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

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  • I think a 1M trimmer could replace both resistors.
    My band:- http://www.youtube.com/user/RedwingBand

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    • TelRay, I misinterpreted post #351 to mean that you now had a 750k/250k voltage divider at the reverb driver input. That may clear up some of what I've been posting.

      As to the 30p to 47p capacitor, yes, that would be across the input voltage divider series resistor where Helmholtz suggested trying a 500k value.

      I wouldn't chase recovering reverb HF at the 1.5k/560p shunt across the reverb transformer primary. As I mentioned, it amounts to about 1dB cut at 10kHz. Maybe a tenth of a dB more if you remove the resistor all together. This SHOULD be insignificant. So if that circuit is helping to mitigate the problem as it is, leave it as it is.

      Your interpretation of the modification I outlined is spot on.
      "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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      • This could be significant because the 560k/1.5k shunt across the reverb transformer primary really doesn't dull the reverb tone significantly. It only demonstrates a 1dB attenuation at 10k!!! (assuming a typical 22k primary impedance on the reverb transformer)
        I wonder how you calculated/simulated this.

        Replacing the reverb transformer with a 22k resistor would give wrong results. The transformer reflects its load to the primary. The load is the reverb transducer, which is mostly inductive. So the load the tube sees is a large inductance in series with some resistance. The impedance of the inductive part rises proportional to frequency. So if primary impedance is 22k at 1kHz, it will theoretically be close to 220k at 10kHz.
        Last edited by Helmholtz; 09-05-2019, 01:53 PM.
        - Own Opinions Only -

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        • Ah! You're right. I'm totally aware of that too, but yes, I used a 22k resistance as the load. Substituting a 220k load (theoretical, but probably close enough) demonstrates a much more significant -3dB at 10kHz. Which will be slightly audible through a guitar speaker, but probably still significant as we know from experience.

          A 280pf cap rolls off 1dB at 10kHz. If it still squelches the oscillation that might help the loss of HF on the reverb a little.

          And, TelRay, increasing the resistance in series with that shunt capacitor actually rolls off MORE HF because it changes circuit impedance and lowers the knee frequency of the cap until the resistance value increases to the point where it's competing with the load. At which point the effectiveness of the circuit is lost. So I probably wouldn't try increasing that resistance to restore HF to the reverb.
          "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


          • In principle the RC circuit across the reverb transformer primary works like the tone control in a guitar albeit at a higher frequency, the transformer primary impedance replacing the frequency dependent impedance of the PU.
            Reducing resistance attenuates HF until the "tone cap" and the inductance develop a lower frequency resonant peak.

            As inductances (of primary, transducer, and leakage) and self-capacitance of the transformer (typically around 100pF) are not well known*, resonant frequencies and treble attenuation effects are hardly predictable, but I also assume that increasing the series R above 1.5k won't do much in the audio range. In any case you won't get more treble than without a shunting capacitor - except if the cap produces a "beneficial" resonant peak in the kHz range .

            *Main problem as my measurements show is that especially the primary inductance of the transformer, which shunts the reflected load, is not constant but strongly drops with increasing frequency.


            The strongest reverb treble increase I would expect from wiring a treble "bleed" capacitor across the upper voltage divider resistor, as Chuck suggested. But as he already noted, there is a risk that this might bring back the oscillation.
            Last edited by Helmholtz; 09-05-2019, 04:18 PM.
            - Own Opinions Only -

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            • Originally posted by Chuck H View Post
              increasing the resistance in series with that shunt capacitor actually rolls off MORE HF because it changes circuit impedance and lowers the knee frequency of the cap until the resistance value increases to the point where it's competing with the load. At which point the effectiveness of the circuit is lost. So I probably wouldn't try increasing that resistance to restore HF to the reverb.
              Clarification: the resistance never had a impact on the oscillation I was measuring with the oscilloscope (video on post# 322 with an without the cap and different R values up to 4 K), the 560 pF cap certainly did (same video on post #322)

