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 Helmholtz View Post
    As the reverb transformer secondary is grounded to the chassis via the RCA jacks, it is important that these make perfect chassis contact.
    Those 'star' washer teeth can get oxidized where they meet the chassis, it's good that you mentioned this. I would also do all the pots, cleaning up the washers and where they contact the chassis.

    Originally posted by Chuck H View Post
    Two channel Fenders with reverb and vibrato are spaghetti monsters and that's why you see many HF ground caps and such in the silver face line. That doesn't mean we shouldn't try to "fix" it
    I couldn't find it right now, but I recall Justin Thomas posting recently about SF (bassman?) and how critical the lead dress is. I even think he was dealing with a squeal or similar harsh glitchiness.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    Comment


    • hahaha... guilty as charged. those solderings could look a lot sexier. they did initially but as a moved things around trying different configurations they started to degrade. will definitely clean them up.
      i remember taking all the RCA jacks out and cleaning them about 6 months ago will give it another go. same for the pots.
      i like the idea of re-defining the grounding around the reverb circuit.
      still pending on my side to try the voltage divider and the faraday cage (already got the roll).
      i have a gig again this week so, not sure if I will open the amp again before next weekend but will definitely keep you guys posted as soon as i do.
      thx!!!

      Comment


      • still pending on my side to try the voltage divider
        I consider the voltage divider a kind of emergency solution, because it not only reduces grid drive but also rolls off highs. This could be compensated by a "bright cap" across the upper resistor....

        I recommend to first try to connect the 560p cap to the cathode(s) (top of 470R) instead of point A. And then try a resistor of 1..5k in series with the 560p as this will recover some reverb treble.
        - Own Opinions Only -

        Comment


        • been a bit busy lately and had no time to open up the amp again... nevertheless I've been experimenting with new sounds. Hope you guys like it.

          Comment


          • That's fun. And a little hypnotic.
            "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


            • hi all! i'm back... vacation, a couple of days out sick and some work priorities

              i am going to implement this one today:

              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

              Comment


              • Hi all,
                was experimenting with the proposal below using different R values on the resistor highlighted as 1,500 Ohm



                In the video below the first frame shows the original oscillating (without 560 pF cap), the second frame the remaining bump with the 560 pF cap and then the effect of several resistor values from 500 to 4,000 Ohm
                (also tried a 10,000 Ohm R not shown here with the same result)

                The conclusion is that those R values show no effect/change in the “bump”



                PS: also tried a couple of different R values switching the 560 pF cap to the “A” point with the same result

                Comment


                • The conclusion is that those R values show no effect/change in the “bump”
                  Alright, but I think it was worth a try. And you might like the reverb sound better with a series resistor.

                  This thread has been going on for so long that I lost track of modifications, test results and open proposals.
                  I think what is missing is reverb drive reduction by a voltage divider at the V3 grid.

                  If the "bump" in the video shows worst case I would consider it purely cosmetical, hardly having audible effect.

                  And please don't forget to always post scope and amp settings.
                  - Own Opinions Only -

                  Comment


                  • sure thing, it definitely was worth trying. thanks a lot for suggesting it.
                    About the "bump", I cannot consider it only a cosmetical effect as it is the only visible effect I see on the scope that correlates with what that "farty" sound I hear when I connect the reverb (that noise around 9KHz you guys can see on the video in posts #268 - humbuckers - and #278 - single coils).
                    As for the sound, I did not really do an audible test.
                    Your memory is good, the voltage divider was the last thing on the list that's still pending (and the Faraday cage)
                    Thx!

                    Comment


                    • And also the sig gen settings, and the circuit node that the scope probe was connected to
                      Looks like you were feeding it a squarewave. or otherwise overdriving V3?

                      Plus, is the schematic in post #322 accurate, ie no grid stoppers on V3?
                      My band:- http://www.youtube.com/user/RedwingBand

                      Comment


                      • Sure thing, as I am always using the same 400 Hz Sine signal, Channel volume at 7.5+, Master Volume at 3-4 with the scope placed at the SPKR OUT I didn't feel like repeating myself. But you are right, it makes it easier for you guys who are surely following multiple threads (and this one has been long).
                        Also scope settings are .5mV/DIV .2 ms/Div

                        Originally posted by pdf64 View Post
                        is the schematic in post #322 accurate, ie no grid stoppers on V3?
                        That's correct, the grid stoppers are not in the original circuit but we gave them a try between posts #75 and #85
                        However, at that time we were fighting against the screech that was solved by lead dressing
                        It is also true that at that time I did not have a scope to evaluate the impact of the grid stoppers on the (later identified) parasitic oscillation.

                        So the question now is which way to go first: voltage divider or re-installing the grid stoppers

                        Comment


                        • I'm not convinced that the issue really is parasitic oscillation.
                          I think that either may help, but, with the settings used, as the reverb drive level is almost certainly way too high (and overdriving V3), a voltage divider to reduce the signal level at V3 grid is probably the best option.
                          My band:- http://www.youtube.com/user/RedwingBand

                          Comment


                          • the parasitic oscillation was the general consensus until now, however always happy to consider other options.
                            the channel volume is a bit high as those are the settings i needed to se the “bump” or the oscillation on the scope.
                            maybe you can give a listen to the videos in posts #268 - humbuckers - and #278 - single coils
                            specially on #278 the ch vol is at 7 and works great (overdriven sound) until i plug the reverb in

                            Comment


                            • well!!!... the voltage divider worked and i cannot see the “bump” on the scope anymore (all amp, signal generator and scope settings as above)
                              tweaked the pots around and couldn’t reproduce it anymore, so... it’s a winner!




                              I do not know how it sounds yet but will post when i do
                              thanks!!!

                              UPDATE 1: i thought that the reverb was not working at all when actually it is but sounds like it is at a super low volume (close to 0) when the pot is at 10. maybe i need to experiment with different R values for the voltage divider, like 750 Ohm on the ground side and 250 on the cap side?
                              Last edited by TelRay; 07-28-2019, 10:06 PM.

                              Comment


                              • Originally posted by TelRay View Post
                                well!!!... the voltage divider worked and i cannot see the “bump” on the scope anymore (all amp, signal generator and scope settings as above)
                                tweaked the pots around and couldn’t reproduce it anymore, so... it’s a winner!




                                I do not know how it sounds yet but will post when i do
                                thanks!!!

                                UPDATE 1: i thought that the reverb was not working at all when actually it is but sounds like it is at a super low volume (close to 0) when the pot is at 10. maybe i need to experiment with different R values for the voltage divider, like 750 Ohm on the ground side and 250 on the cap side?
                                Those resistance values are WAY too low for the high impedance of the circuit. This makes the circuit actually divide against the signal impedance rather than on it's own as a voltage divider. You need to use values in the hundreds of thousands of ohms for this. Like a 220k series resistor behind the existing 1M load.

                                EDIT: So... That's the reason the nasty is gone. Further mitigation is likely happening because you've basically shunted some higher HF virtually to 0V right off the sending triode. It's like turning the reverb off AND adding a low pass filter to the dry signal the way you have it now.
                                Last edited by Chuck H; 07-29-2019, 12:21 AM.
                                "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

                                Working...
                                X