Announcement

Collapse
No announcement yet.

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

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Originally posted by Chuck H View Post
    If it were mine though... I have to admit that I would rebuild it with a more ideal lead dress and ground scheme. Though it can't really ever be ideal because of how long the front panel layout is on those amps. Some preamp controls will always have long leads and be subject to later circuitry.
    Thisis probably why I've seen the front-panel wiring bundled, laced, braided, twisted, or wrapped tightly by an only-grounded-at-one-end piece of small-gauge wire on SF amps. The layout apparently had been known to be an issue & these were attempts to fix it.

    Sometimes we see "desperation measures" in amps, ss an acknowledgement of less-than-ideal build quality; but it's cheaper & faster than Chuck's ideal solution of tearing the whole amp down & redesigning it to work well. But, we're not a factory, so cost & time may not be such large concerns to us.

    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


    • I suggest to remove the 120pF bright cap from the master vol; it will tend to emphasise the higher order, discordant harmonics created by the overdrive.
      A lower value master vol pot, eg 220k may be less affected by residual parasitic capacitances than 1M, leading to a smoother tone at low settings.
      My band:- http://www.youtube.com/user/RedwingBand

      Comment


      • As stock, the master vol has a bright cap (to a track tapping on a 4th terminal); is that still in circuit at all?
        Good point!
        Something I overlooked as my 1972 SF Dual Showman Reverb doesn't have the bright cap at the master vol.
        I would definitely remove it.
        - Own Opinions Only -

        Comment


        • Originally posted by pdf64 View Post
          I suggest to remove the 120pF bright cap from the master vol; it will tend to emphasise the higher order, discordant harmonics created by the overdrive.
          A lower value master vol pot, eg 220k may be less affected by residual parasitic capacitances than 1M, leading to a smoother tone at low settings.
          duly noted! removing the 120 pF cap added to the list!

          Comment


          • I tend to think that the whole concept of bright caps across vol. pots is based on a misinterpretation of the Fletcher-Munson ear sensitivity curves.
            - Own Opinions Only -

            Comment


            • Originally posted by Helmholtz View Post
              I tend to think that the whole concept of bright caps across vol. pots is based on a misinterpretation of the Fletcher-Munson ear sensitivity curves.
              Or because it sounds good a lot of the time

              But I agree that it's probably not needed for a Twin Reverb master volume pot. If the amp seems too dull without the cap TelRay could try a lower value cap and/or a resistor in series with the cap to get a little back.
              "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


              • Originally posted by Chuck H View Post
                Or because it sounds good a lot of the time

                But I agree that it's probably not needed for a Twin Reverb master volume pot. If the amp seems too dull without the cap TelRay could try a lower value cap and/or a resistor in series with the cap to get a little back.
                Yes, I was speaking of the master vol. pot, as this controls over all loudness.

                A bright cap produces a treble boost by bypassing the the upper/series part of the voltage divider with a cap. As volume setting is increased, this effect reduces, the HP corner frequency shifts and at full volume the effect is zero. In other words, low volume sound will be (much) brighter than full volume.
                The Fletcher- Munson curves of equal loudness don't provide justification of such behaviour.

                For preamp/drive volume pots the effect may be desirable, especially if additional highs are required at lower settings. Also the highs reduce with increased drive, which is welcome before distorting stages.

                I like the bright cap to be switchable.
                - Own Opinions Only -

                Comment


                • Originally posted by TelRay View Post
                  thx guys, I will record a “better” quality audio with more “normal” settings in a couple of days
                  i think that if we conclude “that’s how the SF sounds like” I won’t be able to push the VIBRATO Ch VOL beyond 5 if I don’t want to fry those highs.
                  On the other hand, the NORMAL channel is the cleanest thing I’ve ever heard with the Ch VOL at 10
                  Might make sense to also post a recording with the Normal channel for reference.

                  Typically Twins are used for clean amplification and not to produce distorted sounds. For distortion its better to use a good pedal.
                  It is normal that the Vibrato channel distorts beyond 5. But with full master vol. this will be incredibly loud.

