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

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  • #61
    Originally posted by TelRay View Post
    hey man, I am willing to give it a try to see what's the effect. Can you help me out understanding how is it done?

    Ultimately I want the amp sounding good as intended in the original schematic without adding too many "fixes in the mix"

    thx!
    To be fair, you've removed the boost switch***, so it's already not as per the original schematic. On that note...

    Notice in your last video that the controls that will abate the screech when turned still produce a noteworthy pop and audible gain reduction in the range that they also stop the oscillation. So this would be those controls introducing negative feedback, that would be countering the positive feedback that is responsible for the instability. I also suspect that there is enough channel interaction with V1 in it's socket that negative feedback from that channel also serves to stabilize the condition. ***It may well be that the boost switch wiring also served to stabilize the amp because it's leads proximity to others producing some negative feedback. My assessment so far...

    The amp as it is is inherently unstable. There are two possibilities. One is that there is a problem with the decoupling function of the power supply. This could be due to a miswire (even a minor one that could be missed the first ten times you look it over) or it could be that this particular model amp bordered on instability as it was made. Many silver face Fender amps were actually. In production they may have found that it worked acceptably when new and fully stock. As in boost circuit in place and V1 installed. Change the formula with new caps, filters, boost removed, V1 removed, any associated lead dress changes, etc. and the anomalies that were keeping the original design stable are gone. My opinion on the matter so far...

    Look over the power supply filtering one more time to be sure that it's absolutely correct and decoupling between nodes. If you replaced the old filters with stale ones that could also be an issue. If it's determined that the power supply is correct and the new caps are working properly I say DITCH THE CIRCUIT. If it happens that the amp is only barely stable anyway then you probably never will get good overdrive tones from it in my experience. It will always seem harsh or otherwise odd and off the mark. You have a chassis, pots, tube sockets, tubes, transformers, speakers and a cabinet. Everything you need to build a really great amp.

    Should it come to this you could count on a lot of support here regarding lead dress and grounding schemes to (almost surely) make it MUCH more stable and therefor eminently more useful than it ever was, even when it was new. Not to mention losing the old circuit board. Which I consider another problem waiting to happen anyway.

    It's a big undertaking if you've never built an amp though. JM2C on this one.
    "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


    • #62
      Originally posted by Chuck H View Post
      ......If you replaced the old filters with stale ones that could also be an issue. If it's determined that the power supply is correct and the new caps are working properly I say DITCH THE CIRCUIT. If it happens that the amp is only barely stable anyway then you probably never will get good overdrive tones from it in my experience......
      Agreed! IMO, the master volume Twin was Fender's attempt at a spork, which is neither a very good spoon or fork. You won't get a good overdrive sound out of it without a stomp box.
      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

      Comment


      • #63
        Originally posted by pdf64 View Post
        Great, you might wish to reconsider the grid stopper mod for V3 suggested previously.
        Originally posted by TelRay View Post
        hey man, I am willing to give it a try to see what's the effect. Can you help me out understanding how is it done?
        He mentioned them in post #11 (22K). Grid stopper resistors are mounted right on the socket pins, in series. So you disconnect the wire from the grid, then connect the resistor between wire and socket pin.
        Because he said 'stoppers' plural, I assume he meant one on pin2 and another on pin7.
        Hopefully he will correct me if he meant to leave the 2 grids tied together and just use one grid stopper.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #64
          Chuck (and everyone),

          A couple years ago I had a MV Bassman 50 that someone had hack-modded into a "Marshallized" mosquito-chainsaw machine. I gutted it & did my best to rewire it to a clone of my AB165. I left the MV out in the process, and had to change some of the grounds.

          Sure enough, I had a really nasty attack on almost every note on tge E & A stings, it distorted more than should be expected, andvjust seemed "off" compared to my AB165. I put it back to the stock circuit & got 90% of the way thete but still had a huge "thud" attack on low notes. I had left one ground wire where I'd had it. I had no idea where it went originally in 1975, until I found some good pics of the same amp online. I put that one ground wire where it was in 1975 & it was a great amp.

