Cathode bias vs fixed bias

Discussion in 'Amps and Cabs' started by strings4v, Jan 14, 2015.

  1. strings4v

    strings4v Member

    May 11, 2013
    So cathode bias is aka self bias if I am correct.... when you switch power tubes you don't have to bias plate voltage yourself.
    Now would you guys choose cathode bias amps (given that you don't have an ability to bias tubes by yourself) over fixed bias amps just for that reason?
    Most of the big glass amps need biasing when swapping tubes and such.
    Since I don't have a decent tech to take amps to locally, I wanted to know if this should be a consideration factor when buying an amp in the future..
  2. critter74

    critter74 Supporting Member

    Dec 13, 2008
    Your Girlfriends House, USA
    Well it's a lot more complex than that and not any old amp can be changed to cathode bias if it's fixed. Some can but some can't.

    In short- learn to bias. It's relatively easy especially if you buy one of the many tools like a Bias-Rite kit or the like. Just follow the instructions and safety guidelines.

    Or get your amp biased once using a set of tubes you know you like. Know what the rating for them is and then always order the same tube with the same rating (preferably from the same dealer as differnt vendors may have differnt ways of doing tube testing so can come up with slight differnt rating scales). Then it's just pop in a new set when needed. Not perfect as bias slips as tubes age, but it should be fine.
  3. Blue Strat

    Blue Strat Member

    Aug 2, 2002
    Sterling, VA (not far from Washington DC)
    +1. Just get a bias probe, 1000s of guitar players with no tech skills have learned to do this.
  4. saltydogg

    saltydogg Rock & Roll Enthusiast Gold Supporting Member

    Oct 18, 2008
    Morris County, NJ
    I eventually bought a set bias-probes, only because I didn't want to wait a month to get my amp back from the tech.

    Saves time and money, and it's not difficult.
  5. RJLII

    RJLII Member

    Aug 5, 2009
    Two of my amps are Cathode bias, one is fixed bias. I bought a bias probe and it takes 5 minutes to bias the tubes on the fixed bias amp if/when I swap them.
  6. Silent Sound

    Silent Sound Member

    Jun 8, 2012
    If you don't want to learn to bias an amp or mess with it, cathode bias would be the way to go. There are sometimes some sound differences between the two. Fixed bias amps tend to be a bit stiffer. The reason most big bottle amps are fixed bias is because you can get more power from a fixed bias amp. Big bottles and the parts necessary to run them generally cost more, so if you're building an amp with them, it's probably to gain more power.

    Personally, I'd just get the amp you like, and if it happens to be fixed bias, then get a cheap probe and learn to bias that amp. It's something you can learn to do in an afternoon.
  7. Guinness Lad

    Guinness Lad Silver Supporting Member

    Nov 11, 2004
    On top a mountain of Chocolate Chips
    If you have cathode bias and use random grades of tubes (rating wise) the amp will sound different depending on what tubes you select (even same manufacturer).

    It wont be easy at all to get consistent results unless you find a set you like and then by many backup pair of the same grade. For example groove tube el84 grade 4-6. Even this will vary a bit...
  8. teemuk

    teemuk Member

    Apr 21, 2009
    Cathode bias tries to establish the operating point automatically by generating the bias voltage across the cathode resistor. This works only if the tube falls within certain tolerance of current draw so cathode biasing doesn't neccessarily remove the need for bias adjustment. At least the bias should be checked when tubes are swapped and if you find a need to adjust bias it will usually be more difficult than with fixed bias amps that tend to have adjustment trimmers.

    No. Because there are other reasons too. (Due to largish cathode voltage) cathode bias always robs something away from maximum output power with given tubes, plate voltages and bias. So, one issue is efficiency, which is better with fixed bias. Second issue are very different clipping characteristics: Cathode biasing is less prone to bias shifts during grid conduction, so cathode biased circuits will generally have less crossover distortion in "deep overdrive" than fixed bias amps. Their clipping characteristics are also less abrupt so if you aim for "soft clipping" then cathode bias is a better choice than fixed bias. The cathode bias resistor also generates feedback signal to the cathode, and if not bypassed will introduce gain compression at signal peaks, further softening the clipping.

    So it's basically about choosing different clipping characteristics of the power stage as well as slightly different efficiencies of the amp.
  9. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

    Oct 14, 2005
    Berkeley, CA
    There are important differences in sound, feel, harmonic content and breakup between fixed/ajustable-bias and cathode-bias amps.

    I encourage you to buy an amp based upon its sonic character. It is easy to learn how to bias an amp, and the tools are available.

    - Thom
  10. teefus

    teefus Silver Supporting Member

    Mar 15, 2004
    there is a tone and feel difference between similarly tubed cathode and fixed bias amps. i wouldn't choose one because it is easier or less perceived maintenance.

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