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Twin Reverb - Amp circa 2005 Trem issues

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  • Twin Reverb - Amp circa 2005 Trem issues

    Hello folks,
    I have a cust that would like the Trem on this amp to be more like the 60's versions. The trem on this amp basically just turns the drive to the power tubes on and off in a rather sharp transition, instead of it being a slower more smooth slope-type transition from on to off (or lower volume).
    I really am too busy to take time to mess around with values in the ckt at this stage, so I thought I'd put it out there as I have to figure someone else has addressed this issue.
    So, anyone messed with this issue & found a solution?
    The trem ckt is similar to this Master volume version (this does not have master vol).
    See also scope shot: Upper is drive on 6L6 grid, lower is pulse to the neon bulb in the bug. Click image for larger version

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    Thanx, Glen
    Attached Files

  • #2
    I have found that the abrupt modulation waveform is often due to the characteristics of the LDR inside the light bug. When that is the case, changes to the other components in the trem circuit won't fix the issue. In those cases, I try different light bug assemblies. Sometimes, from different suppliers.

    Comment


    • #3
      Originally posted by Mars Amp Repair View Post
      Hello folks,
      I have a cust that would like the Trem on this amp to be more like the 60's versions. The trem on this amp basically just turns the drive to the power tubes on and off in a rather sharp transition, instead of it being a slower more smooth slope-type transition from on to off (or lower volume).
      I really am too busy to take time to mess around with values in the ckt at this stage, so I thought I'd put it out there as I have to figure someone else has addressed this issue.
      So, anyone messed with this issue & found a solution?
      The trem ckt is similar to this Master volume version (this does not have master vol).
      See also scope shot: Upper is drive on 6L6 grid, lower is pulse to the neon bulb in the bug. Click image for larger version

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ID:	993489
      Thanx, Glen
      looking at the schematics, replace the 15k resistor on the wiper of the balance control to 33K.

      Comment


      • #4
        Originally posted by sparkies View Post

        looking at the schematics, replace the 15k resistor on the wiper of the balance control to 33K.
        Thanx for the tip. Unfortunately, this amp is not exactly like the schemo I attached. I'll pursue the idea, tho.

        Comment


        • #5
          Your scope pic only shows one negative going pulse.
          Could you go to a larger time base setting to see the periodicity?

          I suspect the neon has gone up in breakdown voltage causing a discontinuous modulation with a long "dead time" between pulses..
          - Own Opinions Only -

          Comment


          • #6
            Originally posted by Mars Amp Repair View Post

            Thanx for the tip. Unfortunately, this amp is not exactly like the schemo I attached. I'll pursue the idea, tho.
            No problem. That resistor valid values are 12K to 56K depending on the opto that is used. I never knew why they didn't put that somewhere on their print.

            Comment


            • #7
              Originally posted by sparkies View Post

              looking at the schematics, replace the 15k resistor on the wiper of the balance control to 33K.
              What balance control, and how does it relate to the opto? The amp in question is from 2005, the schematics are for a silver face that the owner would prefer the trem circuit emulate.
              At least that's how I understand the issue. In either case, I don't see how altering the bias (idle current) would affect this style of tremolo circuit?
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #8
                Originally posted by g1 View Post
                What balance control, and how does it relate to the opto? The amp in question is from 2005, the schematics are for a silver face that the owner would prefer the trem circuit emulate.
                At least that's how I understand the issue. In either case, I don't see how altering the bias (idle current) would affect this style of tremolo circuit?
                Agreed. At first glance someone may think that the 15K resistor from the bias balance pot wiper has something to do with the tremolo because it seems to connect to the LDR in the light bug. However, the resistor is really just connected to ground and, as g1 points out, has nothing to do with the tremolo circuit.

                Comment


                • #9
                  Originally posted by Tom Phillips View Post
                  Agreed. At first glance someone may think that the 15K resistor from the bias balance pot wiper has something to do with the tremolo because it seems to connect to the LDR in the light bug.
                  I always hated how Fender drew that ground and some other grounds in the SF schematics. I can see how at quick glance you might presume it was bias wiggle style tremolo. But then there would be no opto.

                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #10
                    Originally posted by g1 View Post

                    What balance control, and how does it relate to the opto? The amp in question is from 2005, the schematics are for a silver face that the owner would prefer the trem circuit emulate.
                    At least that's how I understand the issue. In either case, I don't see how altering the bias (idle current) would affect this style of tremolo circuit?
                    The resistor on the wiper changes the signal amplitude in the vibrato circuit.
                    Last edited by sparkies; 02-09-2024, 01:58 AM.

                    Comment


                    • #11
                      Originally posted by Tom Phillips View Post
                      Agreed. At first glance someone may think that the 15K resistor from the bias balance pot wiper has something to do with the tremolo because it seems to connect to the LDR in the light bug. However, the resistor is really just connected to ground and, as g1 points out, has nothing to do with the tremolo circuit.
                      No, its the -50 bias voltage drop The real question is if he will need to change the 3.3K to something like 2.2K

                      Comment


                      • #12
                        Originally posted by sparkies View Post

                        The resistor on the wiper changes the signal amplitude in the vibrato circuit.
                        The wiper resistor goes directly to ground. I've circled it in red in this image. It has nothing to do with the vibrato circuit.

                        Click image for larger version

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                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #13
                          Originally posted by g1 View Post

                          The wiper resistor goes directly to ground. I've circled it in red in this image. It has nothing to do with the vibrato circuit.

                          Click image for larger version

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                          Its annoying they do have callouts on their schematic.

                          The blue bias volage to the vibrato.
                          Click image for larger version

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                          • #14
                            Originally posted by sparkies View Post

                            Its annoying they do have callouts on their schematic.

                            The blue bias volage to the vibrato.
                            That is the bias voltage only. That 4th pot terminal is a 'tap' where the bias is fed to the pot. The route to the 2M2 is just a 'kick' to get the vibrato running when using the footswitch. The vibrato action is at point 'B' of the intensity pot. It modulates the signal coming out of V1B on the way to the phase inverter.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • #15
                              The end of your blue line goes to ground, as g1 states. You can't have bias voltage at the same point as a ground.
                              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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