                  Generally a master volume only makes sense if the amp is able to produce a nice preamp distortion, which a Twin is not. Also distorted reverb sounds nasty.
                  BF Twins never had a master volume and hardly anyone misses it.
                  - Own Opinions Only -

                  Comment


                  • Originally posted by Helmholtz View Post
                    BF Twins never had a master volume and hardly anyone misses it.
                    And that's the truth brother! The only thing for which I ever found the master volume useful, you could clear up some hiss noise by turning it down to about 7. Below that, the gain structure makes for unpleasant distortion.
                    This isn't the future I signed up for.

                    Comment


                    • Originally posted by Helmholtz View Post
                      I tend to think that the whole concept of bright caps across vol. pots is based on a misinterpretation of the Fletcher-Munson ear sensitivity curves.
                      Maybe the original rationale for them was to counter the treble roll off of bigger speakers, and the culmulative HF limitation caused by cascading stages with Miller capacitance and significant interstage impedances, most notably 1M vol pots?
                      I like a multi-throw switch for various values of bright cap, as different values suit different guitars, speakers, and control settings.
                      My band:- http://www.youtube.com/user/RedwingBand

                      Comment


                      • Originally posted by pdf64 View Post
                        Maybe the original rationale for them was to counter the treble roll off of bigger speakers, and the culmulative HF limitation caused by cascading stages with Miller capacitance and significant interstage impedances, most notably 1M vol pots?
                        I like a multi-throw switch for various values of bright cap, as different values suit different guitars, speakers, and control settings.
                        But why boost treble only at low master settings?
                        - Own Opinions Only -

                        Comment


                        • Hello all, as promised, video with better audio on NORMAL and VIBRATO CHANNEL at different CHANNEL Volume settings



                          the problematic bit is the third example (at 0m30s)

                          Notice that for the last part (arpeggio) I need to bring the channel volume down to 3 to get have a super clean sound (the last one has the the same settings + REVERB at 5)
                          Additional notice, the amp is super quiet (no hum) at any setting.
                          There's no noise reduction, compression, etc. Just tried to achieve the same overall volume levels with the MASTER CHANNEL VOLUME

                          (there's a tiny bit of clipping with the normal channel sections, but these are coming either from the MIC being very close to the amp or the small pre-amp i used before the audio card input)

                          The amp has currently the 2x 47K gridstoppers on V3
                          the 25uF/2.2K filter
                          All tubes in, etc...
                          Last edited by TelRay; 03-08-2019, 12:40 AM.

                          Comment


                          • hey people!
                            any comments on the video?
                            haven’t been able to pull the 120 pF cap out of the master volume yet but will do over the weekend
                            best!

                            Comment


                            • Originally posted by TelRay View Post
                              any comments on the video?
                              haven’t been able to pull the 120 pF cap out of the master volume yet but will do over the weekend...
                              I think that seems ok now. The vib channel has 3 stages, more gain, so will overdrive at a lower channel vol setting than the normal channel does.
                              I assume that the master vol is set pretty low? If so the amp, especially any overdrive from the vib channel, will probably sound nicer when you lift the master's bright cap.

                              Originally posted by Helmholtz View Post
                              But why boost treble only at low master settings?
                              Sorry I missed this at the time.
                              I can't think of a rationale for fitting a regular bright cap to a master vol.
                              But the one on that SF TR is connected to a tap on the track (I seem to remember that electrically it's about halfway, ie at the 500k-500k midpoint), rather than the wiper. So there's no big high freq boost at low master vol settings.
                              So maybe the reason for it was to try and even out the freq response at the output of the pot, in the presence of Miller capacitance from the next stage (LTP)?
                              Last edited by pdf64; 03-13-2019, 05:55 PM.
                              My band:- http://www.youtube.com/user/RedwingBand

                              Comment


                              • any comments on the video?
                                As far as I can tell from the new video, the amp seems to sound normal now and shows no signs of something being wrong anymore.

                                I think there are good reasons why some amps (like some favorite Fender Tweed and Blackface models) stood the test of time and eventually get/got re-issued.
                                - Own Opinions Only -

                                Comment

                                Working...
                                X