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

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  • BTW, I can't remember everything now, and we might have covered this, but... Are you getting this problem with ANY configuration of elements in the test. That is, reverb tank in or out of the cabinet, amp chassis in or out of the cabinet, testing through the dummy load and the speakers, etc.
    "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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    • Originally posted by Chuck H View Post
      Those resistance values are WAY too low for the high impedance of the circuit..
      OMG... you are totally right. i don’t know what the hell was i thinking. i used 2x 500 Ohm resistors instead of 2x 500 K as described on #284 and #307

      please forget what i posted :P

      Comment


      • Originally posted by Chuck H View Post
        BTW, I can't remember everything now, and we might have covered this, but... Are you getting this problem with ANY configuration of elements in the test. That is, reverb tank in or out of the cabinet, amp chassis in or out of the cabinet, testing through the dummy load and the speakers, etc.
        thx for the very pertinent questions, mister

        in the past i took the reverb tank out the cabinet and the problem was still there
        in regards of the dummy load... i remember using the scope with the speakers connected but not to reproduce the oscillation or bump (because those are seen at a pretty high volume)
        so far i have always measured the oscillation with the board out of the chassis. however the board is seating on the amp's cabinet (proximity to the speakers magnet should be about the same, not the angle though if that has any influence). I still need to place the aluminum layer to close the faraday cage (haven't done that yet, sorry)
        now I have put the right 500 KOhm resistors in the voltage divider, the oscillation does not show... however there's still a difference in how the amp overdrives with the reverb tank connected and not (connected shows the ugliness we've heard in the past). still trying to determine if it is better or the same (will take some time).

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        • When I rebuild my SC200PLUS preamp to SLO100, i have that sound on my presence control. It was a positive feedback. I switched load wires from my OT to the pairs of KT88 and problem is solved.
          Last edited by Chuck H; 08-02-2019, 01:00 PM. Reason: Poster surely didn't mean "PT" so I changed it to OT.

          Comment


          • Originally posted by Vucinic View Post
            I switched load wires from my OT to the pairs of KT88 and problem is solved.
            hey, thank you for jumping in!
            i will need to look into that circuit to understand what you did (or if you want to add further details for a newbie)

            Comment


            • thx again Vucinic for sharing,
              I've taken a look to the SC schematic and I believe I can be more precise with my question now:



              - did you remove the old wires (A and B) an installed new ones?
              - did you flip the wires? instead of A connected to 1 and B to 2, switched them to A to 2 and B to 1?

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              • STOP!!!!

                I think we've already covered this in this thread somewhere, but a quick run through didn't reveal it. So...

                There's an easier way to tell if your OT is in incorrect phase. All you have to do is disconnect the NFB loop. One wire. If the amp get's louder and the problem remains then you DO NOT have to swap your OT leads. If the amp doesn't change much or gets quieter and the problem goes away then you may have the OT wired in incorrect phase. Only THEN you would proceed to swap the OT leads to the power tubes.
                "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


                • thx Chuck!
                  of course I wouldn’t start swapping wires without being 100% sure, many (maaaany) thanks for the warning
                  yessss, you are right and we did cover / checked the Negative Feedback. There was some confusion on my side because I wasn’t hearing a noticeable change when lifting the 820 R and that was because the NF is evident at high (near to 10) master volume settings.
                  we did very extensive and educational measurements to check it worked properly.
                  that being said and understanding the two issues are related (happy to learn they are) there will be no OT wire swapping

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                  • Now one thing I don't think we've discussed is phasing of the reverb drive transformer, and whether it could possibly make any difference.
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    Comment


                    • Originally posted by g1 View Post
                      Now one thing I don't think we've discussed is phasing of the reverb drive transformer, and whether it could possibly make any difference.
                      you are totally right! we have not. totally open to suggestions on how to evaluate that and furthermore if i need to specifically test the reverb transformer in any way.

                      thx!

                      Comment


                      • now I have put the right 500 KOhm resistors in the voltage divider, the oscillation does not show... however there's still a difference in how the amp overdrives with the reverb tank connected and not (connected shows the ugliness we've heard in the past). still trying to determine if it is better or the same (will take some time).
                        Using two 500k resistors as voltage divider reduces grid drive by 50%. This might not be sufficient. You may try to reduce the value of the lower resistor (connected between grid and ground) to increase drive attenuation.

                        V3 grid drive should be low enough to prevent V3 from producing unwanted distortion (= additional higher harmonics) that may trigger the damped oscillation (ringing).
                        Last edited by Helmholtz; 08-08-2019, 04:01 PM.
                        - Own Opinions Only -

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                        • Originally posted by Helmholtz View Post
                          ...try to reduce the value of the lower resistor (connected between grid and ground) to increase drive attenuation
                          I finally did that... and it worked! I am not able to hear any differences between the REVERB cable CONNECTED and DISCONNECTED. Vielen Dank Herr Helmholtz for making the suggestion.
                          After trying with a 500 K resistor on each side I had to go down to 230 K (grid to ground) and 835 K (actual measurements from the multimeter). [Pic below with resistors marked in YELLOW]



                          Up to this point I am at a point in which I have a Fender Twin REVERB (before the reverb tank could not be connected without triggering the undesired "glitch")... THANK YOU ALL!!!

