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

  • hope this is more understandable. Due to the irregularity of the waves I have looked for the voltage between the highest peak and lowest valley.
    I would say the 20 uF eCap reduces the voltage in half

    Comment


    • I would say the 20 uF eCap reduces the voltage in half
      Agree, signal as well as ripple appear to be reduced by aproximately 50%. That's just what you would expect from doubling the filter capacitance at node A.

      Now I wonder what you see doing the same with the MV at zero, meaning zero output. This should reduce the "jumpiness" (ripple voltage) and avoid signal injection by the power tube screen currents. The idea is to separate/isolate the influence of V3 on the power supply signal voltage.
      - Own Opinions Only -

      Comment


      • Originally posted by Helmholtz View Post
        Agree, signal as well as ripple appear to be reduced by aproximately 50%. That's just what you would expect from doubling the filter capacitance at node A. Now I wonder what you see doing the same with the MV at zero, meaning zero output. This should reduce the "jumpiness" (ripple voltage) and avoid signal injection by the power tube screen currents. The idea is to separate/isolate the influence of V3 on the power supply signal voltage.
        thx, mister.
        1.- Add 20 uF / 500 V Cap we'll have to wait to see the influence of the MASTER VOLUME as I had to put the amp back together for a show tonight

        3.- 560 pF cap I have some update on placing the 560 uF in different positions that I will post later on today.

        6.- Faraday Cage in case this is of use for someone else in the future, I have found at Lowes a nice mill flash roll (thanks for pointing me in the right direction because I did not know this denomination and would have only search for Aluminum foil). 14 in by 10 ft 200um thickness) for $14

        https://www.lowes.com/pd/amerimax-14...iABEgLIhfD_BwE

        Comment


        • 3.- 560 pF cap: moved it around following the evolution of the Fender schematics. The conclusion is that there is a difference between having the cap or not in the circuit but the effect shows to be exactly the same regardless of the cap being connected to POINT "A" or the GRID RESISTOR



          2.- .0022 uF Cap try the different positions described on Fender Schematics, already did that and the position never showed to influence the oscillation

          Comment


          • I guess the pictures are taken at the amp's output?

            The conclusion is that there is a difference between having the cap or not in the circuit
            Looks as if the 560p mainly lowers the resonant frequency.

            but the effect shows to be exactly the same regardless of the cap being connected to POINT "A" or the GRID RESISTOR
            Yeah, that's what I expected after you mentioned that you changed the CATHODE (not grid) resistor from 470R to 2k2 in parallel with 25µF. The result may have been different with the single 470 Ohm cathode resistor. ( Not easy to keep track of your modifications.)

            2.- .0022 uF Cap try the different positions described on Fender Schematics, already did that and the position never showed to influence the oscillation
            Ok, but having the cap at the reverb output jack should improve/boost the treble response of the reverb signal.
            - Own Opinions Only -

            Comment


            • Originally posted by Helmholtz View Post
              I guess the pictures are taken at the amp's output?
              cooorrect... this was the agreed verification method for this specific modification (560 pF) on the post in which I listed all the recommendations in order
              (Verification: scope at the SPK OUT)

              Originally posted by Helmholtz View Post
              Yeah, that's what I expected after you mentioned that you changed the CATHODE (not grid) resistor from 470R to 2k2 in parallel with 25µF. The result may have been different with the single 470 Ohm cathode resistor. ( Not easy to keep track of your modifications.)
              hahahah... I know, and I am even changing things around at a much slower pace than you guys are suggesting.
              just to re-cap:
              The amp originally had the 2K2R in parallel with the 25 µF cap, when I bought it (they were modern components so I understand this was a modification done by a previous owner). As I was having problems in that area I reverted to what the schematic showed, placing the 470R.
              On this thread it was suggested to go the "blackface" arrangement of 2K2R in parallel with the 25 µF cap.
              I was actually thinking of placing the 470R again in the circuit and see what happens moving the 560 pF cap around.

