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Screaming Bright Switch Cap??? - 1974 Fender SF Twin Reverb Master Volume Push Pull Switch

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  • Is the reverb tank housing well grounded?

    I also recommend to ground the speakers' chassis.
    - Own Opinions Only -

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    • thank you man!

      Originally posted by Helmholtz View Post
      Is the reverb tank housing well grounded?
      verification method being... probe continuity between the reverb chassis and the amp head chassis?
      grounding is provided by the RCA plugs ring (connecting TANK to AMP HEAD), correct?

      Originally posted by Helmholtz View Post
      I also recommend to ground the speakers' chassis.
      any specific recommendations on how to do it?

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      • verification method being... probe continuity between the reverb chassis and the amp head chassis?
        grounding is provided by the RCA plugs ring (connecting TANK to AMP HEAD), correct?
        Correct. I never rely on old mechanical contacts.

        any specific recommendations on how to do it?
        I use a solder lug over one of the frame mounting screws secured by a serrated washer between lug and frame, solder a wire to the lug and connect to the grounded speaker terminal. And I verify contact by measuring.
        Last edited by Helmholtz; 10-07-2019, 08:29 PM.
        - Own Opinions Only -

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        • Originally posted by Helmholtz View Post
          I use a solder lug over one of the frame mounting screws secured by a serrated washer between lug and frame, solder a wire to the lug and connect to the grounded speaker terminal. And I verify contact by measuring.
          like this? and done independently for every single speaker, right?

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          • I'll add to the discussion: There are amp manufacturers that wire speaker polarity so that amp input phase matches amp output phase, assuming the user wants the phase relationship of input to output unchanged. So, the speaker can sometimes be wired + to ground to retain phase relationship between input and output. In this case, speaker wiring will depend on amp design- number of gain stages and other design choices.
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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            • Originally posted by TelRay View Post
              I'm starting to think this is...
              The harder a symptom is to troubleshoot, the harder the solution is to name unless it's stumbled upon and recognized in the debug process. Once fixed, there may be no way to reproduce. Bad solder joint? Bad component that was swapped out and trashed? Just make sure you wrap this up before Halloween

              ...pure witchcraft. :/
              edit: just remember the other 99,999 of this model worked more or less OK.
              If it still won't get loud enough, it's probably broken. - Steve Conner
              If the thing works, stop fixing it. - Enzo
              We need more chaos in music, in art... I'm here to make it. - Justin Thomas
              MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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              • Sometimes an amp just needs to know it's loved. Pay some attention to it and it starts behaving

                It happens. I once had the guys here chasing (what I thought was) an odd oscillation in one of my builds that turned out to be a cold solder joint that would only become intermittent at two frequencies. It wouldn't have been out of the realm of possibility that in the course of troubleshooting I could have resoldered that joint without knowing that a cold joint was what I was chasing. In that case the problem would have seemed to spontaneously disappear. That or I might have thought that whatever change I made while resoldering that joint had fixed the problem. Even though if I reversed it the problem wouldn't recur. See? Not witchcraft, just coincidence. The way I see it something about your speaker wiring was causing a problem. There's still potential for induced feedback loops even with the tank out or the reverb turned all the way down. It might have been how the leads were routed? Who knows. Short tale...

                On a rather complicated build, that just happened to be a 2x12 with reverb, I had a whistling oscillation when the reverb was turned above five. The only thing that fixed it was grounding the speaker frames. There may have been another solution or design flaw that wasn't diagnosed before I grounded the frames. I don't know and I don't much care. Once you figure in all the variables of induced and invisible circuits that exist within a guitar amp, which even for vintage designs is a very high gain circuit, there are so many electronic principals happening and interacting all at once and differently in any given circumstance that it's near impossible to account for everything ahead of time or even to know where pitfalls might be before they happen. As a guy that pretty much only builds his own designs that's my $.02 on witchcraft.
                "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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                • Originally posted by The Dude View Post
                  I'll add to the discussion: There are amp manufacturers that wire speaker polarity so that amp input phase matches amp output phase, assuming the user wants the phase relationship of input to output unchanged. So, the speaker can sometimes be wired + to ground to retain phase relationship between input and output. In this case, speaker wiring will depend on amp design- number of gain stages and other design choices.
                  thx man, I get that. I guess it might be useful to ask other 70's silverface twins how are the speakers wired


