Announcement

Collapse
No announcement yet.

Mojotone Vibrolux 5f11 Kit: Tremolo Thumping

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Mojotone Vibrolux 5f11 Kit: Tremolo Thumping

    I am having a problem with a Mojotone Vibrolux 5f11 kit I built early last year. When I finished the amp everything worked just fine, including the tremolo. Recently, the tremolo has developed a thumping. It is present with nothing plugged in and the volume and tone controls set at the minimum. It increases in volume as the intensity control is turned up, and it increases in speed when the speed control is turned up. I have tried chopsticking the wires with no change. I have tried different pairs of power tubes and different 12ax7 tubes in place of V1 with no change. Voltages (excluding heaters) are as follows:

    V1

    pin 1: 213
    pin 2: 0.0
    pin 3: 1.61
    pin 6: 217
    pin 7: 0.02
    pin8: 1.69

    V2

    pin 1: 212
    pin 2: 0.0
    pin 3: 1.64
    pin 6: 257
    pin 7: 22
    pin8: 56.1

    V3

    pin 1: 0.0
    pin 3: 404
    pin 4: 407
    pin 5: -38
    pin 6: -38

    V4


    pin 1: 0.0
    pin 3: 404
    pin 4: 407
    pin 5: -38
    pin 6: -38

    I've attached the schematic and some photos of the wiring related to the tremolo. I would appreciate any help in tracking down the source of the problem.
    Thanks
    Attached Files

  • #2
    It could come from a problem in the filtering. If you have another 16 or 22uF/450V capacitor, you can install it in parallel on each node (starting with C) to test.
    You can also raise the bias electrolytic value eventually using another in parallel, making sure it is 50V or higher.

    Comment


    • #3
      Thanks for the reply, Pedro. I tried your suggestions, but there was no change.

      Comment


      • #4
        Can you turn the Tremelo Off with the footswitch?

        Comment


        • #5
          Yes. With the tremolo off, the thumping stops.

          Comment


          • #6
            So it appears to be an intensity or a speed issue.

            Comment


            • #7
              Maybe there’s a bad 0V connection, such that the oscillator and signal circuits are forced to share the same 0V return.
              My band:- http://www.youtube.com/user/RedwingBand

              Comment


              • #8
                Thanks Jazz P Bass. Yes. It seems so. Using a signal tracer, the thumping is present at pins 6 and 8 of V1 and at pin 3 of both 6V6 power tubes. That is without any signal at the inputs and with the volume and tone controls at the minimum.

                Comment


                • #9
                  Pdf64, thanks for your reply. I'm not sure what you mean. Is "OV" output valve? Can you explain what you mean by the oscillator and signal circuits sharing the same OV return? I've gone over the connections to all the pins on the 6v6 output tubes and all seen solid. Which connections would have to be dodgy to cause oscillator and signal circuits to share the same OV return? Again, thanks for your help.

                  Comment


                  • #10
                    Zero volts. Same thing as ground here. If the oscillator and signal circuits happen to share the same ground point, they can interact.

                    If you can hear the thump, then it will be visible on the power tube plates and other points along the signal path.

                    Do we have a schematic for this? I don't want to assume the Mojo project has the identical circuit to the original Fender.


                    FInd out how the thump is geting in. One path could be the trem itself coming through teh intensity pot and onto the power tube bias supply. Another path could be through grounds or power supply.

                    Turning up the intensity makes it stronger, but does turning that control to zero stop it?


                    There is a 0.1uf cap feeding the trem to the depth pot on the Fender drawing. If you disconnect that cap, then the trem effect should disappear. If so, then the trem path is involved. If the thump remains without that cap, then power supply or grounds are more likely the cause.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #11
                      Thanks, Enzo. The schematic is attached to the original post. Turning the intensity pot to zero does stop the thumping.
                      I lifted one end of the 0.1uf cap that feeds the trem to the depth pot and that stopped the thump as you suggested. I also tried moving the ground connection for the oscillator to different places away from the signal ground and it did not change or eliminate the thumping.

                      Comment


                      • #12
                        Got a scope?

                        So we know it is coming through the trem.

                        As you dial down the trem, does the thump go away at a certain point? excess trem signal can thump.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment


                        • #13
                          Are the power tubes reasonably well matched? Even just in regard of cathode current at idle.
                          My band:- http://www.youtube.com/user/RedwingBand

                          Comment


                          • #14
                            Enzo, I don't have a scope. The thump really follows the taper of the pot and disappears, along with the trem effect, near the minimum setting.

                            Pdf64, the tubes seem to be reasonably well matched: 18.2ma and 18.9 ma.

                            Comment


                            • #15
                              Try changing the bias capacitor out.
                              I would use a 50uf/ 50V.
                              (and back it off the resistor)

                              Comment

                              Working...
                              X