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

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  • I agree. That's not at all uncommon in a tube guitar amp. A hi-fi tube amp is another story.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • I always adjust the TMB controls to the point that the sine is least distorted : so that other forms of distortion become more clear.

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      • The oscilloscope has a TRACE ROTATION adjustment screw that I am not able to access with a screwdriver, but when i push it a bit the tilt changes so I am guessing I could achieve a 0 degrees line if I could adjust this.
        That makes sense. Probably the magnets adjusting trace rotation shifted. No problem for now.

        I understand that what you are saying is that it indicates a problem that might be bigger but the oscilloscope is somehow limited in showing the real dimension of the problem.
        Right. Forget "spurious" (should have looked it up myself ). I meant something like suspicious. Actually I'm sure it's HF oscillation: It's spontaneous and shows some HF periodicity.

        so the amplitude of the waveform in the TOP is wider than at the BOTTOM and this represents unwanted distortion. Correct me if I am wrong.
        Any deviation from a perfect symmetrical sine is non-linear distortion and means added harmonic content. I was actually speaking of the min-max asymmetry of the V2, pin 1 signal which is the input to the tone stack, showing that there is already more than one frequency (fundamental + low order harmonics) contained. The tone stack further changes ("distorts") the waveform by changing the relative amplitudes of the different harmonics and shifting their phases, depending on settings.
        Last edited by Helmholtz; 04-10-2019, 03:25 PM.
        - Own Opinions Only -

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        • Originally posted by Helmholtz View Post
          Any deviation from a perfect symmetrical sine is non-linear distortion and means added harmonic content.
          [Rant] Right on. Most electronic musicians, especially guitar players, equate distortion with clipping or similar extreme changes to the signal. The subtle added harmonics in a relatively "clean" amp are technically distortion and I hope that the majority of MEF'ers recognize - or come to recognize - the technical nature of some of these terms. Don't get me started on the term 'tone' [/Rant]

          The human ear is fairly deaf to static changes in harmonic phase relationship. While the scope shows a wildly un-sinus wave, the ear hears a tone with the preponderance of the acoustic energy in the fundamental and it sounds 'like a sine wave'. Any waveform that doesn't have jagged edges doesn't contain a lot of energy in the higher harmonics. Because it looks smooth on the scope (regardless of how wavy it is) it will sound smooth and sine-wave-like. This is what I meant when I said the shape of the waveform isn't concerning to me. It's normal for a tone leaving a tube amplification stage to have some added lower harmonics, 2nd harmonic mostly. If the phases line up (as they will, if measured at the anode) the waveform looks pretty close to a sine wave, maybe a little 'egg-shaped'. Move those phases around, and the wave can look pretty whack. I've got an amp I'm working on with three tube stages before the tone stack. While the tone from my signal generator still sounds like a sine wave, I can adjust the tone stack controls to go from egg-shaped to something that resembles a square wave. But because the wave form has rounded edges ('smooth', from above) it still sounds sine-ish or "clean", as opposed to "dirty" or "distorted" (guitar player definition of distorted). The wave is clearly distorted on the scope, but doesn't sound like what I might expect to hear.
          If it still won't get loud enough, it's probably broken. - Steve Conner
          If the thing works, stop fixing it. - Enzo
          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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          • reading strato56's suggestion of bringing the TMB settings to those that bring up the most noticeable failure mode I decided to re-create the old "SCREECH" sound I got when pulling the V1 tube off the circuit, BRIGHT SWITCH on, etc.

            My intention was to follow the sound path with the scope but I couldn't as every time I would connect the probe the SCREECH would disappear (see video)



            I can play Theremin with this thing!!!

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            • I'd suggest you take a good look for chassis pics and layout of your exact model and pay particular attention to those shielded cables.
              They way those cables are routed doesn't look right to me. Here's an example, although not the same model you can see they are not just loosely draped across the board like what you have.


              Click image for larger version

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              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


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              • Are all the shields of the black coax cables connected to ground at one end?
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                • Originally posted by g1 View Post
                  They way those cables are routed doesn't look right to me. Here's an example, although not the same model you can see they are not just loosely draped across the board like what you have.[ATTACH=CONFIG]53296[/ATTACH]
                  You are absolutely right. I've checked the circuit photos from my amp from before I changed anything and the 2 coax cables were routed together and held in between those two capacitors. I've tried to route them like that and I confirm there's no SCREECH
                  I fully understand now all the comments about instability and lead dressing... it's incredible how sensitive these things are. If I were Mr CBS Fender of the 1970s I would have held those m*****f**** cables with a plastic tie.

