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

  • #16
    Well there's a good possibility that the socket is worn, what with the "tube rolling" mentality***. But it may not be. Can you please report on whether all tubes tried cause the screech. I'm thinking it could be oxide on the tube pins themselves since some of the tubes you've tried pop and others do not. It doesn't come up often, but I've seen it. I had a small bucket full of 12au7 tubes pulled from a Conn organ that sat out in the rain. I had to use one of those brass wire toothbrushes from the hardware store to clean off the pins on most of them to get good contact in any socket. Also, what is the nature of the other tubes you are trying in those sockets? Are they new? NOS? Used but test good? Unknown? Also, it may even be V4 that's microphonic. Granted you are only detecting this issue on the trem channel right now, but I wouldn't rule out the possibility. The screech is very much like some of the worst microphonioc tubes I've heard. You really do need to plug in some known good preamp tubes.

    ***Tube rolling is an internet term used to describe the practice of trying multiple tubes in any given circuit position to find the best sounding combination and/or order for the tubes available. Many sockets take an unnecessary beating with this practice because time and frustration can lead to aggressive pulling and plugging. This is especially bad for amps that have PCB mounted sockets because it increases the risk of failed pads and traces.
    "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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    Comment


    • #17
      I would try 'retensioning' the socket tabs of V1.

      Comment


      • #18
        Hey guys, thank you!

        I have replaced the V1 socket by a new one. SCREECH is still there when I pull the V1 tube out.

        Regarding the question about the TUBES.I have about 12x 12AX7s in various stages of wear. Some NOS Sylvania and Tungsol (that I am just trying now), TAD used moderately, JJs (1 year old but not much use), etc.

        I have tested all of them (both triodes) in a Tube tester I bought some months ago. All of them shown different "health" status between 75-100%. Filament continuity was also tested.



        As far as if any tube is microphonic, I am using probability (tube rolling) to tell me if that could be the case. I took the V1 (NORMAL CHANNEL) and V5 (TREMOLO) tubes out and I am rolling every other tube. As I do not have more than 2 12AT7s when I finished with the 12AX7s I used them also in V3 (REVERB) and V6 (PHASE INVERTER).

        My conclusion is that the problem is not in the TUBES (see table below, every combination produced a SCREECH - edited from previous post, the AMPEREX also did, the settings were not right)

        Attached Files
        Last edited by TelRay; 02-24-2019, 08:17 PM.

        Comment


        • #19
          That's a hell of an effort. Thank you.

          I have to ask why you replaced the V1 socket when it's the V2 socket involved with the channel that is having a problem.?.

          And I have to ask... Are you certain that the problem does not manifest at all with the normal channel?

          Something to know is that the cathode is shared between V1b and V2b. When you unplug V1 you incidentally heat the bias and increase the gain of V2b. I'm not sure why you have V1 removed at this time.

          If you could, please, measure and post voltages on V2. You may have posted them in a previous thread, but new readings would be better.

          Also, you noted that the reverb no longer affects the screeching while now the mid pot does. But the test parameters in the videos are not equal. In the first video you left the volume at max and the reverb did stop the screech set between min and max. And you didn't turn the mid knob at all. In the second video you turned the volume down to just where it manifests the screech at around 3.5. So, if you put the volume at max does the reverb again stop the screech between min and max settings?

          Ok... Chasing parasitic oscillation now since I no longer believe it's tube microphonics. Though it still may be a dirty or bad socket. The voltage measurements may tell us something there.

          It's hard to tell looking into the spaghetti plate that is the silver face amps and the photo angle is such that some face deck wiring is obscured, but I would actually strip out any wiring associated with the boost switch. Having them bundled may have been keeping the amp stable when they were connected to the switch. Now, with leads hanging on one end there is a different relationship between those leads. They are carrying small signal information over a lot of real estate and could be causing instability. So I mean EVERY lead associated with the boost including those from the board or the reverb pan jacks.
          Last edited by Chuck H; 02-24-2019, 09:09 PM.
          "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

          "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

          Comment


          • #20
            Originally posted by Chuck H View Post
            I have to ask why you replaced the V1 socket when it's the V2 socket involved with the channel that is having a problem.?
            Sure. If you remember, the SCREECH can only be generated with the V1 out of the circuit. Additionally, I found that if I move (rocking back and forth) the V1 tube (when in the circuit) there was a "pop" (like disconnecting and connecting something). That gave me the idea that the V1 socket was not good (no other tube made any noise when moving them) and thought that maybe when removing the V1 tube I could be generating a bridge between some of the V1 socket leads (that are more freely to move without being held by the tube).

