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Thread: Biasing

  1. #1
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    Biasing

    O.k. A ll you audiophiles what is the procedure for biasing custom c amp want to change power tubes and I would like to learn procedure any help or advise would be well apreciated thanks .

  2. #2
    I'd give PRS a call to get instructions on which probe from a multimeter goes where, just to be on the safe side. Once you confirm how to insert the probes, it's just a matter of setting the correct amperage scale on the meter, and adjusting the trim controls on the amp's chassis. Takes a couple of minutes and you're done.

    It's literally that simple.

    I've done it with the insert points on a Two-Rock, but before advising anyone on a PRS amp, I'd want to call PRS because voltages are involved, and the insert points look a little different.
    I saw ten thousand talkers whose tongues were all broken...

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  3. #3
    Senior Member andy474x's Avatar
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    If your amp is similar to most other PRS amps I've seen, it will have two small, red, circular test points on the back, one for each power tube, and a common black test point between them. There will also be an adjustment pot, which looks like a standard input jack, but inside is a knob that will accept a small flathead screwdriver, this is for adjusting the bias. I have a cheap multimeter that I got at Radioshack, and the probes fit inside the test points perfectly, almost like an instrument cable jack. The red probe goes to the red points, and the black probe to the black point, although if you mix them up it doesn't really matter, it will just read negative instead of positive, for example, -35 mV instead of +35 mV.

    Warm the amp up on standby, and then turn it full on (make sure your speaker load is connected). Turn your multimeter on to the mV setting, and insert the probes into the test points and get your reading for each red bias point individually, leaving the black probe in the common black test point for both. They may be a few mV different depending on how well your tubes are matched, which is OK. Take the average of the two, and compare that to the PRS recommended spec for your amp. If it's off by more than a couple of mV, use a small, flathead screwdriver with a plastic handle to adjust the bias pot, until you get to the right readings.

    I always make sure to only handle the probes and screwdriver by their plastic coatings, just to be on the safe side so I don't get shocked.

    I'm no amp expert, but it seems to work and I haven't electrocuted myself yet. Which is pretty much the standard I live my life by.
    -I'm no expert, but it seems to work and I haven't electrocuted myself yet. Which is pretty much the standard I live by.

    SE Custom 24 25th Anniversary - SE Akesson+57/08's - SE 30 Head/Cab

  4. #4
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    Thanks for the info most appreciated

  5. #5
    Senior Member vchizzle's Avatar
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    Quote Originally Posted by andy474x View Post

    I'm no amp expert, but it seems to work and I haven't electrocuted myself yet. Which is pretty much the standard I live my life by.
    That should probably be your new signature.

  6. #6
    Senior Member andy474x's Avatar
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    Quote Originally Posted by vchizzle View Post
    That should probably be your new signature.
    Haha, I think you're right!
    -I'm no expert, but it seems to work and I haven't electrocuted myself yet. Which is pretty much the standard I live by.

    SE Custom 24 25th Anniversary - SE Akesson+57/08's - SE 30 Head/Cab

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