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

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  • I am surprized not to see any 120Hz ripple. What is the peak-to-peak signal voltage?

    Are you sure you measured at point A (in little square), the "448V" supply that connects to the 20µ/500V filter cap and the TR2 choke?
    Last edited by Helmholtz; 05-10-2019, 02:01 PM.
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    • From your measurements:

      Gcl=7.78/1.9=4.1 ( you used the FB voltage by mistake)
      Go=7.23
      Go/Gcl=1.76

      Means that Go is 76% or 4.9dB higher than Gcl.

      The scope signals only show a Go/Gcl ratio of roughly 4/3. The reason for this is that the NFB signal adds to the input signal and increases the apparent Gcl. This effect should reduce at full MV setting.

      In fact with this MV wiring the effective feedback ratio and Gcl change with the MV setting and the theoretical formulae can't work properly as they assume the same PI input signal with and without NFB.
      Last edited by Helmholtz; 05-10-2019, 04:12 PM.
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      • Originally posted by Helmholtz View Post
        Means that Go is 76% or 4.9dB higher than Gcl.
        Thx man for the re-calculation and correction

        Does this close the NFB chapter of this "book" (post)? With the following conclusion:

        - NFB is working
        - the effect of disconnecting the 820R will be only noticeable at close to 10 MV settings

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        • Originally posted by Helmholtz View Post
          I am surprized not to see any 120Hz ripple. What is the peak-to-peak signal voltage?
          Are you sure you measured at point A (in little square), the "448V" supply that connects to the 20µ/500V filter cap and the TR2 choke?
          If you guys have learnt anything about me in the past 256 posts is that: I am not sure of anything
          But certainly more sure than on post #1 and thanks to your support.

          To answer your question, I did place the sope right here:



          With the exception that... many posts ago @ #85 we decided to eliminate the 560 pF cap connecting V3 pin 1-6 to "A" in favor of the "blackface" 2.2K / 25uF 25V arrange replacing the 470R connecting V3 3-8 to ground (compliant with Fender DWG 010182 Rev C, which is the one before the addition of the PULL BOOST to the MV)

          (old photo with still original components, for illustration only)



          the scope settings were:



          the SCOPE was set at 20 mV/DIV so I think the answer to your question is 4.75 DIV * 20 mV/DIV = 95 mV (after turning the VIB CH VOL up to 10, biggest waveform)

          is that right?

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          • Originally posted by TelRay View Post
            Thx man for the re-calculation and correction

            Does this close the NFB chapter of this "book" (post)? With the following conclusion:

            - NFB is working
            - the effect of disconnecting the 820R will be only noticeable at close to 10 MV settings
            In essence, yes.

            What is the max output power into the 4 Ohm dummy load just before clipping?
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            • Originally posted by TelRay View Post
              If you guys have learnt anything about me in the past 256 posts is that: I am not sure of anything
              But certainly more sure than on post #1 and thanks to your support.

              To answer your question, I did place the sope right here:



              With the exception that... many posts ago @ #85 we decided to eliminate the 560 pF cap connecting V3 pin 1-6 to "A" in favor of the "blackface" 2.2K / 25uF 25V arrange replacing the 470R connecting V3 3-8 to ground (compliant with Fender DWG 010182 Rev C, which is the one before the addition of the PULL BOOST to the MV)

              (old photo with still original components, for illustration only)



              the scope settings were:



              the SCOPE was set at 20 mV/DIV so I think the answer to your question is 4.75 DIV * 20 mV/DIV = 95 mV (after turning the VIB CH VOL up to 10, biggest waveform)

              is that right?
              Point A is correct. Can't read the scope settings from your picture (always make sure that the red var(iable) knobs are in their locked (=calibrated) positions).

              Two proposals:

              1)Reinstall the 560p cap and see what happens to the HF. The cap acts as the simplest version of a snubber. It might be necessary to wire a resistor of some k in series.

              2) Solder an additional (good) E-cap of 20µ/500V between point A (positive) and the ground side of the V3 cathode resistor. Ideally there should be no signal voltage at point A.
              Last edited by Helmholtz; 05-10-2019, 05:59 PM.
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              • Originally posted by Helmholtz View Post
                1)Reinstall the 560p cap and see what happens to the HF. The cap acts as the simplest version of a snubber. It might be necessary to wire a resistor of some k in series.
                Mission #1 completed!

                this is the "beneficial" effect of the 560 pF cap, installed as per the drawing corresponding to this amp's schematic seen with the SCOPE connected to the SPKR OUT. I would say it "almost" kills it... only a visible bump where the ripple was before

                Last edited by TelRay; 05-11-2019, 02:49 AM.

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                • following with Hemholz's proposal #1 do you guys recommend I put a 5K resistor in series with the newly re-introduced 560 uF cap?
                  In that case, should the Resistor be placed between the TUBE and the CAP [POSITION 1] or between the CAP and "POINT A" [POSITION 2]? (might be a super rookie question, yes. I understand one position is High Pass and the other Low Pass)



                  for proposal #2, does this "Solder an additional (good) E-cap of 20µ/500V between point A (positive) and the ground side of the V3 cathode resistor" mean this?:



                  and, is this still necessary if the oscillation is killed completely y proposal #1 (after adding the resistor)?
                  Last edited by TelRay; 05-14-2019, 09:08 PM.