            I believe that by removing the V1 tube, the additional gain on V2 makes the problem more evident (by creating the SCREECH, which - i believe - an extreme example of the high frequency distortion I hear when playing the guitar at higher VIBRATO CHANNEL volumes

            Originally posted by Chuck H View Post
            And I have to ask... Are you certain that the problem does not manifest at all with the normal channel?
            No. When playing the NORMAL CHANNEL the guitar does not even enter in overdrive at CH VOL = 10 (it's super clean all the way). So I am guessing that because of the lower gain on V1 I cannot reproduce the SCREECH.

            One might wonder what would happen if I increase the gain on V1 by pulling V2 out of the circuit... maybe I'll try

            Originally posted by Chuck H View Post
            Something to know is that the cathode is shared between V1b and V2b. When you unplug V1 you incidentally heat the bias and increase the gain of V2b. I'm not sure why you have V1 removed at this time.
            Yepp... that cable bridging pin 8 of V1 to the same pin on V2.
            Well, the idea of taking out V1 was one of the many attempts to do something that would reduce that high freq distortion. I wanted to try if putting more gain on the V2 tube and the guitar breaking up at a lower channel volume setting would make any difference. It didn't.
            Since it wasn't either better or worst I didn't put V1 back (hence saving some $$$)

            Originally posted by Chuck H View Post
            If you could, please, measure and post voltages on V2. You may have posted them in a previous thread, but new readings would be better.


            (have not found any difference by measuring V2 with V5 in the circuit and no V1)

            My conclusion:
            isn't it strange that pulling V1 out the voltage on V2-8 drops from 1.9 to 1.4V? (I am almost sure it should go up... that additional gain on V2 statement)
            Also V2-6 drops from 231 to 193V

            Originally posted by Chuck H View Post
            Also, you noted that the reverb no longer affects the screeching while now the mid pot does. But the test parameters in the videos are not equal. In the first video you left the volume at max and the reverb did stop the screech set between min and max. And you didn't turn the mid knob at all. In the second video you turned the volume down to just where it manifests the screech at around 3.5. So, if you put the volume at max does the reverb again stop the screech between min and max settings?
            You are right, given the right knob position combinations I've found that the SCREECH can be eliminated by moving the MIDDLE, REVERB and TREMOLO INTENSITY KNOB out of the MIN position (this last one even when V5 is not in the circuit!)

            Originally posted by Chuck H View Post
            It's hard to tell looking into the spaghetti plate that is the silver face amps and the photo angle is such that some face deck wiring is obscured, but I would actually strip out any wiring associated with the boost switch. Having them bundled may have been keeping the amp stable when they were connected to the switch. Now, with leads hanging on one end there is a different relationship between those leads. They are carrying small signal information over a lot of real estate and could be causing instability. So I mean EVERY lead associated with the boost including those from the board or the reverb pan jacks.
            Yes, I had the same feeling and stripped down every wire that had to do with the PULL BOOST (meaning I did not just tape them and left them hanging around, they are totally out). Additionally have also changed the MASTER VOLUME potentiomenter.
            Last edited by TelRay; 02-25-2019, 12:14 AM.

            Comment


            • #21
              And has anyone thought to check for conductive eyelet board?
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #22
                Originally posted by Enzo View Post
                And has anyone thought to check for conductive eyelet board?
                thx Enzo!

                i lifted the upper board board to check if there were any leads in between the the two a few days ago. I could only separate them by 1/4 inch but haven't seen anything strange.
                do you mean the the actual material could be conductive?

                Comment


                • #23
                  It's normal for the current through V2b to increase when v1 is pulled. The voltages drop because there's more current through the tube.

                  You omitted grid pin measurements. They're probably zero (or whatever your meter reads at null). If there's any significant voltage there it could be a problem. Looking at your cathode and plate voltages I don't suspect that though.

                  As to conductive circuit borard... OHHhhhh YEs!.!.! That's a real thing. Those old, black paper fiber boards can become conductive in a couple of ways. One is that they're made of paper fiber and have been known absorb moisture in humid environments. In these cases it's often possible to use a hair drier to bring things back to normal. It doesn't take much voltage to throw off things like preamp tube bias. An extra .2 or .3 volts on a grid or cathode is enough to make a big difference. Another way they become conductive is time and pressure under high voltage conditions. Why do you suppose those boards are black? Because they're pigmented with carbon. repeated soldering or long exposure to HV (which is common in these amps) can start to form conductive pathways in the carbon pigment. In this case the board needs to be replaced or the affected eyelet points must be floated off the board. I, personally, have had a disproportionate number of the black fiber boards I've experience demonstrate some degree of conductivity.