                          After experimenting for a while and confirming I could not hear a difference I went to the second step, turning up the REVERB volume. What I found is that it is too low (even at 10) or, as previously discussed, also too dark. My question here is: installing the voltage divider is also affecting the Reverb volume? (it would be logic, right?). Or is it only the 560 pF cap darken effect (that almost helped killing the oscillation seen on the scope)?

                          That triggered the next modification, installing a 1,500 Ohm Resistor in series with the 560 pF cap. [Highlighted in green in the picture above].

                          I believe it has recovered a little bit of brightness (not volume) but still far from being chimey. As I understand I can take now to actions (separately or in parallel).

                          - increase the value of the resistance so that I decrease how efficiently the 560 pF cap cuts the high frequencies off
                          - change the 470 K resistor to increase the amount of REVERB signal that is mixed back with the guitar signal (actually this resistor was 220 K when I bought the amp)



                          PS: if you recommend I open a new thread for "Reverb tank Tone" I will be more than happy to close this one and open a new one.
                          Last edited by TelRay; 08-28-2019, 11:51 PM.

                          Comment


                          • I don't think a new thread is appropriate because we still have your amps particular sensitivity to the circuit to manage in the course of finding a circuit that delivers satisfying tone.

                            I have to wonder whether it's both the voltage divider at the reverb drive input or the 1.5k/560pf shunt OR BOTH that are responsible for ridding the amp of it's anomalous oscillation.

                            First... Don't reduce the value of the 470K resistor. There isn't enough load following that resistor to make a significant difference to the reverb output. But there IS enough difference in the load of that resistor and and the 100k pot following a 3.3M series resistor for a change there to attenuate the dry signal. So reducing that resistor value will reduce dry signal more than it will increase reverb. Unless the gain of the dry signal is not of any concern this isn't a good place to make the adjustment.
                            "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 don't think a new thread is appropriate because we still have your amps particular sensitivity to the circuit to manage in the course of finding a circuit that delivers satisfying tone.
                              Great feedback as usual, Chuck... thx for that!

                              What I read is that, even though we've found a way to kill the symptom (the harsh highs with the REVERB tank connected) we have not yet identified the root cause.
                              I am more than happy to continue the investigation (what about G1's doubts on the REVERB transformer's phase or how to check if the transformer is OK?)

                              Originally posted by Chuck H View Post
                              I have to wonder whether it's both the voltage divider at the reverb drive input or the 1.5k/560pf shunt OR BOTH that are responsible for ridding the amp of it's anomalous oscillation.
                              Well, even though in the picture above it looks as if I had done the two mods at the same time, I haven't.

                              The chronology is the following:

                              - installed a voltage divider with 500 K at each side (I'm omitting the failed attempt with the 500 Ohm resistors)
                              [can still hear the defect when connecting the reverb]

                              - changed to 230 K and 835 K as described above
                              [could not tell a difference between connecting or not the REVERB tank]

                              NOTE: both testings above done with REVERB Volume at ZERO

                              Only AFTER that (good sound with REVERB tank connected), I turned up the REVERB Volume.

                              What I found is:

                              - REVERB VOLUME too low (I can only hear the Reverb's tail after I stop playing, cannot really tell if the REVERB is ON while playing)
                              - REVERB TONE too dark

                              AFTER that I included the 1,500 Resistor (highlighted in GREEN above)
                              [volume still low, maybe a little brighter reverb tone]

                              Last night I changed to a 5,200 Resistor (measured)
                              [Reverb tone a little brighter but reverb volume still low]

                              That's why I am looking into:

                              1) Brighten the tone a bit more (I guess trying something closer to 10,000 R?)
                              2) Increasing the REVERB wet signal mixed with the VIBRATO Channel signal (I am still wondering if the voltage divider had also reduced the REVERB volume as a side effect, we are ultimately driving less V3, right?)


                              Originally posted by Chuck H View Post
                              Don't reduce the value of the 470K resistor. There isn't enough load following that resistor to make a significant difference to the reverb output. But there IS enough difference in the load of that resistor and and the 100k pot following a 3.3M series resistor for a change there to attenuate the dry signal. So reducing that resistor value will reduce dry signal more than it will increase reverb. Unless the gain of the dry signal is not of any concern this isn't a good place to make the adjustment.
                              loud and clear

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                              • (I am still wondering if the voltage divider had also reduced the REVERB volume as a side effect, we are ultimately driving less V3, right?)
                                Yes, of course. I was aware of this consequence when I proposed the voltage divider but hoped that there would still be sufficient reverb signal at higher settings (I personally never felt a need to set reverb higher than 3 on a Fender amp.) Modifying an existing design typically means compromising...

                                we have not yet identified the root cause.
                                Too me the root cause is reverb drive signal distortion (harmonics) at V3 plates exciting a resonance in the reverb transformer primary. And I am convinced that the behaviour is typical for many Fender amps. The voltage divider at its grid reduces this distortion. But it also reduces reverb volume and high frequency content.

                                Will post some ideas to somewhat brighten reverb the next days even though possibilities are limited.

                                Do you use the .002µ cap at the reverb return?
                                - Own Opinions Only -

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