Amp biased?

Discussion in 'Other Guitars, other instruments' started by telepathetic, Aug 21, 2011.

  1. telepathetic

    telepathetic Tele-Afflicted

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    As a young lad in the late 60's I'd occasionally replace tubes in an amp when needed. In the 70's I went solid state. When I returned to the world of tube amps 20 years ago I went to replace tubes and was told "You'll have to get the amp biased". I've paid to get this done a few times.
    My question is, what is biasing? My other question is, how do you do it and can you do it yourself?
     
  2. imsilly

    imsilly Friend of Leo's

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    Putting it a very nontechnical way it's adjusting the voltage supply to your power tubes so they operate correctly. If this is not done right the tube can overheat and be destroyed. Another problem is that you can also under supply the tube with voltage and it won't operate efficiently. You can use biasing to alter the overall tone of your amp as well.

    Many amps especially back in the day were cathode biased in the power section meaning they didn't require adjustment. Others were fixed bias that was non-adjustable. Most modern amps have a fixed adjustable bias, which requires someone to re-bias the amp when new power tubes are installed.

    You can bias amps at home if you have the equipment and the expertise. You can use special bias adapters that fit into your tube slots and then run to a multimeter. Some amps even have an outlet for a multimeter to help you bias yourself. A lot of the time a tech will just go inside the amp and do a few measurements while it's running to check the voltages are correct. Safer if you don't know what you are doing to leave it to a tech because in all those cases you are dealing with an amp fully powered up and with lethal voltages.
     
  3. vinman

    vinman Tele-Meister

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    I feel the same as you telepathetic. A tube went, you changed the tube and that was that. I didn't even know about setting up a guitar back then. I just changed pickups or replaced strings. If I knew then what I know now.....
     
  4. donh

    donh Tele-Afflicted

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    In the early 70s a sound company rented my bass rig for a Gotto Barbieri (now known by me forever after as Gotto the Barbarian) concert and the bass player blew it up. The tech that fixed it mentioned bias setting but had no real knowlege of it or (he then thought) the tools to measure it with. I asked about and found no-one who had a clue in the Ann Arbor area at the time, so I was left to my own devices.

    A number of months passed, the amp was working "ok", but I knew it used to be better and in frustration I got Messing About.

    sidebar: 'biasing' a tube amp is simply picking the amount the output tubes are turned on in their rest state. Not enough bias and they have a serious lag before they get going, too much bias and they burn up.

    So I was slowly turning the the bias control while playing the bass and observing the tubes. I noticed that in one direction the amp sounded like **** and in the other the tube would turn a nice cherry red but it sounded great! So I picked a middle ground where it sounded great but the tubes ran nice w/o the red plates.

    Then I kept learning and once I realised how to measure things, I measured discovered that I had set the bias perfectly. By ear and eye. It was a little hard on the tubes (please note that that set of tubes lasted fine for years after), but I ended up with a fine-sounding bass amp when no-one else could help me for love or money.

    To answer your question: I explained biasing a bit above, I use tools (a scope and a meter) but temper that with ears and eyes, annnd yes you can do it yourself if you are fearless and willing to accept the consequences of failure.
     
  5. andrewnelles

    andrewnelles Tele-Meister

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    +1 one on that, I've also set bias by ear, then months later measured it properly and found it to be within the acceptable range.
     
  6. gypsymoth

    gypsymoth Tele-Meister

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    +2 eyes and ears get you pretty right.

    standard warning, don't try this at home, kids.
     
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