              Initially I understood that, even if it did not help killing the oscillation (that little bump remaining where the oscillation was before) it would be beneficial to brighten the REVERB WET sound. That's why I put initially a 1.5K R and later a 5K (which is on the circuit today)

              ORIGINAL COMMENT:
              Originally posted by Chuck H View Post
              AI don't think the resistor is there to make the 560p circuit across the reverb transformer MORE efficient. It's there to make it less efficient so there's less HF loss in the reverb sound. In other words, if the bare 560p cap is doing the job maybe less of that would still do an adequate, but lesser job with less HF loss. It's a matter of compromise. The resistor facilitates the compromise.
              COMMENT AFTER VIDEO ON POST #322:
              Originally posted by Helmholtz View Post
              Alright, but I think it was worth a try. And you might like the reverb sound better with a series resistor
              I will be happy to remove the 5K Resistor it that is the consensus today.

              Thanks guys,

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              • Originally posted by TelRay View Post
                I will be happy to remove the 5K Resistor it that is the consensus today.
                More important... Try a lower value capacitor in that circuit. A gain of 2dB at 10kHz isn't much, but as long as the oscillation is still gone it can't hurt.
                "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


                • This is starting to rival the Trump threads. 367 posts and still cipherin'?
                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                  • It appears to be a labour of love. I admire TelRay's tenacity.
                    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
                      It appears to be a labour of love. I admire TelRay's tenacity.
                      that’s a nice way of saying: “this guy is a pain in the ass, but t least he knows what he wants”
                      hahahahaha...

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                      • Originally posted by TelRay View Post
                        that’s a nice way of saying: “this guy is a pain in the ass, but t least he knows what he wants”
                        hahahahaha...
                        Nah. I can think of a dozen threads started by new guys that just wanted to fix their amp which simply petered out at some point. Never to hear from that member again. There are other members here that have stuck around and can tolerate about a month on a project before the thread withers. Then a year or two later they start a new thread opening with "Well I'm just getting back to this project..."

                        Tenacity was surely what g1 meant And you're pretty fast on the uptake, so you're not a PITA. When you get more time to peruse more of the forum you'll see what I'm talking about. There are a lot of guys that ask questions and just ignore instruction if they don't already believe in the responders line of thinking. Even becoming rude insisting that they know better than us what the problem is, they just don't know how to fix it. Treating members of the forum like appliances that aren't responding correctly. (Pressing the buttons HARDER and saying "Why won't it work right!")
                        "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 Chuck H View Post
                          Treating members of the forum like appliances that aren't responding correctly. (Pressing the buttons HARDER and saying "Why won't it work right!")
                          Great. Now I have water all over my keyboard
                          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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                          • Exactly what Chuck said. Telray you are a shining example of how to get the most out of this place.
                            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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                            • thank you guys, really
                              i was kidding as i always felt paid attention to, listened to even when my knowledge is limited, etc, etc
                              your willingness to help is (capital) AWESOME, it’s been a great help and also a learning process
                              a forum is only as good as its people (both those looking for knowledge and those providing it) and you guys rock

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                              • hello y'all!

                                received my Mouser order and I was about to start the modifications described above. In the process, i wanted to set up the cabinet and the main board in a different way so I can do modifications and try them out quickly with everything (speakers+reverb tank) connected. In that process I disassembled the speaker plug (to add an extension cable) and... the Speakers polarity is inverted :/

                                This means: the WHITE cable in the photo below goes from the TIP of the speaker plug to the NEGATIVE speaker terminal, from which another WHITE cable wires it in parallel to the NEGATIVE terminal on other speaker (the black goes from RING to POSITIVE and to POSITIVE)

                                I would have expected TIP to POSITIVE speaker and then to the other speaker POSITIVE terminal.

                                Do you think this is just something I need to solve for the sake of a CONVENTION in the direction of the speakers movement or it could have had an effect on the poltergeist effects we have been discussing along this thread?

                                Last edited by TelRay; 10-05-2019, 10:44 PM.

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