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

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  • #76
    Well it's bright, to be sure. But I don't hear the same nastiness. Is that with V1 still out? Hows the instability as per the previous tests?
    "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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    • #77
      Originally posted by pdf64 View Post
      The HT node that's put in question by the screech is the one that supplies most of the preamp stages V1 V2 V4, noted at 345V on the schematic. The ESR of its ecap may be excessive, or a bad connection to 0V chassis common.
      so.. it's this one:



      I've measured everything I could here:
      voltages and connections to the chassis (when you see 0mV it means both voltage and continuity to the chassis has been checked)



      the only value I have no reference for is the one in yellow (+221V)
      also measured for conductive circuit board and read values between 50 and 350 mV (I guess I shouldn't be worried about this)

      @ pdf64, as per your suggestion "The ESR of its ecap may be excessive", I have no idea how to measure it. these caps are fairly new (under 4 months). but I guess I could put one of the old Mallory back or even a Sprague that was replaced at some point before I got the amp. (we are speaking about the capacitor marked with #1 in the image above, right?)

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      • #78
        Yes, it's the cap marked 1 that would be in question. I didn't see anything wrong with the wiring, but I don't have the amp in front of me. If that cap isn't performing to decouple signal at the power supply node it can allow undesirable feedback and interaction between the amplification stages it supplies.

        Were these actually new filter caps you installed? I ask because I had a problem once installing caps I had on hand that had gone stale with age. I order new caps for each build or repair now and keep no electrolytic caps on hand.

        I had trouble with new caps once too and I've changed brands since then. The new caps I had trouble with were Atoms

        The usual test is to parallel another cap with the suspected cap. This would have to be a known good cap and not an old cap of unknown worthiness. But it doesn't have to be the same value. Anything from 10uf to 100uf of an appropriate voltage rating will do.
        "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


        • #79
          Originally posted by Chuck H View Post
          Well it's bright, to be sure. But I don't hear the same nastiness. Is that with V1 still out? Hows the instability as per the previous tests?
          thx for the comment, it made me realize that the knob settings were not exactly as in the previous videos.
          what I've noticed:
          1) SCREECH: with V1 out I hear a hum that reminds me of what I got when removing V3. so I would venture to say that the grid stoppers helped eliminating V3's contribution to the SCREECH (never mind the instrument connected, the guitar volume is down and the same hum can be heard without the jack plugged in)
          2) GUITAR SOUND:
          2.i with V1 out, high frequency distortion
          2.ii with V1 in, high frequency distortion
          I am not really able to say there's a difference.

          (video in post #75 has been edited to have same conditions i.e: knob settings than in previous videos)

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          • #80
            Originally posted by Chuck H View Post
            Were these actually new filter caps you installed?.
            yes, brand new ordered from Mouser in DEC 2018. So, to be precise, not even 3 months old (in use)
            I guess I could replace it for one of the old caps and see what happens (have no other capacitor at hand)

            One note, maybe of importance: when I put the new Filter Capacitors I turned the power ON and played my guitar as I do not have at hand a variable transformer to apply voltage gradually say, from 0 to 120 VAC.

            Comment


            • #81
              Short of a Variac type thing for bringing up caps gradually to their rated voltage, you can still limit current. You may have read about the "light bulb limiter" here or elsewhere. I admit that I usually just fire up a new build and let it burn off standby for about a half an hour (or until all the clicks and pops stop, whichever comes last). Then I give it the play test. But it's usually recommended to burn any amp with new caps for longer with either a graduated voltage or on a current limiter. I am sorely lacking in patience though.

              I suppose there's nothing to lose tacking one of the old caps parallel in the circuit. As long as it's not bulging or leaking anything. Use one of the caps that you pulled out of the amp and not the originals (which I assume you got with the amp, as you mentioned them). The originals are likely well dried out by now and/or much of the oxide layer that makes them work will have deteriorated from disuse.
              "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


              • #82
                Originally posted by Chuck H View Post
                I suppose there's nothing to lose tacking one of the old caps parallel in the circuit. As long as it's not bulging or leaking anything. Use one of the caps that you pulled out of the amp and not the originals (which I assume you got with the amp, as you mentioned them). The originals are likely well dried out by now and/or much of the oxide layer that makes them work will have deteriorated from disuse.
                The amp had in the doghouse all Mallorys with the exception of one Atom, so I've tried all of them (all 3 measured between 20 and 21 uF with the multimeter).

