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

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  • Yes. I'm aware of that. All JBL D series were opposite phase, unless they were reconed with an aftermarket kit. What's in question is how Fender wired them.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • Just for some more derailing, here is another "phase mystery" from the past:

      Starting from 1960 the magnet and signal polarity of Fender Strat PUs was reversed, so that older and newer PUs were no longer compatible.
      Now one might speculate about a relation to changed speaker phase...(just kidding).
      - Own Opinions Only -

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      • hi all, just to let you know I put the amp back together (without adding the ground connections to the speakers yet) and everything is workings fine.
        As I said earlier on the thread the only additional (good) symptom I see (hear, really) is no hum when engaging the reverb.

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        • I am still just curious about the REVERB VOLUME increase modification proposed by Chuck... and as I already have the needed resistors and caps I am most certainly going to give it a try.
          However, there is still something that can have an impact on the reverb sound and is something Fender changed when going from the MASTER VOLUME model (RIGHT Schematic) into the PUSH PULL MASTER VOLUME one (LEFT Schematic):




          What I currently have is the version on the LEFT (the 470 Ohm Resistor, though I have removed the 560 uF capacitor that appears faded in the pic).
          What change should I expect if I implement the schematic on the RIGHT (25 uF cap across the 2.2 K Resistor)?

          Comment


          • There are some principals involved with the two circuits you're comparing. I'm not studied in electronics or specific formulas involved, but I'll do my best in laymans terms...

            The circuit you have now uses a considerably hotter bias (DC relationship between the grid and cathode) than the earlier circuit. This has the affect of making the grid more sensitive to the input signal. The colder circuit can significantly improve gain with the bypass cap because the cathode elevation is much higher. But it also doesn't amplify as much to the same amount of signal at the grid. The seesaw action isn't obvious, but it's there.

            Since you've had to decrease grid drive to the reverb as part of the stabilizing effort I actually think it's a good idea to try the colder, but cathode bypassed circuit IF you also know that this may mean increasing grid drive again. Who knows anymore with the trouble you've been through. But maybe using the colder bias would allow you to increase grid drive to stock levels without instigating the problem again. Just work carefully so that the advantage you've gained doesn't get lost if you want to reverse any changes.
            "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


            • thx Chuck!!! great explanation

              only one reminder:
              Originally posted by Chuck H View Post
              Since you've had to decrease grid drive to the reverb as part of the stabilizing effort
              I am not currently using the voltage divider or anything else than what’s on the original schematics (with the exception of having removed that 560 pF cap going to point “A”)

              Comment


              • Ok. Good? So why the desire to change the reverb drive tube cathode circuit? If the amp isn't misbehaving anymore and the circuit isn't strangely altered, that's great. But what are you NOT getting from it? I'm thinking more reverb was hinted at. Do you need it or is it just a case of wanting the option of too much so you know you have enough Because that's TOTALLY me. Or is it the tone of the reverb? The character of the reverb (doesn't sound boingy enough, or whatever). This isn't just a repair forum. We've been doing mods here since I can remember (read- a long time). It's cool to ask about circuit differences if you just want to know what it means. If your amp is missing anything you should post about it. I'll get to the point...

                The amp to practice mods and learning on probably shouldn't be one you already like and rely on. I think a lot of members here have made that mistake. You get gear mod/tweak syndrome and before you know it you can't find your way back and, though you know you've lost something you're not savvy enough yet to pin it down and it's gone forever. Ok, maybe that's just me. But it's good advice.

