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

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  • Someone higher up can correct me, but your hot from the plug should go to the center lug of the fuse holder, not the edge. That way there is not a possibility of voltage at the exposed terminal if you have the fuse out. People have shocked themselves while replacing fuses if wired like you have yours. Hot should be the black (& narrower prong) from yourpower cord, but mistakes happen.

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    • Originally posted by Justin Thomas View Post
      hot from the plug should go to the center lug of the fuse holder, not the edge.
      literally a life saving tip, man!
      I will re-check!!! however I have rechecked with old photos (before I even touched anything) and I have followed exactly the wiring I had in the amp when I bought it.
      That been said... happy to change the wiring if you guys think I need to
      thx!!!

      Comment


      • Don't follow the wiring in the amp when you got it... Ever heard of a Friday Fender? Follow tge standards set by the safety labs, not the factories...

        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

        Comment


        • Well spotted Justin
          Originally posted by TelRay View Post
          I decided to use my lunch break to do some work in the amp and finally try lifting the 120 pF cap.

          at low MASTER VOLUME settings (3 and around) the effect is noticeable as the sound is also duller (so the nasty "clip-blips" are not so evident)
          however, as i compensated by pulling the VIBRATO CHANNEL TREBLE up I got more or less the same "clippy-blippy" peaks
          at high MASTER VOLUME settings (around 8) there is no difference in having the cap in or out

          i think that the utility of this cap is finally for the user to be able to keep the same treble settings at low or high volumes..
          The stock wiring of the MV bright cap was to a tap on the MV, not its wiper. The way you've now got it, at low master settings, the bright cap may be boosting high end significantly above what was intended by the designer.
          My band:- http://www.youtube.com/user/RedwingBand

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          • Originally posted by Justin Thomas View Post
            Don't follow the wiring in the amp when you got it...
            Hey man, thank you for pointing this out... I think it's OK now

            Comment


            • That looks gooder!

              Justin
              "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
              "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
              "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

              Comment


              • Originally posted by pdf64 View Post
                ...at low master settings, the bright cap may be boosting high end significantly above what was intended by the designer.
                I agree, mister. I will eventually revert to the original MATER VOLUME pot with the 4th lug.

                Comment


                • I think I am at that point in the project in which I am either lost, "hearing" ghosts or both at the same time... probably this last option is the most accurate description )

                  as I seem not to be able to objectively evaluate what sounds best I decided to make the video below with the same guitar bit played with the different .0022 uF cap placed according to the 3 options shown on the schematics AB763, A1172 and AA1069/270.

                  I let you judge if of these sound better than the other, i don't think they do anymore

                  In any case, the way the amp is clipping / blipping in those transients is ultimately what I want to fix.

                  Comment


                  • Hello all,

                    I've finally ventured into new territory... the OSCILLOSCOPE

                    I might be doing a lot of things wrong... or not, so I will describe all steps to make sure al the knowledge gathered from this forums and some internet videos (there's one from D-Lab that I found enormously instructional following the signal path through a Fender Champ)

                    1) This is how the SINE signal was generated using WAVELAB's Signal Generator



                    after the file was generated I played it in a loop through a MOTU 2408 mk3 soundcard at 96KHz to the amp's VIBRATO CHANNEL INPUT 1

                    2) This is how it looked on the other end after I adjusted the volume in the soundcard to have about 150 mV (about 3 square divisions of 50 mV each on the oscilloscope)



                    and this is where I placed the Oscilloscope's probe (the ground probe was connected to the amp's chassis): V2 Tube Pin 2 (right before the 100 K resistor)



                    3) After that, I moved the probe to the output of the V2 Tube (Pin 1)





                    so far so good, I would say. I put a SINE wave in and see an undisrupted SINE wave on the scope
                    Last edited by TelRay; 04-10-2019, 12:51 AM.

                    Comment


                    • 4) Then to the Center Tap of the VIBRATO CHANNEL VOLUME POTENTIOMETER and I believe here is where trouble starts



                      a little more detail



                      here is where the probe was placed



                      I see the SINE is disturbed now, the CHANNEL VOLUME is not above 5 and the shape of the wave was not getting better at lower volumes
                      Additionally, there is that blip that seems to be OSCILLATION

                      5) just for instructional purposes I moved the probe to V2 pin 6







                      First, please do let me know if all the procedure described above is correct. Second, my (very humble) opinion is that something is inducing OSCILLATION do you share this conclusion? And more importantly, what could it be?

