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

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  • mmm... I still think there's a problem (what I hear 30 sec into the video from post #117)

    to be fair to the amp, I have recorded this with "super normal" settings:
    VIBRATO CHANNEL VOLUME at 5 'TREBLE at 4' MIDs at 3' BASS at 2.5 (BRIGHT SWITCH OFF)
    I still hear that clipping when I push the guitar a little bit (marked on 3 spots in the video)



    PS: I will put the MASTER VOLUME Push Pull Potentiometer and wiring back in and maybe leave the 120 pF cap out. But that clipping was still there before I changed the Master Volume pot.

    Comment


    • Not so sure if the effect is not normal with this amp.
      But does the sound improve if you pull V3?
      - Own Opinions Only -

      Comment


      • no improvement removing V3
        i think it shouldn’t be normal. I expect a bit more of compression coming from the tubes (during the softer passages) and a bit of overdrive when pushing it (not the clipping i hear on those 3 passages)

        Comment


        • I agree it doesn't sound nice, kinda like blips of oscillation.
          How about if the bright cap on the master vol is lifted?
          That will tend to emphasise the higher, discordant harmonics.
          My band:- http://www.youtube.com/user/RedwingBand

          Comment


          • Please post the voltages at the pins of V4.

            Do you have a scope?
            - Own Opinions Only -

            Comment


            • Originally posted by pdf64 View Post
              I agree it doesn't sound nice, kinda like blips of oscillation. How about if the bright cap on the master vol is lifted? That will tend to emphasize the higher, discordant harmonics.
              Yepp... that's how it all started. I've heard those "clipped blips" and started playing around with the amp's settings to see how I could bring that failure mode to an extreme and ended up with that squeak.
              Haven't had the chance to open the amp up again (couple of gigs with the band) but have been playing at fairly high MASTER VOLUME settings (around 8 on a CTS 1 MOhm 15% taper pot... I've measured about 250K at 80% in the past) which I believe would minimize the 120 pF cap effect.
              Nevertheless, i will try to physically remove it from the circuit over the weekend.

              Originally posted by Helmholtz View Post
              Please post the voltages at the pins of V4. Do you have a scope?
              jawohl Hr Komissar!

              VOLTAGES:
              last time I've measured them they were:
              Pin 1: 234 V
              Pin 3: 1.9 V
              Pin 6: 230 V

              SCOPE
              I do have one, never used it though... but willing to learn

              Comment


              • VOLTAGES:
                last time I've measured them they were:
                Pin 1: 234 V
                Pin 3: 1.9 V
                Pin 6: 230 V

                SCOPE
                I do have one, never used it though... but willing to learn
                Thanks, V4 voltages look O.K. What is the plate supply voltage at other side of 100k plate resistors? Should be around 345V.

                Besides scope you will need a 10:1 probe, a sine signal source providing 100mV@400Hz (signal generator or smart phone app? - don't ask me, I don't use a smart phone - maybe someone could advise) as well as some practicing with the scope.
                But only the scope can show if there is actually HF oscillation involved.
                - Own Opinions Only -

                Comment


                • Originally posted by Helmholtz View Post
                  Thanks, V4 voltages look O.K. What is the plate supply voltage at other side of 100k plate resistors? Should be around 345V.

                  i measured this earlier on this post (#77) from the doghouse side at +346 V but re measured it at this point at +341 V today

                  Comment


                  • Originally posted by pdf64 View Post
                    I suggest to remove the 120pF bright cap from the master vol; it will tend to emphasize the higher order, discordant harmonics created by the overdrive.
                    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..
                    Last edited by TelRay; 03-22-2019, 01:04 AM.

                    Comment


                    • hi all,

                      POWER TUBES:
                      just for the record (even though the outcome did not bring anything new to the table) I have put 3 old power tubes (6L6s) in the amp to see if there was anything different... no change
                      as i had 3 I always had to keep one of the new tubes in the circuit but rolled the tubes to eliminate the probability of that tube being KO.

                      I am delaying putting the original MASTER VOLUME push pull potentiometer and the associated PULL CIRCUIT wiring for some days because I have a gig tomorrow (if any of you are in San Francisco let me know!) and because I'd like to replace the COAXIAL CABLES for new ones as well. visited a couple of physical stores and haven't found anything good (either too thin stranded cable or too rigid solid core) and the cheapest I found online and good was this: https://www.mouser.com/ProductDetail...kkkqpn29yWQ%3D
                      and honestly spending $60 in cable (need 7 ft as there are a few of them) I do not know it's going to make any difference sounds a bit excessive. When I was (way) "younger" you'd go into a store and spend $5 for a couple of meters of good instrument cable.

                      Comment


                      • hi all,
                        the GROUND SWITCH is still in the circuit with the old (original) death capacitor.
                        The amp is currently using a 3 prong plug and I've seen a bunch of people getting rid of the GROUND SWITCH functionality mentioning that AC can get to the chassis if the cap shorts.
                        I am wondering if this (AC to chassis) could be happening to a small degree and thus affecting everything that is grounded to the chassis.
                        I can simply remove the GROUND SWITCH + CAPACITOR, however what would be the best way to measure if there is (at least a minimum) amount of AC going to the chassis?

                        Comment


                        • well... removing the GROUND SWITCH + DEATH CAP had apparently no effect

                          to be specific I did this (and took the chance to add a GROUND connection to the onboard AC connector)

                          Comment


                          • Hello all,

                            searching on he topic of oscillation I've seen there is a .0022 uF Capacitor in the reverb section that has the function of filtering out unneeded high frequencies, cutting noise and preventing oscillation.

                            This is what I have in my amp:




                            A1172 silverface circuit: .0022 uF CAPACITOR across the 220K V4 TUBE Reverb Recovery Grid Leak resistor
                            (which is what I have in my amp)



                            AA1069 and AA270 silverface amps: the .0022 uF CAPACITOR is connected from the REVERB INPUT pan to GROUND



                            So, I have switched the .0022uF capacitor position from the A1172 to the AA1069/AA270 position... and it improved!!! Now the "peaks" distort instead of being "clipped". Maybe the issue is still not 100% solved but clearly a step forward. Will try to record some audio and update this post later on today. But wanted to share the news as you guys pointed me in the direction of an oscillation problem and maybe this provides even further ideas. For instance, I would still like to understand the "physics" behind this improvement... why does this help?

                            Comment


                            • In the A1172 version the 2.2nF cap is wired in parallel with reverb output transducer. Together with the inductance of the transducer this constitutes a parallel resonant filter which boosts frequencies around 4..5kHz, thus making your reverb signal sound brighter. Only above around 8kHz the cap increases high frequency attenuation.

                              When the cap is wired across the reverb transformer secondary and the reverb input transducer, instead, it should not have any influence on reverb sound. It might help against parasitic oscillation of the driver tube, though. OTOH you reported that pulling the driver tube did not cure your problem, meaning that the driver tube is not the culprit.

                              What about removing the cap completely? Blackface amps didn't have it.
                              - Own Opinions Only -

                              Comment


                              • Originally posted by Helmholtz View Post
                                OTOH you reported that pulling the driver tube did not cure your problem, meaning that the driver tube is not the culprit.
                                hier richtig du bist, ja...

                                nevertheless the sound i posted on the video on post #121 has those "blips of oscillation" that I do not hear now (there is some distortion, that's not the nicest but better than before).
                                I will test it at high volume at practice tonight and try to update with a new video playing the same way i did on post #121

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