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                  • The additional 20uf cap proposed would be for the power supply circuit to the HV into the reverb transformer. The idea being that any signal on the power supply rail that is interacting may be causing the problem and additional filtration may help eliminate it.

                    The series resistor is a suggestion that would limit the HF roll off from the 560pf capacitor. It's value would be relative to whether or not the reintroduction of this capacitor stops the oscillation, and then the resistor value could be determined that still stops the oscillation, but also limits the HF roll off of that capacitor in the circuit. It's a matter of drawing a line between compromises. But we don't even know if reintroducing the 560pf capacitor will mitigate the oscillation yet.
                    "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 Chuck H View Post
                      But we don't even know if reintroducing the 560pf capacitor will mitigate the oscillation yet.
                      but Mr Chuck, sir.... we do know that re-introducing the 560 uF cap almost completely kills the oscillation (it flattens it out to a remaining “bump”) - see video on post #262
                      my question was more: should i try to completely “iron” the bump by adding the resistor. if yes, where and what value
                      and... if this finally kills that problem, is proposal #2 (20/500 cap) still necessary?
                      thx!!!
                      Last edited by TelRay; 05-14-2019, 05:53 PM.

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                      • Ah. Ok. Well as I said above, I 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.

                        I can't help but think there may be other solutions like lead dress changes, ground scheme changes, additional power filtering (as Helmholtz mentioned) or even just shielding. But if you're happy with the tone with the 560p cap in place and it solves the problem there really isn't much to think about.?. It's not like it's an "improper" circuit of itself. Fender probably used it to solve for the very problem you're experiencing after all.
                        "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


                        • The series resistor is a suggestion that would limit the HF roll off from the 560pf capacitor. It's value would be relative to whether or not the reintroduction of this capacitor stops the oscillation, and then the resistor value could be determined that still stops the oscillation, but also limits the HF roll off of that capacitor in the circuit.
                          That^^

                          But the series resistor - if adequately chosen - would have a second beneficial effect. It extracts/absorbs energy from the HF resonance by dissipation and thus damps or even kills resonance, while a capacitor only might just shift the resonance to a lower frequency.

                          The complex reverb drive circuit consisting of reverb transformer, transducer, cable, wires and parasitic elements is mainly reactive and could even have more than one resonance. Reactive components (inductors, capacitors) just exchange energy while a resistor "eats" it. As the circuit's behaviour is also influenced by the layout and lead dress, there is hardly a one-for-all optimum solution.

                          If the 560pF cap suffices, fine. But a little experimenting with different resistor and cap values could be fun.


                          I recommended to try the additional 20µ/500V filter cap, because there shouldn't be signal voltage on the power supply. A strong effect of the additional E-cap could indicate a somewhat bad (high ESR) filter cap at node A. Easy to try and if not considered necessary, the cap can be used a spare part.
                          Last edited by Helmholtz; 05-14-2019, 06:26 PM.
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                          • hi all,

                            I think it is time for an update on how this thing actually sounds and make sure nobody thinks we are just going for the mathematically perfect amp but there is something still audible going on

                            - I played the guitar with a nice overdrive pedal with the REVERB INPUT disconnected - nice sound
                            - CONNECTED the REVERB INPUT - there is something going on on the High Frequency side which sounds like oscillation
                            - Placed an EQ and cut some dB at 9KHz leaving the REVERB INPUT connected, the sounds cleans up
                            - Exaggerated the effect boosting at the same frequency to make what I hear more evident

                            Here a video with all the previous steps in a sequence



                            Do you guys think that what we have seen on the scope and are trying to kill is responsible for what is heard on the video? (rough calculations on the frequency of the oscillation placed it above 20KHz)
                            Last edited by TelRay; 05-15-2019, 06:07 PM.

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                            • A guitar signal consists of fundamental(s) and harmonics. Distortion increases the harmonics' content. Typically, the more distortion the more high frequency harmonics. A stronger input signal or a harder attack increases distortion and thus high frequency content.

                              I can hear an increase of higher frequency harmonics in your video when connecting the reverb input. This might be increased distortion caused by oscillation, but it is hardly the HF oscillation itself you are hearing. Furthermore HF oscillations don't produce a specific, identifiable sounds on their own. So we can't really be sure about the cause of the effect.

                              I recommend to try my proposals above and report back. I would also expect the effect to reduce by attenuating the grid drive of V3. IIRC, this has already been proposed.
                              Last edited by Helmholtz; 05-15-2019, 06:52 PM.
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                              • I'm not hearing anything I'd really call 'glitchy' in that one. The one that kind of stood out to me was post #121 on page 4.
                                Are you still able to reproduce that glitch?
                                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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