          I will back you on your assessment that the amp is inherently unstabe, and that the boost/master wiring may have helped with stability.

          I would say put the amp back COMPLETELY to stock. That means put the Master & Boost back in. Find some nice pics of the identical amp and follow the layout perfectly. And remember: just because the amp HAS a feature doesn't mean you have to USE that feature.

          Just my 2 cents...

          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


          • #65
            Originally posted by Justin Thomas View Post
            Chuck (and everyone),

            A couple years ago I had a MV Bassman 50 that someone had hack-modded into a "Marshallized" mosquito-chainsaw machine. I gutted it & did my best to rewire it to a clone of my AB165. I left the MV out in the process, and had to change some of the grounds.

            Sure enough, I had a really nasty attack on almost every note on tge E & A stings, it distorted more than should be expected, andvjust seemed "off" compared to my AB165. I put it back to the stock circuit & got 90% of the way thete but still had a huge "thud" attack on low notes. I had left one ground wire where I'd had it. I had no idea where it went originally in 1975, until I found some good pics of the same amp online. I put that one ground wire where it was in 1975 & it was a great amp.

            I will back you on your assessment that the amp is inherently unstabe, and that the boost/master wiring may have helped with stability.

            I would say put the amp back COMPLETELY to stock. That means put the Master & Boost back in. Find some nice pics of the identical amp and follow the layout perfectly. And remember: just because the amp HAS a feature doesn't mean you have to USE that feature.

            Just my 2 cents...

            Justin
            I agree that could "work". But I don't think it's the best amp it could be for the level of work involved. Especially considering that original caps would need to be somewhat duplicated in size to emulate component and lead dress well enough. It wouldn't be much more work to just build a better amp in that box. Some Twins have a reputation for harsh overdrive tone. This is called bunk by owners of Twins that don't have that problem. But that sort of rep doesn't come out of thin air. I think this may be one of those models that doesn't get there. A good clean tone is fine if all you play is clean jazz and/or use a pedal board and never clip the power tubes. But you might just as well get a Roland JC or any other high powered SS amp at that point. A tube amp that won't clip gracefully with a guitar plugged in is a shame to it's genre IMO. It's those models in the silver face line with this problem that gave them all a bad reputation. I wouldn't spend too much time trying to polish a turd. I really don't mean so much offense by that. It's just a fact of life with some SF models. This may be one of them. Why restore an amp to it's former mediocrity?
            "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


            • #66
              I hear that too... I may have missed how much this poor thing has been modded already. If youre gonna start over, though, may as well go full-on turret board.

              On another note, I thought it was the first-series --#68 series with their wonky "mixed-bias" that gave the SF amps a bad rap... And of course, the late SF models were never meant to distort, period. Not without that terrible "turn the reverb recovery into a boost" switch, anyway.

              No offense taken; we just have 2 different ways of polishing a turd!


              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


              • #67
                Originally posted by g1 View Post
                ...Because he said 'stoppers' plural, I assume he meant one on pin2 and another on pin7.
                Hopefully he will correct me if he meant to leave the 2 grids tied together and just use one grid stopper.
                I've done it both ways with the same improvement but it's probably more correct to use a grid stopper per grid. Just remove the wire and link wire to V3 grids, fit the resistors to the grid terminals with their bodies very close to the terminal, twist the other resistor leads together and fit the wire there, with a bit of heat shrink over the joint. Perhaps best to use 22k if shared, 47k if 'per grid', though almost any value of stopper helps.

                The point is that V3 of the later SF models is biased very hot, and with any classic Fender reverb channel, a type 1 master vol (as fitted here) reveals that it overdrives horribly, with the earliest and most significant clipping point being the V3g-k diode. When the signal level reaches sufficient magnitude, that diode clips the top of dry signal path's wave, via a 500pF - 1M high pass filter. Grid stopper/s on V3 act to greatly reduced the degree and harshness of dry signal path clipping there, leading to less depth but much more pleasant, overdrive.

