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Bias help please.

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  • #1

    Bias help please.

    Hello all,
    I'm new to this forum so please go easy one me. I'm trying to bias my Guild Thunderbass and I'm having a bit of a hard time. Here is where I'm at:
    I was going to put a 1 ohm resistor between cathode and ground so I could easily measure the voltage drop. When I got inside I found a resistor already in there. I think it's 5.6 ohms (stripes are Green/Blue/Gold/Silver). When I measure across that I'm getting a reading of 435 on my multi meter. I know i want the bias to be around 38mv but I'm not sure what math needs to be done about the 435mv and 5.6 ohm situation. The plate voltage is 450 and I'm putting in 2 JJ 6L6's.
    Any help would be greatly appreciated. Thank you
    -Joe

    ...Am I correct in thinking that if I wanted 38 mv through a 5.6 ohm resistor I'd want a reading of roughly 213?
    Last edited by hurricanejoe; 12-23-2010, 06:04 PM.
    Tags: None
  • #2
    Green/Blue/Gold/Silver would be 0.56 ohms. Measure it with your DVM. It's hard to get a accurate reading, but you should get something close. Use ohm's law to figure out what voltage you would read at 38mA. E=I*R. 0.038 * .56 = 0.02128V
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      If the resistor colors are being read correctly, the value is 5.6ohms, silver is the tolerance 10%. Current is voltage/resistance (I=E/R) so the current is .07768. Kind of high, is the resistor for each tube?
      "In theory, there is no difference between theory and practice. In practice there is."
      - Yogi Berra

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      • #4
        There is one resistor for each tube and each tube has it's own bias trim.

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        • #5
          Ohm's Law Calculators

          Looks like .2128V
          "In theory, there is no difference between theory and practice. In practice there is."
          - Yogi Berra

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          • #6
            .2128 volts it what I would want to see on the DMM? Thanks.

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            • #7
              I've got no idea what the hell is happening here. maybe I'm reading the meter wrong. If i set it to 2000m (the next lowest setting on my meter is 200m) I'm getting the aforementioned reading of 435 across this resistor. I've adjusted the pots numerous times and it seems that they can't even drop anywhere low enough to get to 213. I'll just bite the bullet and take it somewhere.
              I have also noticed that the amp is making an intermittent popping sound now even with the stand by on!

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              • #8
                New Findings!!
                I found a schematic that says the amp should be biased to 0.414V. It also says that this rating is relevant to 117v line voltage and that the resistors I claim to be 5.6 are (or at least SHOULD HAVE BEEN) 6.8

                Given this, reading 435 seems a bit more acceptable.

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