                No change in the guitar sound



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                • #83
                  Probably not an issue with power supply decoupling then. Some SF models are notorious for mild instability due to lead dress. It's a hell of a lot going on in there with a lot of long signal leads. I've been designing circuits long enough that the concepts of lead dress and grounding scheme just fall into place as I layout a new design and I can see that the SF amps take a lot of risks in these areas. I did make an amp once that about as complex as a typical two channel Fender with reverb and tremolo and I promise you that the layout and lead dress looked nothing like those amps. Most posters here are well versed in these design aspects and if you ever get to rebuilding the circuit I think you can count on a lot of support.
                  "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


                  • #84
                    How about if the V3 circuit is changed to the BF spec, eg 2k2//22uF cathode, get rid of the 560pF ceramic cap across the drive transformer primary?
                    Some more detailed, high resolution photos of the amp chassis gubbins may be useful.
                    My band:- http://www.youtube.com/user/RedwingBand

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                    • #85
                      Originally posted by pdf64 View Post
                      How about if the V3 circuit is changed to the BF spec, eg 2k2//22uF cathode, get rid of the 560pF ceramic cap across the drive transformer primary?
                      thx, man for the suggestion. actually that seems to be a problematic area in which Fender seems to have gotten some interesting changes.

                      1) this is how my amp looked like when I got it:



                      2) as I did not find that configuration (2.2K + 25uF) in the schematic I got directly from Fender (and some others I've checked for amps around the production date of mine) I removed the 25 uF cap, replaced the resistor by a 470 Ohm one and added the 560 pF 3KV cap as shown here (bringing it back to spec):


                      (this is an extraction from DWG 010182 Rev F in which the PULL BOOST MVS is introduced)

                      3) additionally there is another version in the next revision where the 560 pF cap changes position (now is across pins 3 and 6 of V3)


                      (this one is from Rev G in which a hum balance is added, maybe one generation after my amp)

                      4) this one is before the addition of the PULL BOOST to the MVS and shows what you have proposed


                      (this is an extraction from DWG 010182 Rev C)

                      I can give all a try (#2, #3 and #4) and see what happens

                      PS: should I leave the 47K grid stoppers installed or remove them?
                      Last edited by TelRay; 03-03-2019, 10:51 PM.

                      Comment


                      • #86
                        I much prefer the BF version with 2.2k/25µ. Changing the cathode resistor to 470 Ohm unneccesarily increases V3 plate and transformer current by a factor of around three. It also strongly increases the driving power to the reverb transducer. I would expect more "crosstalk" with the 470 Ohm version.
                        - Own Opinions Only -

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                        • #87
                          Originally posted by Helmholtz View Post
                          I much prefer the BF version with 2.2k/25µ. Changing the cathode resistor to 470 Ohm unneccesarily increases V3 plate and transformer current by a factor of around three. It also strongly increases the driving power to the reverb transducer. I would expect more "crosstalk" with the 470 Ohm version.
                          vielen Dank Hr. Helmholtz!!! das mache Ich sofort!
                          thx!!!

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                          • #88
                            Gern geschehen!
                            Ist aber nur meine persönliche Meinung, auch weil ich den Blackface (Hall-) Sound vorziehe.
                            - Own Opinions Only -

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                            • #89
                              Hey now! What's with the German talk that we gringos can't read? You two plotting on us?













                              "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


                              • #90
                                Originally posted by Helmholtz View Post
                                Gern geschehen! Ist aber nur meine persönliche Meinung, auch weil ich den Blackface (Hall-) Sound vorziehe.
                                natur... however, my original idea was: see how this design should sound like (that's why I insist on bringing it back to spec) and after that decide if I want to jump into all the mods around (including Blackfacing). Otherwise I think I won't be able to notice the improvement.

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