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Issue reading voltage drop, OT CT to plate, for biasing output tubes

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  • #1

    Issue reading voltage drop, OT CT to plate, for biasing output tubes

    Hello Everyone

    I am biasing a SLO'ish amp - basically a SLO overdrive channel only with some different values throughout (for tone only) and everything seems fine, including voltages throughout. However, I am running into an issue when biasing the amp.

    With the amp off (of course), the output transformer CT to pin 3 shows proper reading for resistance, about 24Ω per side.
    Plate voltage is 494 VDC.
    Tubes are 5881 (x4)
    Max target bias is .032A per 5881 (or .064A per side)
    Bias voltage output at the moment is -55 VDC

    However, with the amp on, trying to measure the voltage drop between the OT CT and any of the plates will not offer a steady reading. It jumps around, seeming to stay within +/- .3 negative volts. So the reading seems to jump between -.85 V and -1.15 V, or there about. Obviously the difference is between good and not good bias settings.

    Any idea why this could happen? I wonder if it is some oscillation that I can't seem to see or find. If it means anything, there seems to be no aural indication of oscillation, so if it exists it must be outside the hearing range... but a few times I did get hints that there may be oscillation going on... i.e the signal sort of pulsed in and out when I shutdown the amp once, but I was close to the amp with guitar in hand and the chassis open faced up.

    Any insight or ideas welcome. Thanks!
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken
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  • #2
    Fixed. I must have an issue in the fx loop - once I moved it out of the circuit, the readings became stable.
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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    • #3
      It was mentioned here recently that it is a good idea to remove the PI tube when doing this type of bias measurement to prevent oscillation.
      Also, not sure why you are holding a guitar as biasing must be done with no signal applied.
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      ​

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      • #4
        Originally posted by g1 View Post
        Also, not sure why you are holding a guitar as biasing must be done with no signal applied.
        haha. No way, I have waaaay too much healthy fear to have a hand in a powered amp with a guitar around my shoulder... that was just testing the amp between bias readings.

        Removing the PI during a bias reading huh? I will have to research those posts... thanks for the heads up!!
        "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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        • #5
          Originally posted by Gtr0 View Post
          Removing the PI during a bias reading huh? I will have to research those posts... thanks for the heads up!!
          Here is the post I was referring to: https://music-electronics-forum.com/...l=1#post537786
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          ​

          Comment

          • #6
            Originally posted by g1 View Post
            Here is the post I was referring to: https://music-electronics-forum.com/...l=1#post537786
            Awesome. Thanks very much!
            "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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