                      Comment


                      • Actually I am hoping that someone with better command of English will take over. Such explanations are quite awkward and time-consuming for me.

                        Just a few preliminary remarks:

                        - I think you are doing it right.

                        - Your scope signal/sine wave seems to wander upwards. This might indicate a probe ground problem or a power supply problem of your scope (bad filter cap(s)). Do you get a straight ground line if no probe is connected?

                        - What is the bandwidth of your scope?

                        - The "blip" as you call it is actualy spurious HF oscillation. It's amplitude might actually be higher if not attenuated by the scope's limited bandwidth.

                        - The typical remedy measure is "chopsticking" the wiring (bending wires with a chopstick) until the oscillation stops. The reason for the oscillation can be supposed to be unwanted coupling between adjacent wires.


                        Hope this helps.
                        Last edited by Helmholtz; 04-10-2019, 01:46 PM.
                        - Own Opinions Only -

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                        • The 'skewness' of the sine wave happens any time you put the signal though a tone stack. It looks like a sine to the nekked eye, but has a little harmonic action.* As the harmonics are put out of phase by the caps in the tone stack, they shift. Not a biggie. As Herr Helmholtz says, it's the "fuzz" on the waveform that's of concern.

                          *if there weren't we wouldn't need tubes, after all.
                          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

                          Comment


                          • It looks like a sine to the nekked eye, but has a little harmonic action.
                            That!
                            The min-max asymmetry of the "sine" indicates harmonic content (= distortion).
                            - Own Opinions Only -

                            Comment


                            • Originally posted by Helmholtz View Post
                              - I think you are doing it right.
                              Vielen Dank Hr Komissar!

                              Originally posted by Helmholtz View Post
                              - Your scope signal/sine wave seems to wander upwards. This might indicate a probe ground problem or a power supply problem of your scope (bad filter cap(s)).
                              Yepp, noticed that tilt (upwards to the right) too. The scope is a loan from a friend and the first time I use it (or any other scope for that matter)
                              The oscilloscope has a TRACE ROTATION adjustment screw that I am not able to access with a screwdriver, but when i push it a bit the tilt changes so I am guessing I could achieve a 0 degrees line if I could adjust this.
                              I will take whatever findings I discover with this oscilloscope and confirm when I can with a trusted / calibrated scope.

                              Originally posted by Helmholtz View Post
                              Do you get a straight ground line if no probe is connected?
                              Yes, there's a straight line and it has the same tilt as the wave (upwards to the right)

                              Originally posted by Helmholtz View Post
                              - What is the bandwidth of your scope?
                              20 MHz

                              Originally posted by Helmholtz View Post
                              - The "blib" as you call it is actually spurious HF oscillation. It's amplitude might actually be higher if not attenuated by the scope's limited bandwidth.
                              OK, tried to look the meaning of "spurious" as English is not my first language either
                              I understand that what you are saying is that it indicates a problem that might be bigger but the oscilloscope is somehow limited in showing the real dimension of the problem.

                              Originally posted by Helmholtz View Post
                              - The typical remedy measure is "chopsticking" the wiring (bending wires with a chopstick) until the oscillation stops. The reason for the oscillation can be supposed to be unwanted coupling between adjacent wires.
                              I will try that!


                              Originally posted by Helmholtz View Post
                              Hope this helps.
                              Everything you guys suggest helps a lot. I am very thankful for your support.

                              Originally posted by eschertron View Post
                              The 'skewness' of the sine wave happens any time you put the signal though a tone stack. It looks like a sine to the nekked eye, but has a little harmonic action.* As the harmonics are put out of phase by the caps in the tone stack, they shift. Not a biggie. As Herr Helmholtz says, it's the "fuzz" on the waveform that's of concern.
                              *if there weren't we wouldn't need tubes, after all.
                              Thank you for jumping in, man
                              I understand that, in that case, "this" (below) skewed SINE wave is normal when placing the probe after the tone stack.
                              In that case, great... one less thing to worry about



                              Originally posted by Helmholtz View Post
                              - The min-max asymmetry of the "sine" indicates harmonic content (= distortion)
                              ahhhh... so the amplitude of the waveform in the TOP is wider than at the BOTTOM and this represents unwanted distortion. Correct me if I am wrong.



                              In SUMMARY: there are two problems
                              - the "blip" / oscillation
                              - the asymmetry between top and bottom SINE wave

                              Comment


                              • I wouldn't say that the asymmetry is a problem, but rather a *feature* of the triode's transfer curve. It's what gives tubes their 'tube-iness'.
                                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

                                Comment

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