How do you know if your amp needs 'biasing'?

Discussion in 'Amps and Cabs [BG]' started by 110s lover, Apr 25, 2014.


  1. ?? New too tube amps...ive always been solid SS up until a few months ago falling for a Marshall.

    Do you only have to bias tube heads if the tube pack themselves in or if its a guitar amp and you want too convert it to a bass amp or? Educate me :D
     
  2. Hi.

    The "skinny" of it:

    Usually You need to check the bias if new (in that amp) power amp tubes are installed.

    There's many schools of thought whether used tubes should/could be biased.

    A complete explanation of biasing a tube (or SS) amps can be found from the old amplifier textbooks.

    Regards
    Sam
     
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    All amps need to be biased. Occasionally the bias needs to be adjusted as the amp ages. With tube amps, the small signal tubes, like the ones in the pre-amp are self biasing. You don't need to adjust the bias. When you change your power tubes or as they age you need to look at the boas. Some smaller tube amps are cathode biased. These are self biasing like the preamp tubes. Other amps are what is called fixed bias. These need to be biased when the tubes are changed. Ideally, the bias should also be checked during yearly maintenance.

    The bias sets the operating point of the tube, much like tuning a string to pitch. An optimal bias will allow the amp to perform as it was intended. Bias can be set hotter or colder than the optimal setting and that affects how the amp performs. Too cold and the amp can be flat and sterile sounding, too hot and it runs out of headroom and distorts sooner than it should. The bias can be checked and adjusted with a voltmeter. How this is done depends on the amp.

    There is some information on biasing here. The description is geared towards a B-15 but the general principles apply.
     
  4. Thanks for the information guys, I will definatley browse that link posted, cheers.
     
  5. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    All amps need biasing, but cathode bias ones do it themselves (within a range, sometimes you have to change a resistor as valve specs have a pretty wide tolerance now), fixed bias ones need the bias to be fixed in the right place manually. Biasing too hot makes valves not last, idle current at 70% of max dissipation is the standard aim, but with modern valves, if you aren't gonna have it checked up too often, going a bit colder like 60% is a fairly good idea, as the valves drift with age and it gives you some leeway, and they are pushed less hard. If you tinker with cars, its like setting the tickover.

    Edit: Generally SS output devices are consistent enough over their lifetime to not need adjustment, but it is still a thing in the amps, valves are jsut inconsistent and non linear, which is why they sound good.
     
  6. Coolhandjjl

    Coolhandjjl Supporting Member

    Oct 13, 2010
    Appleton
    If you are into vintage Hi-Fi, a 1975 solid state Marantz 250M Power Amp, for example, can have its DC offset and bias adjusted via internal trim pots. once every 20 years is probably sufficient.

    The Mesa Boogie Strategy 400 Power Amp is cold cathode biased, meaning as the tubes age, it self compensates due to the way the circuitry is designed. In fact, even a fresh set of tubes cannot be biased.

    The Fender 300T head does need to be biased, there are connections on the rear to insert multimter probes and adjust.

    So if you do wind up with an amp that requires biasing, no big deal. Lots of tutorials to learn how, or if it's not your thing, any competent amp tech can set it.
     
    Last edited: Apr 25, 2014
  7. FFTT

    FFTT

    Mar 15, 2009
    With fixed or adjustable bias amps, you need to check-adjust bias any time you change power tubes.
    Not necessary when you change preamp tubes.
    The power tubes should be matched as close as possible.

    In Cathode Biased amps, you just need to make sure the power tubes are matched.
     
  8. Coolhandjjl

    Coolhandjjl Supporting Member

    Oct 13, 2010
    Appleton
    +1

    Buy your tubes from a reputable source. Many of the better dealers sell packaged matched sets for most of the popular amps.
     
  9. I remember that beast. The old man had one of them in our house - still there, I think. Didn't know you could/had to re-bias those things. All these years I thought biasing was just associated with the tube/valve world.
     
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Those early Marantz units used Mosfet transistors in the power amps. Mosfets are closest to tubes in how they preform.

    A transistor and a tube are similar in that they both need to be biased to place them in the correct operating range. Just like tubes, if you change a power transistor, you need to check the bias. People being biased against tubes and transistors is something else.
     
  11. FFTT

    FFTT

    Mar 15, 2009
    My poor Marantz 2252-B is in dire need of thorough cleaning and overhaul.
    On the list.

    My tube sources are well vetted for quality and service .
     
  12. Coolhandjjl

    Coolhandjjl Supporting Member

    Oct 13, 2010
    Appleton
    I have two of 'em ;-) Let me know if he ever decides to part with it.


    I always knew there was something special about those amps. No wonder I like their sound so much.
     
  13. FFTT

    FFTT

    Mar 15, 2009
    I used my Marantz as the power amp to drive my Infinity SM-82s for monitors.
    I was also running my Pioneer dual cassette as a bounce deck through the Marantz and all tied to my Mackie 1604
    and my Tascam 488.

    I actually got amazing results all things considered.
     
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I have a 2252 that I bought around 1977. For most of those years it was never shut off. Still use it every day. Probably needs cleaning as well.
     
  15. FFTT

    FFTT

    Mar 15, 2009
    Exactly, you left it run 24/7 ready to track.

    Mine is just gathering dust in the basement, but "one day"!
     
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