              Originally posted by Helmholtz View Post
              Ok, but having the cap at the reverb output jack should improve/boost the treble response of the reverb signal.
              yessss, the REV is a bit dark. but today I cannot use it at all as I had to play the gig last night with the REVERB cables disconnected to avoid those "farty speaker cone" sounds. I guess we will worry about this later

              Today the amp is a beautiful sounding FENDER TWIN. With time it will become a FENDER TWIN REVERB...
              again guys... thank you!

              Comment


              • 4.- Install a Voltage divider replace 1M resistor by 2x 500K

                Reason: Reduce grid drive
                Verification: scope at the SPK OUT

                Before trying this one out I wanted to make sure that what I have understood from Helmholz's recommendation (proposal #1) is correct (as I was reading a bit about out and found out there is an easy mistake to make)

                Comment


                • I proposed a real voltage divider as shown in your proposal #1, as it mainly reduces drive voltage. Your proposal #2 doesn't do much to reduce drive voltage. You may vary the individual resistor values, but the sum of the resistances should be close to 1M= 1000K.
                  Last edited by Helmholtz; 05-25-2019, 12:37 PM.
                  - Own Opinions Only -

                  Comment


                  • To illustrate the evolution of the oscillation and make it easier to historically track changes, please see the photo below.
                    The conclusion is that reverting to the 470R kills the oscillation at the settings I would normally use when playing: VIBRATO CHANNEL @ 6.5 and MV at 3-4 (keep in mind the .002 uF cap is still at REVERB IN as seen on the schematic extract above every oscilloscope photo).
                    If I push the amp above a CHANNEL VOLUME of 7.5 or more... the ""bump"" re-appears. If I keep the CH VOL at 6.5 the ""bump"" does not re-appear by an increase in the MV settings (went up to 10)



                    NOTE: the "bump" above CH VOL 7.5 is not changed by placing the .002 uF cap at the REVERB OUT or changing the 560 pF cap to V3 cathode (pins 3-8)
                    Last edited by TelRay; 05-26-2019, 05:14 PM.

                    Comment


                    • I looked through every page, but I couldn't find a clear shot of the phono/rca jack wiring. The one photo I could find wasn't clear enough to be sure, but it looked like some stuff may have been wired wrong. I'm sure some things have changed since then, but... Can we get a shot of the reverb and footswitch jack wiring?
                      "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
                        I looked through every page, but I couldn't find a clear shot of the phono/rca jack wiring. The one photo I could find wasn't clear enough to be sure, but it looked like some stuff may have been wired wrong. I'm sure some things have changed since then, but... Can we get a shot of the reverb and footswitch jack wiring?
                        sure, mister.
                        in theory you shuold be able to see this







                        Last edited by TelRay; 05-27-2019, 02:09 AM.

                        Comment


                        • @ Chuck, were the photos OK?
                          (just let me know if you want something else)
                          thx!

                          Comment


                          • The photos are fine. And (unfortunately?) wired correctly as far as I can tell. So I'm confused as to why disconnecting the reverb tank input would squash the problem. It's definitely a parasitic oscillation because when it first comes on you can see the time lag due to phase error on the one lobes return and then it's that area that develops the hash at higher gain. So that's a positive feedback. I only have observations at this time, not solutions.

                            Since removing the load from the reverb transformer secondary also reduces current through the reverb transformer I think you may want to investigate any daisy chained grounding or ground locations for specific circuits relative to the reverb. Try separating daisy chained grounds in the signal path for the reverb recovery and the mixer stage and make sure the reverb driver tube circuit and transformer secondary are grounded with their power supply filter and NOT the preamp. Except for the 1M resistor on the grid of the reverb driver tube. Keep that grounded with the preamp (but not sharing a ground lead with another circuit).
                            "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


                            • As the reverb transformer secondary is grounded to the chassis via the RCA jacks, it is important that these make perfect chassis contact. Also I don't like some of your solder joints.
                              I may make sense to dismantle the RCA jacks and clean the contact parts/chassis area with Deoxit (on a Q-Tip). The ground lugs are easier to solder when removed from the chassis.
                              Also clean and tighten the outer "crown" of the RCA plugs.
                              - Own Opinions Only -

                              Comment


                              • I didn't want to say anything But yeah, probably time to think about cleaning up the overworked solder from the repeated work.

                                As to the actual problem, I actually think it may be "normal" for that amp. 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
                                "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