                  Set your meter for low voltage and put the black probe on the chassis. Now stab the board in random places with the red probe. Especially near HV eyelets. You'll almost surely see SOMETHING. I always do. But don't worry about it unless it's more than a few mV.
                  "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

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                  Comment


                  • #24
                    Originally posted by Chuck H View Post
                    You omitted grid pin measurements. They're probably zero (or whatever your meter reads at null). If there's any significant voltage there it could be a problem. Looking at your cathode and plate voltages I don't suspect that though.
                    Yessir, those are 0V

                    Originally posted by Chuck H View Post
                    As to conductive circuit borard... OHHhhhh YEs!.!.!
                    Oh no... sounds like a real pain in the a**. And surely will need to de-solder some wires, that board was not moving much more than 1/4 in when I inspected if there was something below.
                    Sounds like another weekend of fun

                    BY THE WAY:
                    I have removed the BRIGHT SWITCH off the circuit (I thought maybe it was connected to ground somehow or leaking in between leads, etc).
                    After it was out I've tried to reproduce the SCREECH with 3 different 120 pF capacitors (the original one and 2 new ones) by connecting them directly to the potentiometer leads, in all cases I got that ugly noise back.
                    So, it's not the capacitor (by now I should have changed the name of the post ) and it's not the switch

                    Comment


                    • #25
                      Ok... So the middle knob, the reverb knob and the trem intensity knob can all stop the screech. The reverb knob only stops the screech when the volume is maxed. It's not microphonic tubes. If this were just a little less weird I'd think you were F'ing with us, but there's no way you can make this up

                      I'm back to thinking there's a bad ground somewhere. Test grounds from the board eyelets that are supposed to lead to ground to the chassis. Not to the actual ground point.

                      Are all knobs doing what they're supposed to? The "Middle" affects midrange, etc.?.

                      This is a long shot, but... Try lifting one end of the 820R feedback resistor and see if that doesn't change anything.
                      "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

                      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                      Comment


                      • #26
                        Originally posted by Chuck H View Post
                        ...If this were just a little less weird I'd think you were F'ing with us, but there's no way you can make this up
                        hahahahaha... I kid you not!
                        this is the original problem I am trying to solve
                        the whole story about the SCREECH noise is just a way to reproduce this failure mode to the max




                        Originally posted by Chuck H View Post
                        Are all knobs doing what they're supposed to? The "Middle" affects midrange, etc.?.
                        Yepp... the all work as expected, maybe the MID is more MID-LOW, but yes.

                        Originally posted by Chuck H View Post
                        This is a long shot, but... Try lifting one end of the 820R feedback resistor and see if that doesn't change anything.
                        I was wanting to do that with the NFB resistor for a while but did not want to messs up with the amp until I got ot sounding good.
                        I think it will provide more MIDs and probably provide with more overdrive.
                        Will try thay probably on Tuesday and let you know
                        Last edited by TelRay; 02-25-2019, 04:38 PM.

                        Comment


                        • #27
                          You don't have to move the board to test for conductive surface. Just ground your meter to the chassis, then put your red meter probe right on the black material NEAR an eyelet and see if any DC voltage is measured. SImple as that.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #28
                            Originally posted by Enzo View Post
                            You don't have to move the board to test for conductive surface. Just ground your meter to the chassis, then put your red meter probe right on the black material NEAR an eyelet and see if any DC voltage is measured. SImple as that.
                            that's great news, I thought that the board below could also be contributing (i think it is the same material) and that the two needed to be separated.

                            will do it within the next couple of days and let you guys know

                            thx!!!

                            Comment


                            • #29
                              Originally posted by TelRay View Post
                              ...when moving the pots the wire connecting the MID control to the VOL one and then to ground came loose �� (could be an indication of failure)



                              As I recall, Fenders are usually wired with the 0V circuit common connection to the mid control terminal as a short length of wire braid to the control panel brass plate; the wire in yours to the vol control's 0V terminal seems unusual. Is that definitely connected to the chassis / 0V circuit common?
                              My band:- http://www.youtube.com/user/RedwingBand

                              Comment


                              • #30
                                Originally posted by pdf64 View Post
                                ...the wire in yours to the vol control's 0V terminal seems unusual. Is that definitely connected to the chassis / 0V circuit common?
                                Yes, you are right. It is actually the VIBRATO CHANNEL VOLUME potentiometer that is grounded to the chassis and the MID control "shares" that connection by means of the short white wire you see in the photo.
                                I think this is OK according to the schematics (additionally, the same wiring is present on the NORMAL Ch tone stack):



                                That shared connection seems original, as it looks like every other wire around the tone controls and I see no evidence of a previous wire running from the MID pot directly to the chassis.
                                I've checked there is continuity between the VOL lug and the chassis and also the MID lug to the chassis. They were OK
                                Last edited by TelRay; 02-25-2019, 10:27 PM.

                                Comment

                                antalya escort
                                kartal escort
                                sex vidio
                                altyaz?l? porno
                                Working...
                                X