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  • Musicman bias question

    Hi all: I have a Musicman 130 HD amp I am working on. On the schematic it says to adjust the bias pot to 1/2 volt AC at point "Y"
    There are 2ea "Y" points on the schematic
    Can this be right, "AC", or just a typo? I can attain 1/2 VDC at that point but not AC.
    The "Y" point is right off of pin 8 of 2 of the power tubes (EL34's)

    2475-130_and_2275-130.pdf

    Here is a different version of the schematic:

    musicman-2100-130-schematic.pdf

  • #2
    I can see how it's cornfusing. There's a 10 ohm resistor from cathodes of one pair to ground. And another 10 ohm resistor on the other pair of cathodes. And - - - they're connected together! So, effectively you're measuring voltage across a 5 ohm resistor. Half a volt translates to 100 milliamps when we apply the numbers to Ohms Law, V = IR . I being current in amps. Assuming each output tube is passing equal current that's 25 milliamps per tube. Makes sense for this application.

    Something you should be aware of, now 40 - 45 years after manufacture, most of those 10 ohm resistors have drifted. Typically I see 'em 10% high at 11 ohms each, or even more. It's worth measuring them, then applying Ohm's law. Your option to change 'em out to accurate 10 ohm resistors. Or a single 5 ohm. Generally I don't bother, just use the actual resistance at that point & do the calculation. When I do bother, I put in 1 ohm 1% 3 watt wirewounds on each tube, then it's easy to tell which tubes are running hot or cold. Plus the arithmetic is trivial: one millivolt per milliamp.
    This isn't the future I signed up for.

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    • #3
      I don't see AC, I see DC with a little dot after each letter. it is not clear, but I don;t read it as AC. In any case, a technician knows bias is a DC reading.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        The first questions I would ask is which version of the circuit is in the amp that you are working on. Specifically, does it have a vacuum tube circuit driving the output tubes or is it the solid state version. The bias adjustment procedure is different for each version. I want to be clear that I'm asking about the Phase Inverter / Driver circuit NOT the pre-amp section. Both versions have SS pre-amp circuitry.

        Proceed carefully with Music Man amps or things can really get messed up.

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        • #5
          And, make sure your meter is rated for it... My Fluke wasn't, when it was in high-power mode...

          Justin
          "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
          "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
          "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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          • #6
            Here's the factory info:
            Attached Files
            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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            • #7
              It actually says "at" point y. Those two point y's are the same. Those are connection points. HOWVER, I would suggest biasing MM's a little cooler than that. Say, 22-23mA PER TUBE. Go poke around the MM site for guidance. Intro Page Mike

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