                  It might be that... witchcraft, lead dressing, cold solder joint, etc, etc. What I am afraid of is assembling this thing back together.... hahahahahahah

                  Just to make sure of what I was hearing as "good" sound I turned the VIBRATO Channel to 10 and made a video with LEFT Channel REVERB TANK disconnected and connected on the RIGHT channel. I could not hear any difference... which is a good thing



                  Just for info, the sound has not been manipulated in any way (no EQ, compression, etc)

                  Thiking about it, the only other difference I notice (apart from the anomaly not being reproduced) is the REVERB being quieter in terms of hum. I Remember in the past trying to reduce the hum by flipping it 180 degrees in the cabinet, moving it away from the speaker magnets and the amp's transformers, etc. Could that be pointing in the direction of TANK grounding as Helmoltz pointed out?

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                  • To the best of my recollection, Twins were/are always jack tip to speaker positive. Again, to the best of my recollection, the factory wired them that way regardless of speakers used in the amp.
                    Here's the rub. Some of the old Twins had factory installed JBL D120F speakers, which were "backwards" according to standard speaker terminal marking practice. In other words, a positive voltage applied to the positive terminal caused the speaker cone to move inward- not outward. This would indicate to me that Fender didn't care much about speaker phase as long as the speakers were in phase with each other.
                    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                    • like this? and done independently for every single speaker, right?
                      Not quite. Washer and lug should be on the flat edge of the frame under the nut. You can't have anything between speaker and baffle.
                      - Own Opinions Only -

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                      • Originally posted by Helmholtz View Post
                        I use a solder lug over one of the frame mounting screws secured by a serrated washer between lug and frame, solder a wire to the lug and connect to the grounded speaker terminal. And I verify contact by measuring.
                        Originally posted by Chuck H View Post
                        The only thing that fixed it was grounding the speaker frames.
                        This gets a little interesting as to whether we are talking ground, chassis, or common. As far as I know for this model, the speaker jack is not insulated from chassis. So Helmholtz method is essentially connecting speaker frames to chassis ground. But what if the jack was insulated? Would it be better to connect the frames to the neg. lugs, or to amp chassis?
                        Chuck, in the scenario you mentioned, did you 'ground' the frames to the spkr. neg. terminals, or to amp chassis?


                        Originally posted by The Dude View Post
                        Here's the rub. Some of the old Twins had factory installed JBL D120F speakers, which were "backwards" according to standard speaker terminal marking practice. In other words, a positive voltage applied to the positive terminal caused the speaker cone to move inward- not outward.
                        Maybe in those units they wired speaker plug tip to speaker neg. terminals like Telray found in this guy.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


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                        • Originally posted by g1 View Post
                          Chuck, in the scenario you mentioned, did you 'ground' the frames to the spkr. neg. terminals, or to amp chassis?
                          OT final, and therefor speaker negative (and now frames), are grounded to the chassis.
                          "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


                          • Originally posted by g1 View Post
                            ......Maybe in those units they wired speaker plug tip to speaker neg. terminals like Telray found in this guy.
                            I don't recall taking apart a plug on a Twin to see, but I do recall battery testing Twin cabinets with JBL's and the cones did indeed move inward with +battery to jack tip. I remember thinking it was odd that they didn't wire them differently. It's also possible that someone rewired the amps or changed speakers before I worked on them and inadvertently changed polarity.
                            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                            • Would it be better to connect the frames to the neg. lugs, or to amp chassis?
                              Always to amp chassis/ground. So it's essential to identify which speaker terminal connects to amp ground. The principle is to use the grounded frame for shielding the voice coil's electric field. It actually matters only (sometimes) in combo amps.
                              If none of the speaker leads is connected to amp ground (very rare) both speaker wires are floating and none of them can be used for frame grounding.

                              Interestingly some older speakers already had a riveted connection between the minus lug and the frame. This may seem a good idea but prohibited series wiring.
                              - Own Opinions Only -

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                              • I don't recall taking apart a plug on a Twin to see, but I do recall battery testing Twin cabinets with JBL's and the cones did indeed move inward with +battery to jack tip.
                                As already written I can confirm that (some?) JBL D-series speakers were opposite phase.
                                - Own Opinions Only -

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