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Mig 50 cathode follower bias

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  • #1

    Mig 50 cathode follower bias

    My friend has a Mig 50 that he says is way to loud in both channels when volume is on ONE and mostly loud overdriven noise in one of the channels. Looking at the schem I found on the net got me wondering-
    How does this cathode follower get biased or is the schem incorrect? I will be looking at the amp for on Sunday. If anyone has a hint, please let me know. Thanks.
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  • #2
    Check this out:

    The Valve Wizard

    I don't think the cathode follower is the problem. It contributes no gain and FWIW the same exact circuit has been used on millions of perfectly functional guitar amps. Unless it's broken it should be fine just how it is.

    Mig 50's are popular mod platforms. Perhaps this one has had it's gain raised to where it's almost impossible to get a clean tone. Other than that it's possible that the volume pot is the wrong taper for any number of reasons.

    Compare the amp to the schem to see if it's been modified and make sure the volume controls aren't linear taper. You really do want A10 or A15 pots in there but some designers and builders have been known to use a linear taper for this circuit because it behaves more like what the original Marshall amps.

    If your guy is looking for a clean tone he's way off ideal with that amp anyway.
    "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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    • #3
      Most likely someone stuck a linear pot in place of an audio taper pot somewhere. In regards to the cathode follower... An interesting fact is that, if it were not direct coupled and actually biased properly, you would have MORE signal going to the phase inverter. It also generates small amounts of compression (which is the reason for the lowered output), but you can actually make the cathode resistor of the cathode follower section larger to somewhat negate this effect.

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      • #4
        Originally posted by Chuck H View Post
        Check this out:

        The Valve Wizard

        I don't think the cathode follower is the problem. It contributes no gain and FWIW the same exact circuit has been used on millions of perfectly functional guitar amps. Unless it's broken it should be fine just how it is.
        Chuck, Thanks for the link & tips. I would have never thought of the current stealing & compression- I like it!

        exclamationmark- Thanks for the tips, I'll check the pots first.

        Scott

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