                The grid stopper also acts to improve the channel's margin of stability, which can be somewhat marginal at best.

                There are a multitude of circuit 0V common connections to the chassis in these amps; a full service should include re-establishing them all the mechanical ones (slacken off pot/jack/RCA/screw fasteners, rotate any star washers a fraction and retighten), and verification of all soldered ones (close inspection using visual magnification and bright light, whilst giving attached wires / leads a wiggle).
                My band:- http://www.youtube.com/user/RedwingBand

                Comment


                • #68
                  wooow... there’s a lot of new info... thx everyone so much for your contribution
                  will read everything today and try to understand what the next steps shoild be

                  Comment


                  • #69
                    I'm back after doing a lot of reading...
                    thank you guys, this is an overwhelming amount of information. I appreciate your help immensely.

                    As the conversation has somehow expanded and some assumptions were made I think I should re-cap (that does not mean replacing capacitors now... hahahaha):

                    - ORIGINAL PROBLEM: high frequency distortion at VIBRATO CH volume 7+
                    - MODIFICATIONS: I intend to have a working amp with a circuit that reflects the schematic previously shared: Schematic Fender 100 Watts RMS AMP w/ Reverb and Vibrato (the white schematic, not the blue one)
                    - CURRENT MODIFICATIONS:
                    1) added a "variable bias resistor" replacing the 15KOhm resistor by a 10KOhm + potentiomenter (I wanted to understand if the PROBLEM was bias adjustment related)
                    2) removed all the associated PULL BOOST circuitry (wires + potentiometer). as the PROBLEM reminded me of the sound produced by the amp in PULL BOOST mode
                    3) capacitors, resistors, diodes: replaced (kept the originals)

                    EXPECTATIONS:
                    Do I expect a Fender Twin o overdrive my guitar sound like a Marshall? Not at all. I want to be able to push it to a sweet light overdrive tone that can be boosted with the overdrive and distortion stomp boxes I own.

                    ASSESSMENTS:
                    I very much like (even given my - still - poor electronics knowledge) what Chuck H said: the oscillation might be counterbalanced by controls and tubes introducing negative feedback.

                    PENDING TASKS:
                    - check power supply filtering (I understand this as checking capacitors, resistors, voltages and grounding in the doghouse. Please correct me if I am wrong)
                    - decoupling between nodes (I believe this involves checking a low pass filter that has the task of eliminate noise propagation from one circuit to another, I am not able to locate what components exactly are to be checked)
                    - add grid stoppers on V3 (pins 2 and 7, each one with a 47 KOhm resistor - see image below)


                    - check mechanical components 0 V

                    Comment


                    • #70
                      Check the filter caps. I once had a very nasty distortion in a Vibroking if pushed past half on the volume pot. Turned out one of the preamp filter caps failed completely (measured zero uF)

                      Comment


                      • #71
                        Hello all,

                        on the topic of grounding, which is something I think I need to be 100% sure is OK as many of you suggested.
                        There are currently 6 long (6 to 10") coaxial cables (+ 1 more coaxial cable from the PULL BOOST circuit that is currently out of the system).



                        I could classify the uses of these cables in 3 categories:

                        1) running from INPUT JACKs to TUBE sockets
                        the core of the wire is connected to the channel input jack tip (through a resistor) and runs to either the V1 or V2 pin #2
                        the shield of the wire is connected to the channel input jack ground (which is grounded to the chassis through the star washers) and is cut short about 1/2" before reaching the tube (remaining fully in the jacket, no wires sticking out)


                        2) running from TUBEs to CHANNEL VOLUME pots
                        the core of the wire goes from V1 and V2 pins #7 to the wiper lug of the corresponding channel volume pot.
                        the shield is connected to one of the outer pot terminals, which is then connected to the same terminal on the MID control and connected to the chassis through a wire. this shield wire also is cut short about 1/2" before reaching the tube (remaining fully in the jacket, no wires sticking out)


