you have to buy power tubes that will operate within their bias specs, or, as I've done, get the amp modded to be adjustable).

    Why? On the more urgent side of things, you don't want your power tubes to start redplating, then possibly take out some expensive amp components as they fail catastrophically. On the less urgent side of things - you are able to optimize your power section to your taste (hotter or cooler bias settings).

    The mis-matched pairs of power tube question is more complicated, and I'm sure others here have more experience, so I'll let them answer. Just don't let those tubes redplate!

    - T
     
  3. coyoteblue

    coyoteblue Member

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    Thanks for the advice. What do you mean by a "single-ended" amp? Never heard that term before.
     
  4. bob-i

    bob-i Member

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    Single ended means only one power tube. Push Pull means one power tube pushes the speaker, one pulls it back. Push Pull and be in pairs or even 3 tubes per side. Single ended amps are class A, push pull can be class A, class A/B class B or class C. Most commly PP amps are Class A/B.

    Now where Timbre Wolf is close, he's not technically correct.

    There are 2 main ways to bias a tube. Fixed bias meaning it's a constant voltage, and cathode bias, meaning the cathode supplies the voltage. Some fixed bias amps are adjustable, so when the tubes are replaced you should at least check the bias to make sure it's within range. This has nothing to do with single ended or push pull.

    With more fixed bias amps, adjustable or non-adjustable it's a best practice to chack the bias when changing tubes. If you buy tubes that are within the manufacture specs, it may not be necessary.

    If a tube is bias too hot, too much current, it will run hot shorten the life of the tube. If the tube is biased too cold, it will sound lifeless and dry, possibly even create some harsh distortion.

    I hope that helps.
     
  5. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    "Single-ended" means that there is a single power tube, like the old Fender Champ, THD Univalve, Emery Sound Micro- or SuperBaby, Lexicon Signature 284 (to name a few). Here's another compelling amp, with two power tubes in parallel single-ended output configuration: Electrosonic Neptune.

    Just saw bob-i's response (thanks for clarifying for me). I've never heard of push-pull amps with 3 tubes per side - what are some examples?

    - T
     
  6. bob-i

    bob-i Member

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    The Ampeg SVT for one, 6 - 6550's for 300 Watts RMS. There have been others. Most really high powered amps are SS now, there's just no need for a tube amp that powerful.
     
  7. coyoteblue

    coyoteblue Member

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    Does a Princeton Reverb need biasing? At Fender Amp Field Guide the bias of the PR is described as "fixed, nonadjustable" but people do get this amp biased. I just bought two new JJ's rated at 31 mA. Would this fit the range for a PR? Thanks.
     
  8. bob-i

    bob-i Member

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    Personally I'd check it. The Princeton Reverb was engineered for RCA 6V6's. JJ's are quite different.

    That said, put the tubes in, power up the amp and watch in a darkened room. If the plates turn RED, shut it off. If they get a little blue or you just see the orange glow of the heaters you're good.

    This isn't to say that it's the best it will ever be, but it'll be safe and you won't overheat the tubes.
     
  9. Blue Strat

    Blue Strat Member

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    The rating on the tubes is irrelevant unless you've already tested a pair of tubes with the same rating, from the same dealer (every dealer uses different test equipment and/or rating systems) and found them to be in spec FOR THAT AMP.

    The only way to know if the bias is ok is to check the particular tubes in the particular amp.

    The answer the Princeton bias question is YES. It's a fixed bias amp and ALL fixed bias amps need to be checked/adjusted. Some amps are adjustable without modification, others aren't. Princetons don't have a bias pot but this in no way indicates that you shouldn't check the bias and adjust it (by using a different bias set resistor or having a bias pot installed) if necessary.
     
  10. coyoteblue

    coyoteblue Member

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    I've got my answer and it's off to the shop to get my PR biased...thanks for the clarification.
     
  11. doug reichert

    doug reichert Member

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    It's a '73 100 watt. It has had a master volume added, but I don't know anything about the circuit or if even matters. I bought a set of JJ's from Tubes And More. I won't have time to get it into the shop before an upcoming gig, but I'd like to get the new tubes in prior to that. Will it trash the tubes or anything else in this particular amp to run them for a few hours without biasing?
     
  12. Tapp

    Tapp Member

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    Yes, you should get it checked. If you absolutely don't have time to get it checked then look at the tubes (with the amp warmed up at volume) in the back and make sure they're not red plating. Inside the tube you'll see a grey or black metal structure. If the tubes are drawing too much current then sometimes small red circles will form on the plates (hot spots) and this will surely be a short life for those tubes. Keep in mind though, you could still be cooking your tubes without red plating so the best option is to have the amp checked.

    Tapp
     
  13. riffmeister

    riffmeister Gold Supporting Member

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    "Is biasing really necessary?"

    No, and brushing your teeth is not really necessay, either. ;)
     
  14. minesaguinness

    minesaguinness Member

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    +1 The Princeton Reverb is a non-cathode biased amp (meaning it is not self biasing and the cathodes are connected directly to ground). HOwever, in original form there is no bias adjust potentiometer in the amp.. so the 6V6 GT power valves (tubes) need to be within the original spec.... a good amp tech can advise.

    Cheers
    Brian
     

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