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70s Bassman 100 bias mod question

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  • 70s Bassman 100 bias mod question

    Customer wants to add a bias level adjustment to the CBS bias balance in this amp. What would be the way to go about this? Would replacing the 3.3K with a 5K or 10K pot do it?

    https://www.thetubestore.com/lib/the...-Schematic.pdf
    It's weird, because it WAS working fine.....

  • #2
    It would work, but I wouldn't do it that way. If the pot goes bad, you lose bias all together. I'd change the 15K for a pot and resistor. It's much safer. See this:

    https://robrobinette.com/Silverface_...t_Bias_Balance

    Edit: I should add, I wouldn't do it at all. There's nothing wrong with the circuit as is and there are plenty of old Fenders still out there running without bias modification. But if you must,.........
    Last edited by The Dude; 03-02-2019, 01:32 AM.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      I don't know if it's Mister Thud or Mister P. Bass who had the plan for both, assuming your chassis already has other holes added.

      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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      • #4
        Originally posted by The Dude View Post
        It would work, but I wouldn't do it that way. If the pot goes bad, you lose bias all together. I'd change the 15K for a pot and resistor. It's much safer. See this:

        https://robrobinette.com/Silverface_...t_Bias_Balance

        Edit: I should add, I wouldn't do it at all. There's nothing wrong with the circuit as is and there are plenty of old Fenders still out there running without bias modification. But if you must,.........
        If you wired the pot as a rheostat at least some bias voltage wouldn't be dependent on the wiper connection. Just thinking out loud.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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