Did I get the right multimeter to bias my Allen Old Flame?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Phil M, Aug 10, 2006.

  1. Phil M

    Phil M Member

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    I know some of you guys have done this so I wanted to check in; I'm pretty sure I can use this one that I bought at Radio Shack. It's a "15-Range Digital Multimeter. Measures AC and DC voltage up to 500V, DC current up to 200mA and resistance up to 2MΩ"

    I have bought some Tung Sol 5881s that I will be replacing the old 5881s with.

    Thanks.
     
  2. tonezoneonline

    tonezoneonline Member

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  3. VaughnC

    VaughnC Supporting Member

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    Assuming the amp is not cathode biased, if the amp has 1 ohm resistors in the in the power tubes cathode circuit you'll be ok. You need to measure the idle voltage across these resistors (1mv=1ma) and calculate what the safe maximum current draw is for that particular circuit & tubes used. The actual bias voltage applied to the tube grids won't tell you much about what the tubes are actually doing.

    Otherwise, you'll have to either install the 1 ohm resistors or buy a bias meter that has adapters (that have 1 ohm resistors built in) that insert between the output tubes and their respective sockets. An actual bias meter is safer for the non-techie types because it doesn't expose one to the the high volatge inside the chassis.
     
  4. dbeeman

    dbeeman Gold Supporting Member

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    At least the newer old flames have bias test points on the back. They are "fixed" bias.

    Phil should be fine with the Ratschack meter. Just be sure to let tha amp warm up for 5 to 10 minutes before setting the bias and check it again after a few hours of playing.
     
  5. Dave C

    Dave C Gold Supporting Member

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    As the other posters have noted .....you'll probably be fine with this meter , but if you're going to do much amp work you'll need a meter that can handle more than 500v. The plate voltage on certain old Marshalls ,Musicman, Ampeg, Hiwatt and maybe Orange amps can definitely exceed the capacity of your meter.
    Dave C
     
  6. Phil M

    Phil M Member

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    I was only planning on doing my Allen (and perhaps future Allens since I like this one so much). I figured this would be easy since the jacks are on the outside and there are instructions in another thread. http://www.thegearpage.net/board/showthread.php?t=154449&highlight=Allen+Bias

    It would only be $10 to get the next model up, but I don't know if I'll ever bias any other amps ...

    Thanks.
     

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