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Fender custom shop 68 Twin reissue Nasty HF Oscillation

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  • #16
    Helmholtz asked earlier what happens when you unplug the reverb send to the tank. I'm just repeating the request so it doesn't get missed. You could also unplug the reverb drive tube.

    EDIT: Ah... Helmholtz was reposting at the same time.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #17
      thanks for the replies...

      yes the oscillation certainly occurs before the power amp.
      Pulling the reverb driver ECC81 doesn't help neither does pulling the opposite ECC83 from the other channel.

      included are some pics of the oscillations at different frequencies before it goes completely haywire.
      Attached Files

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      • #18
        Gut pics?
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment


        • #19
          If this model has the big capacitor on the board near the preamp, did you replace that one too? Or just the caps in the doghouse?
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

          Comment


          • #20
            so here are the guts...

            obviously this is after some changes I made like separating common cathodes, redressing the wiring from the power transformer, etc.
            nothing has hurt so far but nothing has helped either
            Attached Files

            Comment


            • #21
              all the filter caps have been replaced.

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              • #22
                neither does pulling the opposite ECC83 from the other channel.
                Which tube is this in the schematic?

                At which circuit point were the scope pics taken? What was the time base setting (horizontal deflection coefficient)?
                - Own Opinions Only -

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                • #23
                  Helm is considering the same thing I am. Divide and conquer. Trace the signal path to find the first place in the circuit where the oscillation shows itself. Oh, and the scope settings.
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "If you're not interested in opinions and the experience of others, why even start a thread?
                  You can't just expect consent." Helmholtz

                  Comment


                  • #24
                    Originally posted by BiBi View Post
                    so here are the guts...

                    obviously this is after some changes I made like separating common cathodes, redressing the wiring from the power transformer, etc.
                    nothing has hurt so far but nothing has helped either
                    If you actually replaced C34 you obviously used the same Illinois cap crap as before (sorry Illinois people, love your area and tradition).
                    - Own Opinions Only -

                    Comment


                    • #25
                      Originally posted by Helmholtz View Post
                      If you actually replaced C34 you obviously used the same Illinois cap crap as before (sorry Illinois people, love your area and tradition).
                      It's alright. I don't think they're made there
                      "Take two placebos, works twice as well." Enzo

                      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

                      Comment


                      • #26
                        I don't think they're made there
                        Doesn't matter, headquarters are responsible for product quality from all production sites. I know that you can get identical quality out of anywhere if you care, invest and check. Quality management is the key - and installing qualified/experienced quality managers with direct reporting. Also quality improves if the factory is charged with after sales costs (closed feedback loop).
                        - Own Opinions Only -

                        Comment


                        • #27
                          Originally posted by Helmholtz View Post
                          Doesn't matter, headquarters are responsible for product quality from all production sites. I know that you can get identical quality out of anywhere if you care, invest and check. Quality management is the key - and installing qualified/experienced quality managers with direct reporting. Also quality improves if the factory is charged with after sales costs (closed feedback loop).
                          Absolutely. I think the humor I intended must have been missed. And yes, I've had enough experience with Illinois caps that I avoid them. Maybe they make some really good ones, but those aren't going into any guitar amplifiers I'm aware of.
                          "Take two placebos, works twice as well." Enzo

                          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                          "If you're not interested in opinions and the experience of others, why even start a thread?
                          You can't just expect consent." Helmholtz

                          Comment


                          • #28
                            I think the humor I intended must have been missed.
                            I think I got it. But it's such a shame that a US (Illinois) based company with a long tradition got that bad a reputation.
                            Last edited by Helmholtz; 06-15-2019, 10:18 PM.
                            - Own Opinions Only -

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                            • #29
                              Originally posted by Helmholtz View Post
                              I think I got it. But it's such a shame that a US (Illinois) based company with a long tradition got that bad a reputation.
                              Hopefully that reputation is just with us guitar amp guys. By that I mean that it's the amp manufacturers that choose what capacitor to pay for. If a long standing, reputable company gives you the lowest bid that doesn't automatically mean your getting their the product that earned them that reputation. I can go to the local deli and buy a grilled Ruben and it'll be delicious. Or, at the same deli I can save some money and just get the bologna sandwich.
                              "Take two placebos, works twice as well." Enzo

                              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                              "If you're not interested in opinions and the experience of others, why even start a thread?
                              You can't just expect consent." Helmholtz

                              Comment


                              • #30
                                Thanks for the heads up on the Illinois caps-- I had one 22/500 from them still for some reason, but I didn't realize they had such a bad reputation.
                                I replaced it with another and it didn't help however.

                                By the other channel ECC83, I meant I can pull V1 while using the "vibrato" channel or V2 while using the "normal" channel and there is no change in the oscillation.

                                Scope setting was at ,1ms for the photos as I started at around 40Hz on the generator. The scope was connected directly to the dummy load (directly from the amp output jack is the same)
                                Like I wrote earlier, I am quite certain the problem is situated around the grid of V4b. The previous amp's problem was corrected here while and I can remove the following tubes and still see
                                oscillation on V4b's anode. Grounding V4a's grid and separating the cathodes here have not shown any improvement.

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