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

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  • #91
    I would caution against taking schematics to be perfect, and adjusting circuits to match, unless you are sure someone made changes. There are often variants or especially transition models that won't completely match.
    It's not impossible that this unit came from the factory with the 2K2 rather than the 470.
    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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    • #92
      Originally posted by g1 View Post
      I would caution against taking schematics to be perfect, and adjusting circuits to match, unless you are sure someone made changes. There are often variants or especially transition models that won't completely match.
      It's not impossible that this unit came from the factory with the 2K2 rather than the 470.
      so I’ve been told hahahaha...
      in this particular case the 2.2 K was a modern resistor and the 25uF cap was a Sprague that’s why I concluded someone has modified it before (for all the good reasons maybe)
      thx man!

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      • #93
        Originally posted by Chuck H View Post
        Hey now! What's with the German talk that we gringos can't read? You two plotting on us?
        i would never do that!!!
        at least until the amp is fixed... I mean

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        • #94
          well... I think I've done every variation: with the 560 pF in different positions as per the variations on the schematics, added the cap and changed the resistor, etc

          i'll let you judge for yourselves... my ears are tired
          question is: what configuration would you leave?

          Last edited by TelRay; 03-04-2019, 04:00 PM.

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          • #95
            Originally posted by TelRay View Post
            i would never do that!!!
            at least until the amp is fixed... I mean
            Seems we used "inappropriate language". That's a violation of the forum rules and might have consequences .
            - Own Opinions Only -

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            • #96
              i keep looking for “questionable wiring” and came up to the tone stack and effects pots

              TREMOLO SPEED
              watching at the faceplate i’ve found the opening for the tightening nut is smaller than for other knobs... in fact there’s no nut holding the pot (it’s moving free when removing the knob)



              i’ve seen the 100K resistor to ground is bridging the wiper and outer lug.
              again, I could be reading the schematics wrong (I’m learning, guys)




              BASS CONTROL
              (this is like this on the VIBRATO side, not in the NORMAL channel)




              I’ve disconnected both from the lugs that i “think” they shouldn’t be connected to but heard no difference in the guitar sound.

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              • #97
                A quick note on grid stop resistors.
                The design "rule of thumb" from the golden age of tubes was to use 8/gm for the gridstop value for immunity to parasitic oscillation.
                For a 12AX7 that means 5K minimum for as new spec tube and about 7K2 for a worn tube at "replace me now" gm value.

                I routinely put 10K gridstops on all 12AX7 triodes in builds I do.

                There is absolutely no point in going larger than 10K. If a 10K grid stop does not fix oscillation problem then it is not parasitic oscillation and you should start looking for positve feedback via power rails or bad grounds.

                OR
                Bad design - like more than 2 cascaded gain stages running from a single power supply node (Example: the burn channel of a Supersonic, a soon as the bypass cap on that node ages a little, it sqeals like a pig at higher gain settings).

                Cheers,
                Ian

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                • #98
                  Originally posted by Gingertube View Post
                  There is absolutely no point in going larger than 10K. If a 10K grid stop does not fix oscillation problem then it is not parasitic oscillation and you should start looking for positve feedback via power rails or bad grounds
                  thx Ian!
                  I understand in your view the 47K we used is an overkill and if it was an oscillation problem it should have been solved by now (or additionally by adding the 2.2K and 25uF)
                  I am assuming that the gm value for 12AT7s (that’s what’s fitted in V3) is the same as the 12AX7
                  I can relate to what you say as we have not found any noticeable changes by doing this so far (apart from being enormously educational for me)
                  I keep looking for wrong wiring, etc (like the example in the previous post)
                  thx!
                  Last edited by TelRay; 03-05-2019, 02:15 AM.

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                  • #99
                    question is: what configuration would you leave?
                    I listened carefully with a good set of Sennheiser cans and tend to prefer the versions with the 560pF cap connected. Mainly because those sound less bright.
                    Actually, I don't like any of the sounds. Too bright, too harsh and scratchy for me. Makes it hard to concentrate and decide.

                    But to be honest, the amp sounds quite typical for a SF Twin with treble on 10 and zero middle and bass. These amps just don't handle distorted sounds very well.
                    - Own Opinions Only -

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                    • thx man, I know the settings are extreme but i want to concentrate on the frequencies i am havin a problem with
                      i do hear those “fried highs” when trying to get a “sweater but slightly overdriven” sound
                      sorry the phone soumd recording might not be up to the quality of your headphones
                      i will try to record something decently when i get back home at the end of the week
                      i am up to the point in which i like how the NORMAL channel sounds (though there’s not even a bit of overdrive at 10)... pls help! hahahhaaj...

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                      • Originally posted by TelRay View Post
                        I know the settings are extreme but i want to concentrate on the frequencies i am havin a problem with
                        i do hear those “fried highs” when trying to get a “sweater but slightly overdriven” sound
                        FWIW when I said it sounded better than before I had taken the extreme settings into account. Sort of knowing that was what you were up to. And Helmholtz is right that it sounds pretty typical of a SF TR. Later models included 2.2n caps from the power tube grids to ground for stabilization. I'd rather go with the preamp grid stops you've installed. You could actually do better with a VERY FASTIDIOUS tear down and rebuild, but if you reinstall the first preamp tube (which I believe must be a stabilizing force in the design), use more nominal tone settings and relegate the amp to primarily clean tones then I think the grid stoppers have gotten you there. If it were mine though... I have to admit that I would rebuild it with a more ideal lead dress and ground scheme. Though it can't really ever be ideal because of how long the front panel layout is on those amps. Some preamp controls will always have long leads and be subject to later circuitry.

                        JM2C as a designer.
                        "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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                        • gm for a 12AT7 is a bit over twice that of a 12AX7 so you can get away with 4K7 for gridstops.
                          Cheers,
                          Ian

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                          • As stock, the master vol has a bright cap (to a track tapping on a 4th terminal); is that still in circuit at all?
                            For some reason, it’s tricky to get a nice overdrive with a Fender reverb channel. However, a type 2 master vol (before the power tube grids) can give a much smoother result, and a more overdrive depth if required.

                            I advised 47k grid stoppers in order to ‘isolate’ the dry signal path from the V3 g-k diode clipper. They’re way higher than necessary for actual grid stopper function but that just leads to a bit of high frequency roll off in the reverb driver, which I didn’t think would be a bad thing, perhaps even beneficial.
                            My band:- http://www.youtube.com/user/RedwingBand

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                            • thx guys, I will record a “better” quality audio with more “normal” settings in a couple of days
                              i think that if we conclude “that’s how the SF sounds like” I won’t be able to push the VIBRATO Ch VOL beyond 5 if I don’t want to fry those highs.
                              On the other hand, the NORMAL channel is the cleanest thing I’ve ever heard with the Ch VOL at 10

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                              • Originally posted by pdf64 View Post
                                As stock, the master vol has a bright cap (to a track tapping on a 4th terminal); is that still in circuit at all?.
                                thx man,
                                the MASTER VOL pot has now a non push pull pot with 3 lugs and the 120 pF cap is attached to the wiper and the outer lug.
                                i will fit in the original pot in a couple of days (that’s the one with the 4th lug and the cap attached to it)

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