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Marshall JVM Stock and Satriani Variant Negative Feedback Question

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  • Marshall JVM Stock and Satriani Variant Negative Feedback Question

    Looking at both variants of the amp there is a cap bypassing the negative feedback resistor. 120p for the stock model and 100p for the Satriani model. This is not something I've seen on any other Marshall. Excuse my ignorance if I'm incorrect but that would seem to allow certain frequencies to bypass the resistor, massively increasing feedback above that threshold. What's its purpose?
    Last edited by greengriff; 02-14-2021, 12:42 PM. Reason: fix typo

  • #2
    it's called a speed up capacitor,its role is to stabilize the power amp against hi freq peaks especially with difficult signals like square waves,not much required in a guitar amp but for sure in hifi,that shows the designer has studied and wants the product to behave correctly all the time,which is typical of big brands mass producing,they always include filters in input and output to eliminate any inside or outside disturb potentially harmful.

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    • #3
      Thank you very much.

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      • #4
        Originally posted by greengriff View Post
        Looking at both variants of the amp there is a cap bypassing the negative feedback resistor. 120p for the stock model and 100p for the Satriani model. This is not something I've seen on any other Marshall. Excuse my ignorance if I'm incorrect but that would seem to allow certain frequencies to bypass the resistor, massively increasing feedback above that threshold. What's its purpose?
        SCHEMATICS??????????????????????????
        Juan Manuel Fahey

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        • #5
          Originally posted by J M Fahey View Post

          SCHEMATICS??????????????????????????
          Here's the JVM210. NFB resistor and cap down below PI tube on pg.2
          Attached Files
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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          • #6
            Here are the schematics for the JVM410H and the JVM410HJS model.
            Attached Files

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            • #7
              Do both amps use the same output transformer ? One schematic says D5118, but the other doesn't say.
              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
              REMEMBER: Everybody knows that smokin' ain't allowed in school !

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