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

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  • I'm not hearing anything I'd really call 'glitchy' in that one.
    Agree, but it seems strange that connecting the reverb transducer should change the frequency response/content of the direct signal at all. Would be interesting to see the influence on a square wave signal. Maybe a capacitive coupling between adjacent wires or a spill-over via the power supply.

    Question to the OP: Not very probable, but is there a connection between the green reverb transformer secondary wire and the junction between the 1k and 470k resistors (as with the original Master Volume Boost Switch MVS)? If yes it should be removed!
    Last edited by Helmholtz; 05-15-2019, 09:20 PM.
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    • Originally posted by Helmholtz View Post
      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.
      Jawoll!!! will do!

      can you please confirm the placement of the resistor and cap for proposal #1 and #2 as shown on post #263? want to make sure i got it right.

      I understand that for proposal #1 the resistor needs to be placed between V3 pin 6 and the 560 pF cap to act as a Low Pass filter (it would be a High Pass filter if I put it after the cap, right?)
      You have originally proposed a 220 pF cap with a 5K Resistor, according to one of those online RC Filter calculators the cut-off frequency of that arrange is 145 KHz.
      If I now have a 560 pF resistor to get the same cut-off frequency I would need to start with something like a 2K Resistor (142 KHz).

      For proposal #2 I just need to make sure that the placement of the ecap and polarity is right (as described in the schematic con post #263)

      Originally posted by Helmholtz View Post
      I would also expect the effect to reduce by attenuating the grid drive of V3. IIRC, this has already been proposed.
      Are those the 47K resistors added as grid stoppers on V3 on post #75?

      vielen dank, sir!!!
      Last edited by TelRay; 05-16-2019, 05:29 AM.

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      • Originally posted by g1 View Post
        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?
        mmm... i might get lost in the terminology but more than "glitchy" it sounds "farty" (is that even a word?)... sounds like a farting speaker cone
        I can try to reproduce the same sound as on that post #121. Will put it here possibly tomorrow using the same clean guitar sound I used back then.

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        • can you please confirm the placement of the resistor and cap for proposal #1 and #2 as shown on post #263? want to make sure i got it right.
          The placements of the caps as shown in post #263 are correct. You may try to connect the lower end (marked "2") of the 560pF cap to the V3 cathodes (pins 3&8) instead of point "A" , as done in other versions of the Twin. Maybe this works even better.

          I understand that for proposal #1 the resistor needs to be placed between V3 pin 6 and the 560 pF cap to act as a Low Pass filter (it would be a High Pass filter if I put it after the cap, right?)
          The order of components in series wiring doesn't matter, won't change anything.

          You have originally proposed a 220 pF cap with a 5K Resistor, according to one of those online RC Filter calculators the cut-off frequency of that arrange is 145 KHz.
          Not much sense calculating a cut-off frequency using simple RC calculation. The RC network is wired in parallel to the reverb transformer primary, which behaves mainly inductive. So we have an LCR damped resonant filter (with partly unknown component values) that doesn't behave like a simple RC filter.
          I just proposed ballpark values that are unlikely to influence the audio range. The idea being to progessively increase capacitance and/or decrease resistance until the oscillation stops (no real calculation involved).

          For proposal #2 I just need to make sure that the placement of the ecap and polarity is right (as described in the schematic con post #263)
          Right. I would actually start with proposal #2.

          Are those the 47K resistors added as grid stoppers on V3 on post #75?
          The 47k grid stopper doesn't do much to reduce grid drive. It just makes grid current distortion smoother and reduces HF response. Reducing grid drive can be achieved by replacing the 1M grid leak resistor by 2 resistors of e.g. 500k in series and connecting the grid(s) to the midpoint between the resistors, thus producing a voltage divider.
          Last edited by Helmholtz; 05-16-2019, 03:48 PM.
          - Own Opinions Only -

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          • Originally posted by Helmholtz View Post
            Question to the OP: Not very probable, but is there a connection between the green reverb transformer secondary wire and the junction between the 1k and 470k resistors (as with the original Master Volume Boost Switch MVS)? If yes it should be removed!
            Just re-posting this very pertinent question.
            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 Helmholtz View Post
              Question to the OP: Not very probable, but is there a connection between the green reverb transformer secondary wire and the junction between the 1k and 470k resistors (as with the original Master Volume Boost Switch MVS)? If yes it should be removed!
              sorry I missed that one. The answer is, no. The green cable goes to the REVERB IN RCA jack tip and from it a .0022 uF cap connects it to GROUND (as described on post #133, as per schematics of AA1069 and AA270 silverface amps)

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              • Originally posted by TelRay View Post
                sorry I missed that one. The answer is, no. The green cable goes to the REVERB IN RCA jack tip and from it a .0022 uF cap connects it to GROUND (as described on post #133, as per schematics of AA1069 and AA270 silverface amps)
                I don't think connecting the .0022µ cap to the Reverb In jack makes sense. It obviously doesn't help with the oscillation. Actually I never saw it in any version of the Twin before. Is it shown on an original schematic? My AA270 schem doesn't show this wiring.

