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plate resistor problem....?

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  • plate resistor problem....?

    this amp is a jcm900 model 2100 hi gain master volume amp....static noise comes through the speaker even with all volume controls turned off. does a bad preamp plate resistor cause noise like this with the volume controls turned down? wha might be the problem?

    thanks for your help

  • #2
    Have you changed all of the preamp tubes ? The last one I did that had this problem was that ill fated rectifier on the heaters. These amps have a little transformer buzz anyway but what your describing seems more problematic.


    • #3
      static/popping is still present with different tubes swapped into every position. and like i mentioned before, the noise is present with all the volume/gain/preamp controls set to zero.

      it seems that the noise eventually stops once everything gets good and hot, however.

      any ideas? all the resistors getting high voltages measure to spec.


      • #4
        the volume/gain controls have no effect on the static. Bass/Mid/Treb controls have no effect as well. Moving the presence control does effect the background brightness while the static is occuring


        • #5
          If the controls have no effect on the noise, then the source of the noise is located in the circuit AFTER the controls. You need to narrow it down even farther. One way to do this is to start pulling tubes one at a time. Pull the phase inverter tube and see if the noise goes away. If not, the source of the noise is after the PI; if it does, the source of the noise is before it. In that case put the PI tube back in and pull the next tube in line before the PI; if the noise does not go away then the source of the noise is located between that tube and the PI, if the noise does go awy, then the source is loicated before that tube. You can go backwards like that untill you further isolate where in the circuit the source of the noise is located.

          Then there are more steps you can take to narrow it down further.