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68/69 Marshall plexi Superbass choke

Discussion in 'Marshall Amps' started by Adam Rogers, Jun 11, 2021.

  1. Adam Rogers

    Adam Rogers New Member

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    Anyone know what the typical ohm values are for a late 68 early 69 Superbass? Getting what I think are pretty low readings.
     
  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Ohm values aren't what matters. You need to be able to read the inductance.

    No worries, inductance meters are cheap enough. 20 bucks will get you a usable LCR meter.
     
  3. Adam Rogers

    Adam Rogers New Member

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    Ok. My tech tells me the readings are low compared to what he's seen. perhaps I'm describing it wrong but I think he mentioned 32 ohms(?)
     
  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    What you want is probably a 3 or 5 Henry choke. Don't go by measured resistance.

    Check with Classic Tone or Mercury Magnetics.
     
  5. Adam Rogers

    Adam Rogers New Member

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    Cool. Thanks much.
     
  6. Adam Rogers

    Adam Rogers New Member

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    and what would the effects be of a shorted choke? there isn't any noticeable background noise.
     
    Last edited: Jun 13, 2021
  7. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Is the tech measuring the resistance with the choke removed electrically from the circuit? The choke should be about 105 ohms but if it’s in the circuit, the power transformer windings and the balance resistors across the screen filter capacitors are parallel resistance and will reduce the measured resistance.
     
  8. Matthews Guitars

    Matthews Guitars Well-Known Member

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    With a shorted choke, for one, the choke should run hot. But it'll cause all measured voltages in the amp to be higher than spec.

    Some amps don't have chokes. I've heard of some amps being modified to remove the choke. But if that's done then a ballast resistor needs to be added to control inrush current on power-up.
     

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