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Setting the Bias on a Tube Amp

Discussion in 'Amps, Mics & Pickups [DB]' started by GregL, Oct 19, 2009.


  1. GregL

    GregL

    Oct 6, 2009
    How difficult is it to set the bias on a tube amp? Is it necessary after tubes are replaced (Traynor manual says "Yes").
    As a card carrying member for the criminally un-handy, I'll probably need a tech to do this. Are such techs easy to locate. I live near Philadelphia, PA.

    Thanks all.
     
  2. hdiddy

    hdiddy Official Forum Flunkee Supporting Member

    Mar 16, 2004
    Richmond, CA
    *crickets*
     
  3. symcbass

    symcbass

    Nov 8, 2007
    Scotland
    Make sure any Tech is reputable. I had one set the bias on my amp, and totally fried It. Amp came back to me sizzled, tech vanished, and I was left to have the amp totally repaired as It had been fried!!!!!Mucho expensivo!!!!!!!GRRRRRRRRRRRRRRRRRRRRRRRRRRRR!
     
  4. PB+J

    PB+J

    Mar 9, 2000
    arlington va
    It depends entirely on the amp. Some amps are "self biasing." some amps require biasing, and it has to be done by soldering resistors. Some amps have a potentiometer you can adjust, but you need a meter that measures the current to each tube. Some modern amps have little ports where you can plug a multimeter in.

    Tube amps store HUGE voltages--I would not attempt it unless you have a good idea what you are doing

    I also don't think it's necessary. Buy power tubes tat a re matched and forget about it.
     
  5. seamonkey

    seamonkey

    Aug 6, 2004
    HiFi Hobbyist constantly adjust the bias. Tubes wear and so need adjusting over time.
    The MarkBass Classic 300 is the only tube amp has a micro-controller that constantly biases the tubes. And let's you pick "Hi-Fi" versus "Long-life" bias settings. I'm guessing for double bass you'd want an amp biased "Hi-Fi" It might be hard to find a tech that knows how to. But check Craig's list - there seems to be plenty of hobbyist and clubs that are willing to help a guy out for a fair price.
     
  6. johnz

    johnz

    Apr 26, 2008
    Ashland, OR
    Hi Greg,

    PB&J is right in all particulars and especially about high voltages. If you don't know what you're doing, please don't polka mitten ze screwdriver nor twist-em mitten-ze pliers ;)

    I took a quick look here (http://www.lynx.bc.ca/~jc/traynorSchmArchive.html) and it appears that Traynor amps are pretty close to old Fender designs. When those amps were designed, we (er, the USA) produced GOBS of tubes. Nobody gave much thought to matching the tubes because the specs. were held so tight that it wasn't an issue. Not so much today. So, I also agree with PB&J, that you should get a matched set of tubes so you're assured that the tubes will have the same gain.

    But, the issue of bias is slightly different from the issue of matching tube gain. The reason you have two output tubes (depending on your amp, you may have to "pair" of output tubes) is so that each one only has to work half the time. One tube amplifies on positive excursions (from 0 up), while the other tube (or "pair) only amplifies on negative excursions (from 0 "down"). With this strategy you get a lot more power out of a given set of tubes because they are able to "rest" for half a cycle (this cuts down on the heat which limits the power output). But, you don't want the tube to turn fully off because it takes it a little time to turn on and that results in distortion. The bias adjustment sets how much the tubes stay on in absence of a signal. The manufacturer specifies a bias setting high enough to minimize the (cross-over) distortion and low enough no to fry the amp (sorry to hear about your experience SYNCBASS!). For what it's worth, in my experience with musical instrument amps, if the bias is adjustable, it generally applies to both tubes at once.

    My inclination is to set the bias because I have no faith that the gain spec of modern tubes is "close enough" to the ones manufactured when the amp was built.

    Setting the bias should take less than a 1/2 hour. The majority of the time will be in pulling the amp out of its case. The tech will put a meter on the bias test point in the circuit, turn on the amp, let it warm up for a minute or five and then tweek the bias pot until the meter reads the proper voltage. The amp goes back in its case and you're good to go. It's no big deal (assuming the tech can read a schematic :rolleyes:); but, once again, there are lethal voltages inside that chassis. So, please don't mess with it unless you know what you're doing.

    Sorry to be so wordy - I hope that helps more than it confuses!
     
  7. GregL

    GregL

    Oct 6, 2009
    Thank you John, thank you all. I spoke with Traynor, and their tech said if all else fails I could send the amp to them for tube biasing. No way I'm touching the insides of that amp. All I know about electricity is not to piss on an electric fence.

    The Traynor tech said the power tubes (4 of them) need to be biased, but not the pre-amp (2) or splitter tube (1). Said the power tubes can last a couple years, so I'll address that when it happens.

    I read that the Traynor amps are not super loud, but since I intend using this at home, in small venues, I don't think that's a problem. As you get older (63) you get nostalgic. Right now I'm taking my pictures with an old-fashioned 4X5 large format camera.

    BTW, I'm very impressed with the technical drawings Traynor makes available.
     

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