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Fender custom shop 68 Twin reissue Nasty HF Oscillation

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  • Fender custom shop 68 Twin reissue Nasty HF Oscillation

    Hello everyone and thanks for having a look here,

    As the title says, I have a 68 Twin reissue Custom Shop on my bench, actually the 2nd one in two weeks with a similar problem of HF oscillation. This circuit is essentially the same as the 65 Twin reissue, except that they have paralleled the two input sections so the "custom" channel also has access to reverb and tremolo. Oscillation occurs first at around four on the volume control and appears as a heavy squiggle on a sine wave on the oscilloscope. Past this, there comes a point where the amp simply explodes into full oscillation and becomes unusable. The owner of the amp first noticed the problem as a sort of background noise to certain notes but actually didn't ever the play the amp very loud and may not have noticed the oscillation beforehand.

    The amps have grid stoppers on nearly every tube grid, preamp and output. I believe they must have noticed an issue in the factory and made some adjustments (This amp also has a reworking of the traditional tremolo section as in Europe LDRs are outlawed. It would have been nice if this was the source of some trouble, but as I removed the tube and the power to the LDR replacement circuit it isn't the case.)

    The source of the trouble appears to be at the tube used for mixing the reverb return and dry signals. Grounding the grid here removes most, but not all, of the oscillation. Also, when I removed the phase inverter and output tubes, oscillation still occurs at the anode of this tube. Leads from the PCB to the socket aren't particularly long, and there is a grid stopper of 10k on the grid of the mix as well as the reverb return! I have tried to reposition these wires (it worked on the first amp) but here there is no difference. As well, a different tube makes no difference. I also tried to snubber the grid with a capacitor from grid to ground, but this only helps with such large values that the amp would end up sounding completely flat.

    Obviously there is a gremlin in this amp, but I can't seem to find it! If anyone has some ideas, I would be most appreciative...
    Attached Files

  • #2
    What is the approximate oscillation frequency?

    What changes if you short R73 (10 Ohm). Or if you wire a 0.1µ cap in parallel with R73?

    What changes if you disconnect the reverb in/send cable?
    - Own Opinions Only -

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    • #3
      Originally posted by BiBi View Post
      This amp also has a reworking of the traditional tremolo section as in Europe LDRs are outlawed.
      LDR's outlawed in Europe? First I've heard of this. Anyone have additional info on this statement?
      Logic is an organized way of going wrong with confidence

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      • #4
        https://echa.europa.eu/substances-re...ed-under-reach
        See cadmium sulphide
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          Grid resistors in each triode are associated with the incorporation of the two modules that replace the neon/LDR. This happens in all since they started installing it eleven years ago.
          Have you checked the filter capacitors? In some units they degrade quickly and create strange problems.

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          • #6
            Originally posted by Pedro Vecino View Post
            Grid resistors in each triode are associated with the incorporation of the two modules that replace the neon/LDR. This happens in all since they started installing it eleven years ago.
            Have you checked the filter capacitors? In some units they degrade quickly and create strange problems.
            IC caps...if one fails it can easily lead to feedback.

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            • #7
              Agree with the above. I've had a few with this exact symptom. All were found to have at least one bad filter cap.
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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              • #8
                While you are in there: I'll bet that the power tube grid stoppers are on the board instead of the sockets. This works OK on the RI amps even with JJs, but not on the Tone Master, at least with JJs. So I'd short the ones on the board and add them to the sockets. Worst case scenario is that all should be well if anyone puts JJs in it.

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                • #9
                  its a feature, not a bug, as all twins have that oscillation.
                  As far its the resistor or capacitor of the feedback loop.

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                  • #10
                    I'm not sure how to approximate the oscillation frequency. It appears on the top of the sine waveform at around 4 on the volume control then gets larger until it explodes into all out oscillation.

                    Shorting R73 and bypassing both have no effect.

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                    • #11
                      I've now replaced the filter capacitors but that also has no effect. (I'm not exactly sure what the reasoning behind it is either)
                      I have also separated the common cathode connection between the reverb return and the mix, also to no effect.

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                      • #12
                        This amp is built in the traditional Fender style in that case: grid resistors go from pin 8 to pin 5, g2 resistors from 6 to 4

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                        • #13
                          no this is not a feature. It goes into full oscillation after 5 on the volume control. I know what you are talking about as all Fender designs are prone to oscillate, but this is really exceptional and unusable.

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                          • #14
                            Lastly, I have also tried tightening all bolts and connections to ground. No luck here either.

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                            • #15
                              Also, when I removed the phase inverter and output tubes, oscillation still occurs at the anode of this tube.
                              This means that the oscillation is produced before the power amp.

                              One of the usual suspects is the reverb driver V3. Does the oscillation stop when you pull it?
                              - Own Opinions Only -

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