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Thread: 70s Bassman 100 bias mod question

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    Supporting Member Randall's Avatar
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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

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    It's weird, because it WAS working fine.....

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    Supporting Member The Dude's Avatar
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    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,.........

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    Last edited by The Dude; 03-02-2019 at 03:32 AM.
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    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

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    Bent Member Chuck H's Avatar
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    Quote 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.

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