                So, if you want your amp do something better than it does now, ask about it specifically. If you want to play with amps you should try to find success in every project and never risk undoing it.
                "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


                • Grid stoppers still on V3?
                  Is there still a master volume control?
                  My band:- http://www.youtube.com/user/RedwingBand

                  Comment


                  • Thanks Chuck and PDF... the circuit is in fact back to stock (as per Fender schematics, apart from the minor exception of the 560 uF cap not being there anymore) and everything is working well (the anomaly has disappeared and I keep my fingers crossed because it is not clear when / why it disappeared, however it was around the time in which I started questioning the speakers phase / polarity) [that means grid stoppers, voltage divider, etc... all gone]

                    I remember that back on post #86 there was a brief discussion on the topic and Helmholz expressed:

                    "I much prefer the BF version with 2.2k/25µ. Changing the cathode resistor to 470 Ohm unnecessarily increases V3 plate and transformer current by a factor of around three. It also strongly increases the driving power to the reverb transducer. I would expect more "crosstalk" with the 470 Ohm version."

                    I read "driving" more as "compressing/saturating" more... which is good because I like a "dense" reverb. If that is the case, I will leave it as it is with the 470 Ohm resistor (which is what the schematic for this specific model shows).

                    My question was more towards if that difference (470 vs 25/2.2) had any influence on the REVERB tone.

                    To the point made by Chuck "what are you NOT getting from it?" I would say, two things:

                    - I'd like a bit more of REVERB or more BRIGHTNESS in the REVERB (need to try both to decide). (Even though when I reverted to the original schematics by taking the voltage dividers out the REVERB recovered some volume, as expected)
                    - (point not discussed yet) I'd like a deeper VIBRATO (I always thought the tremolo was not reaching - let's say - 0 dB before coming bouncing back to the set volume even with the INTENSITY knob at 10)

                    Again, the amp is sounding great and it is back to stock. And what I am discussing is clearly a mod. I could gladly start a new thread however as the increase REVERB volume mod was something proposed during this discussion I wanted to ask about the 470 vs 25/2.2 versions before doing anything else.

                    As always, relying on your wisdom

                    Thx!

                    Comment


                    • Master vol?
                      My band:- http://www.youtube.com/user/RedwingBand

                      Comment


                      • Originally posted by pdf64 View Post
                        Master vol?
                        yessir, MASTER VOLUME was always in the circuit (what I removed a long time ago was the couple of wires providing the PULL ugly distortion... and never put them back)

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                        • That's how the switch should have been labeled, "PULL UGLY".

                          If you go back to page one... You posted a schematic that is accompanied by a "wiring diagram". Can you tell us if your amp circuits look to be consistent with either one. And if yes, which one? They are quite different.
                          "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


                          • hahahaha... sometimes “ugly” is good but i haven’t found a use for “that ugly” yet

                            quick answer: the schematic that reflects the current amp is the white one (that’s the original Fender one, god knows which good intentioned soul came up with the blue layout) with the two exceptions mentioned before:
                            - no 560 uF cap to point “A”
                            - no “PULL” wires

                            Comment


                            • Originally posted by TelRay View Post
                              with the two exceptions mentioned before:
                              - no 560 uF cap to point “A”
                              - no “PULL” wires
                              Ok. It's hard to say for sure from the schematic what remnants may be left over from the old pull switch. I know you removed the wires. did you remove anything else? There is likely other circuitry in the amp that's sole purpose was to facilitate switching to ugly mode and it detriments the clean mode (and reverb). If I point it out in the schematic, do you think you'd be able to locate it in the amp?
                              "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


                              • - I'd like a bit more of REVERB or more BRIGHTNESS in the REVERB (need to try both to decide). (Even though when I reverted to the original schematics by taking the voltage dividers out the REVERB recovered some volume, as expected)
                                The single most effective measure to increase reverb brightness is the .002µF cap wired across the 220k grid leak resistor of V4A (as mentioned several times) assuming that the circuit is completely back to stock. If that isn't enough I would try a different tank. Any BF or SF amp should have reverb in abundance.

                                While I am happy that the original problem (mysteriously) disappeared, this is one of the more frustrating threads to me as it is not clear what actually solved the problem (maybe a bad ground contact at one of the reverb jacks?) and all the well founded proposals turned out to be superfluous. Wasted a lot of time.

                                Lesson learned: Always check chassis ground contacts first.
                                - Own Opinions Only -

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