                OTOH, the cap makes sense on the reverb output - especially when using some cap (560p) at the V3 plates, which produces a lowpass effect. THe reason is that having the cap across the reverb output transducer regains some reverb treble that might have attenuated by the V3 plates cap. In addition the .0022 cap attenuates unwanted HF content with this wiring.
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                • Originally posted by Helmholtz View Post
                  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.
                  Originally posted by g1 View Post
                  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?
                  I think I will try to satisfy both posts by trying to play something like on post #121.
                  Trying to be as fair to the amp as possible here I've changed guitars from Humbuckers (a Gretsch White Falcon) to Single Coils (a Fender Jazzmaster), connected the guitar straight into the amp (no distortion = no added harmonics at the input and changing pickups I believe also not such a hot input signal) and use the following settings: CH VOL 7, TMB (4,3,4) and MV 3
                  You can hear it on the video below, my conclusions are the following.
                  I do not hear those "blips of oscillation" in loud passages as in post #121 anymore (as reported several posts back, even if playing with the Gretsch)
                  I still hear the difference between the REVERB INPUT being connected or not (as on the video in which I used the overdrive)
                  At the end of the video i looped the sections in which I can hear that High Frequency "distortion, blip... etc" and took several dB at 9KHz and then boosted at the same frequency to make the problem more evident

                  I will follow the modifications proposed in the last posts and report back


                  Comment


                  • Originally posted by Helmholtz View Post
                    I don't think connecting the .0022µ cap to the Reverb In jack makes sense. It obviously doesn't help with the oscillation. Actually I never saw it in any version of the Twin before. Is it shown on an original schematic? My AA270 schem doesn't show this wiring.
                    OTOH, the cap makes sense on the reverb output - especially when using some cap (560p) at the V3 plates, which produces a lowpass effect. THe reason is that having the cap across the reverb output transducer regains some reverb treble that might have attenuated by the V3 plates cap. In addition the .0022 cap attenuates unwanted HF content with this wiring.
                    I remember that on post #133 I was following Rob Robinette's fixes introduced by Fender for what he called:
                    "Reduce Reverb Noise & Oscillation: This .002uF cap from reverb pan input to ground was used in the AA1069 and AA270 silverface amps. I prefer the A1172 fix shown at top because it cleans up the pan input and output."
                    LINK here (you might need to scroll down until you find the Reduce Reverb Noise & Oscillation section"

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                    • Originally posted by TelRay View Post
                      I remember that on post #133 I was following Rob Robinette's fixes introduced by Fender for what he called:
                      "Reduce Reverb Noise & Oscillation: This .002uF cap from reverb pan input to ground was used in the AA1069 and AA270 silverface amps. I prefer the A1172 fix shown at top because it cleans up the pan input and output."
                      LINK here (you might need to scroll down until you find the Reduce Reverb Noise & Oscillation section"
                      Thanks, found it.

                      Seems the AA1069 and AA270 Bandmaster Reverb had the .0022µ cap across the reverb input, while the corresponding Twin versions AA769 and AA270 (https://www.thetubestore.com/later-f...amp-schematics) had the cap at the reverb output - strange.
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                      • I'm not hearing that farty glitching anymore. You did say this was with the ch.vol. around 7 and lower master setting, so I'm going to suggest what you now have is normal.
                        The pull boost was just horrible and no one ever used it (except as 'horrible' effect).
                        But the master was also not very well liked, many either ran it up full all the time, or removed it. In that era, Fender did not know how to do distortion.
                        I would say that you are no longer dealing with a 'fault', just trying to optimize the quality of the distortion. That is ok to do, but you may be at a point where it is as good as it gets without a major re-design. If you ever have a chance to try another SF Twin w/Master, compare it at similar settings.
                        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
                          I'm not hearing that farty glitching anymore. You did say this was with the ch.vol. around 7 and lower master setting, so I'm going to suggest what you now have is normal.
                          I agree with you Mr G1.... partially
                          I like the current slightly overdriven sound I am getting from the amp, it's pushy and clear and I would never expect any more grit coming from a Twin. That has improved noticeably in the last 100 posts
                          But... I'd like to get the same great sound with the REVERB tank connected, and that "dirt" I hear around 9 KHz in the latest videos (the one with the overdrive on post #268 and the clean tone with the single coil pickups on post #278) prevents me from considering this case closed. While this could be "just tolerable" for live performance it is still not suitable for studio work.
                          After I have solved this issue I will consider I have restored this amp to its former glory and will eventually think about any tweaks in the future.
                          Would you agree? Please say "Yes" because I still need your help

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                          • Ok. I missed the part that the reverb was not connected. Carry on with those mods and post your results.
                            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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                            • trying to get things in order to dig into the amp again today, I wanted to prioritize what to do and how to check that the modification is effective

                              1.- Add 20 uF / 500 V Cap

                              Reason: there shouldn't be signal voltage at the power supply
                              Verification: place scope on point "A"


                              2.- .0022 uF Cap try the different positions described on Fender Schematics

                              Reason: the cap across the REVERB OUTPUT transducer regains some TREBLE - especially when using the 560pF cap at the V3 plates, which produces a LOW PASS effect. It also attenuates unwanted HF content with this wiring.
                              Verification: scope at the SPK OUT


                              3.- 560 pF cap try the different positions described on Fender Schematics

                              Reason:
                              Verification: scope at the SPK OUT


                              4.- Install a Voltage divider replace 1M resistor by 2x 500K

                              Reason: Reduce grid drive
                              Verification: scope at the SPK OUT

                              5.- add Resistors (around series with the 560 pF cap

                              Reason: extracts energy from the HF resonance by dissipation and thus damps or even kills resonance
                              Verification: scope at the SPK OUT

                              6.- Faraday Cage add Aluminum foil to the enclosure

                              Please jump in if you have any other scientific verification methods

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                              • You might want to avoid aluminum foil. Torn or loose foil could be a problem. Aluminum coil stock is available at any home center, sold as flashing.

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