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

  1. #281
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    I'm not hearing that farty glitching anymore. You did say this was with the ch.vol. around 7 and lower master setting, so I'm going to suggest what you now have is normal.
    The pull boost was just horrible and no one ever used it (except as 'horrible' effect).
    But the master was also not very well liked, many either ran it up full all the time, or removed it. In that era, Fender did not know how to do distortion.
    I would say that you are no longer dealing with a 'fault', just trying to optimize the quality of the distortion. That is ok to do, but you may be at a point where it is as good as it gets without a major re-design. If you ever have a chance to try another SF Twin w/Master, compare it at similar settings.

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  2. #282
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    Quote Originally Posted by g1 View Post
    I'm not hearing that farty glitching anymore. You did say this was with the ch.vol. around 7 and lower master setting, so I'm going to suggest what you now have is normal.
    I agree with you Mr G1.... partially
    I like the current slightly overdriven sound I am getting from the amp, it's pushy and clear and I would never expect any more grit coming from a Twin. That has improved noticeably in the last 100 posts
    But... I'd like to get the same great sound with the REVERB tank connected, and that "dirt" I hear around 9 KHz in the latest videos (the one with the overdrive on post #268 and the clean tone with the single coil pickups on post #278) prevents me from considering this case closed. While this could be "just tolerable" for live performance it is still not suitable for studio work.
    After I have solved this issue I will consider I have restored this amp to its former glory and will eventually think about any tweaks in the future.
    Would you agree? Please say "Yes" because I still need your help

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  3. #283
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    Ok. I missed the part that the reverb was not connected. Carry on with those mods and post your results.

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  4. #284
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    trying to get things in order to dig into the amp again today, I wanted to prioritize what to do and how to check that the modification is effective

    1.- Add 20 uF / 500 V Cap

    Reason: there shouldn't be signal voltage at the power supply
    Verification: place scope on point "A"


    2.- .0022 uF Cap try the different positions described on Fender Schematics

    Reason: the cap across the REVERB OUTPUT transducer regains some TREBLE - especially when using the 560pF cap at the V3 plates, which produces a LOW PASS effect. It also attenuates unwanted HF content with this wiring.
    Verification: scope at the SPK OUT


    3.- 560 pF cap try the different positions described on Fender Schematics

    Reason:
    Verification: scope at the SPK OUT


    4.- Install a Voltage divider replace 1M resistor by 2x 500K

    Reason: Reduce grid drive
    Verification: scope at the SPK OUT

    5.- add Resistors (around series with the 560 pF cap

    Reason: extracts energy from the HF resonance by dissipation and thus damps or even kills resonance
    Verification: scope at the SPK OUT

    6.- Faraday Cage add Aluminum foil to the enclosure

    Please jump in if you have any other scientific verification methods

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  5. #285
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    You might want to avoid aluminum foil. Torn or loose foil could be a problem. Aluminum coil stock is available at any home center, sold as flashing.

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  6. #286
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    Perfect and reasonable order/sequence of steps

    Regarding "6.- Faraday Cage add Aluminum foil to the enclosure", you may tack some household aluminum foil to a large enough cardboard and place it on the chassis (making sure it makes contact) to test its effect outside the cabinet.
    The additional shielding may actually reduce oscillation and unwanted coupling issues, so also check with scope at SPK OUT.

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    Aluminum coil stock is available at any home center, sold as flashing.

    I understand the thicker/heavier foil is called flashing, right?

    What would you call the alu sheet metal used in Marshall tops?

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  8. #288
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    Quote Originally Posted by Helmholtz View Post
    I understand the thicker/heavier foil is called flashing, right?

    What would you call the alu sheet metal used in Marshall tops?
    I don't think the heavy foil is flashing. At least here, flashing is a building material, most often seen at roof-edge, so more like the marshall stuff.

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  9. #289
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    TE=Helmholtz;529871]I understand the thicker/heavier foil is called flashing, right?

    What would you call the alu sheet metal used in Marshall tops?[/QUOTE]

    In the States aluminum foil is thin (easily torn by hand) rolls used to cover a casserole or a potato for baking.

    Flashing is a mm or two thick with some strength and rigidity to it.

    I could imagine aluminum foil torn/ unglued hanging down by the high voltage parts. That was my concern.

    Sorry for any confusion.

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