                        3) running from CIRCUIT EYELETS to the MASTER VOLUME potentiomenter (two cables)
                        3.i core wire comes from V6 pin #2 through a .01 uF ceramic disc cap to the master volume's wiper lug
                        shield wire goes out of one of the outer pot terminals and is cut short about 1/2" before reaching the eyelet (remaining fully in the jacket, no wires sticking out)

                        3.ii core wire from the circuit eyelet to the other outer pot terminal
                        the shield wire comes from a circuit eyelet to join the shield wire of 3.i on the other outer terminal

                        I have added all those shield wires in red to the following layout



                        my question is: following the logic of how the shield wire on the coaxial cables is being used in cases 1 and 2, shouldn't the shield wire in 3.i be connected to ground?

                        These shield wires coming out of potentiometers and jacks and connected to nothing on the other end are not described in any schematic, so it's difficult for me to understand the difference stated above.

                        Additionally, I would like to replace those cables for new (high quality) ones (some of them looked like they suffered from several sessions of soldering). Any recommendations on what / where to buy bulk cable? (physically in the bay area would be great, otherwise online)

                        Thx!
                        Last edited by TelRay; 03-02-2019, 04:58 PM.

                        Comment


                        • #72
                          Remember what Enzo mentioned about looking at the circuits. The shields from the channel mix to the master and from the master to the PI input cap are not grounded, per se', but they do pass through a small 100 ohm feedback shunt resistor to ground (and alternately through the 820 ohm feedback series resistor which goes to a load of only a couple of ohms from ground). To the signal chain this is nearly grounded. I think it's an odd choice to put it there instead of ground. Perhaps this backs my notion that the design is inherently unstable and having the master and it's shields on the feedback node instead of ground helped to stabilize it.?.

                          Regarding shields in general, you do want only one end grounded (or virtually so) to avoid having a ground loop hum being impressed on the center, signal lead.

                          You could try an actual ground for the master circuit and it's shields to see if it helps I suppose. Nothing to lose and you can always put it back like it was.

                          Where to find shielded lead? Well you order it from Mouser in huge spools of course I actually do have a couple of spools of shielded lead wire (one conductor and two) but I've used all kinds of stuff. My favorite is the thin, molded end mono audio cables used for cheap stuff. But don't just start hacking up everything you have with a thin black cord on it. Most doo dad audio cables aren't mono and so will have more than one conductor. You can use them, of course, but it bothers my aesthetic sensibilities. YMMV. I've even used plain ol 75 ohm coaxial cable. Pretty much any shielded cable will work without enough difference in capacitance per foot to matter.
                          "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


                          • #73
                            Originally posted by Chuck H View Post
                            I think it's an odd choice to put it there instead of ground...You could try an actual ground for the master circuit and it's shields to see if it helps I suppose. Nothing to lose and you can always put it back like it was.
                            Thx Chuck!
                            That's what I thought too, however I did connect the shield to ground... nothing improved (or got worse for that matter)
                            It was worth the try as I keep thinking that a "magic" wire not grounded properly will reveal itself an solve all my problems
                            thx for the advice on the cable!

                            I guess GRID STOPPERS on V3 is the next stop...

                            Comment


                            • #74
                              You may find Merlin's document on grounding useful http://www.valvewizard.co.uk/Grounding.pdf
                              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.
                              My band:- http://www.youtube.com/user/RedwingBand

                              Comment


                              • #75
                                GRID STOPPERS installed (recycled the old original 47 KOhm resistors, that were at
                                47.1 and 48.8 KOhm)



                                I think I want to hear some improvement, however not sure I do



                                thx, pdf64... will read that and continue chasing grounding issues
                                Last edited by TelRay; 03-02-2019, 10:49 PM.

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