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                  • Originally posted by Helmholtz View Post
                    Are all the shields of the black coax cables connected to ground at one end?
                    yessir they are, I've asked a similar question and analyzed that in post #71... OMG this thread is long!

                    Click image for larger version

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                    • alright! with the "mystery" of the SCREECH being solved (I cannot still understand how could Fender design a lead dress in which just moving one cable that could snap at any moment such a terrible noise could be produced) I think that now it's time to fine tune for other sources of oscillation.
                      Chopstick tapped around many components with no result until I found this unplugging the REVERB INPUT... interesting
                      (this is with the Scope's probe plugged at the VIBRATO CHANNEL VOLUME pot center tap.

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                      • Does unplugging the reverb input also cure your sound problem?

                        Does the HF blip also disappear when pulling V3?

                        Oscillation issues sometimes stop when the chassis is covered with a grounded metal plate. Fender amps typically use a steel mesh stapled to the top of the cabinet, which acts as a ground plane. So there may be no more oscillation when the amp is mounted in its cabinet.
                        - Own Opinions Only -

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                        • Originally posted by Helmholtz View Post
                          Does unplugging the reverb input also cure your sound problem?
                          In the past I remember I had a bigger problem in this area and cannot recall how I got rid of it (see old video from at least 3 months ago)



                          The current problem (as in the video on post #121, those "oscillation blips") are not gone neither be pulling V3 out or unplugging the REVERB IN

                          Originally posted by Helmholtz View Post
                          Does the HF blip also disappear when pulling V3?
                          Yes, the SCOPE shows no oscillation when pulling V3
                          V3 socket is new, V3 tube is new
                          tried 2 other older 12AT7s and the oscillation was there
                          tried 2 12AX7s (just for the sake of experimentation) and there was less oscillation (I understand the tubes are different, I wanted to see the effect)

                          Originally posted by Helmholtz View Post
                          Oscillation issues sometimes stop when the chassis is covered with a grounded metal plate. Fender amps typically use a steel mesh stapled to the top of the cabinet, which acts as a ground plane. So there may be no more oscillation when the amp is mounted in its cabinet.
                          I don't see any metal on the top of the amp, just cheap wood
                          Last edited by TelRay; 04-12-2019, 07:46 PM.

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                          • I don't see any metal on the top of the amp, just cheap wood
                            So your amp cabinet is no longer original. Any amp needs a closed shielding (Faraday) cage for the electronics. You may use heavy aluminum foil attached to the top of the amp cabinet.
                            Last edited by Helmholtz; 04-12-2019, 08:45 PM.
                            - Own Opinions Only -

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                            • The current problem (as in the video on post #121, those "oscillation blips") are not gone neither be pulling V3 out or unplugging the REVERB IN
                              Yes, the SCOPE shows no oscillation when pulling V3
                              The above statements seem contradictory. I guess there is no clear correlation between the oscillation blips and the sound issues?
                              - Own Opinions Only -

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                              • Originally posted by Helmholtz View Post
                                The above statements seem contradictory. I guess there is no clear correlation between the oscillation blips and the sound issues?
                                At some point in time (3 months ago - the video filmed vertically with the guitar sounding and that big OSCILLATION noise when connecting and disconnecting the REVERB IN plug) there was an audible symptom. As I say, I don't remember how I fixed it but "that" problem is not audible anymore.
                                I am guessing that the tiny oscillation seen today with the oscilloscope shows that that problem is not cured 100% from a SCIENTIFIC point of view (measuring with an instrument) but almost solved from a PRACTICAL (what you hear... or "don't" hear in this case) one.

                                Maybe this thing is amplified further down the line in the amp or something else is added. So far I got as far as putting the SCOPE's PROBE in the VIBRATO CHANNEL VOLUME and did not go any further as I am not sure of where to go apart from the obvious SPEAKER OUT.
                                Would be great if someone could point me in the direction of having the SCOPE PROBE POINTS mapped for a Fender Twin (or Bassman, etc).

                                Great advise on the Aluminum foil, man. When I am done "fixing" the amp I will definitely do that.

                                Thank you (and the rest) for being so helpful, patient and educational.

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