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5f11 input grid stopper experiment.

Discussion in 'Shock Brother's DIY Amps' started by Kevin Wolfe, Jan 6, 2021.

  1. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    I was reading back over some old post and found a thread started by @jsnwhite619 where we discussed reducing grid stopper values and decided I’d give it a shot. Since I was planning to redo the inputs on this amp to eliminate a ground loop that was bugging my OCD, I picked a 5f11 as the victim.
    In some of the Valve Wizard writings he suggest going as low as 10k and adding a cap between 100pF and 470pF between the grid and ground. He also goes into the maths regarding Miller Capacitance with a formula to calculate the actual outcome with a given tube.
    Using the 3 inputs of the 5f11, I mounted some 27k’s at the jacks and a length of shielded cable from their juncture to pin 2 at V1, with a 120 pF silver mica cap from there to a ground lug at the tube socket. I decided on those values only because those were some of the choices I had on hand in my parts bin. I didn’t work out the formula since I knew I wasn’t going as low as 10k.
    The outcome: Well, first off, getting rid of the ground loop reduced the noise some more on an already quiet amp. Secondly, there seems to be no added RF or microphonic noise in the input circuit. Sound wise, I’m getting a volume level at around 5 that I was getting up around 8 on the volume pot, so mission accomplished, I think. 93867A1A-9174-405E-A9E4-E33992A43FB8.jpeg
     
  2. sds1

    sds1 Tele-Afflicted

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    I had been wondering to myself lately if that grid stopper would affect signal in, at it is in series with your guitar volume pot wiper. Thanks for the answer.

    I have been using the Valve Wizard 10k with cap to ground on basically everything, because it seems optimized, but not being aware of the attenuation effect on input signal.

    Whoops.
     
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  3. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I also use the valvewizard 10k + cap.

    If implemented correctly it does not attenuate the input signal.
     
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  4. Mongo Park

    Mongo Park Tele-Afflicted

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    So with the basic fender 2 input like a 5f2a I could use 10k and 120pf
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    If fender two inputs are what I think they are, the 10k's are in parallel (so 5k eff.), which is said to be low for pedals.

    And the capacitance can be increased to have a lower bandwidth, decreasing noise.
     
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  6. Mongo Park

    Mongo Park Tele-Afflicted

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    Well I don’t use pedals but could go say 20k or whatever is close to that. thanks for the help.
     
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  7. James Knox

    James Knox Tele-Holic Silver Supporter

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    Can’t quite tell from the pic. How did you ground the cap? To a solder lug on the Tube Socket bolt?
     
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  8. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    I know, it didn’t show up in the picture but its a regular ground lug attached with the tube socket screw.
     
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  9. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    I wondered if the audible change was a figment of my imagination. It would be interesting to do a before and after with a signal generator and a scope to actually measure any changes in signal amplitude or headroom at onset of distortion.
     
  10. timfred

    timfred Tele-Meister

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    Check out Mesa Boogie schematics. No input grid stoppers or caps at all on many of their amps including most of the Mark Series.
     
  11. James Knox

    James Knox Tele-Holic Silver Supporter

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    Interesting...
     
  12. chas.wahl

    chas.wahl Tele-Meister

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    Wait! How does the 5F11 input circuit even work when input #1 is used?

    upload_2021-1-7_10-59-43.png
    I'm sort of clueless regarding such things (trying to learn), but to me it appears that if you plug into #3, there's one 68k in the signal path to grid, and a 68k plus the parallel 68ks (effectively 34k) to ground (112k effective); if #2, there are 2 68ks parallel to grid (effective 34k) and the same 112k to ground; but if you plug into #1, there are 3 68ks in parallel to grid (effective 22k7), but no path to ground (infinite impedance??). What's with that?
     
  13. schmee

    schmee Doctor of Teleocity

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    Is that the amp layout that is known to be incorrect? .. I can't remember if it's the Harvard or 5F11 or.....
     
  14. mrriggs

    mrriggs Tele-Meister

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    The guitar pickups are the path to ground.
     
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  15. chas.wahl

    chas.wahl Tele-Meister

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    It's from the schematic on Fender Amp Field Guide (Narrow Panel Vibrolux, 5F11), apparently identical to the one that @King Fan just posted on a parallel thread here. And the layout posted just above that by @brucerbc appears to be wired just like the schematic, to my eye.
     
  16. chas.wahl

    chas.wahl Tele-Meister

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    Whoa! Learn something new every day. So the guitar one chooses, and whether pickups are circuited in parallel or in series, or singly, determines the input's impedance (solely, in the case of input #1)?
     
  17. mrriggs

    mrriggs Tele-Meister

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    Yes, the impedance of the guitar matters. Even though the voltage dividers on the input jacks are simple resistors, they interact with the mostly reactive (inductance) of the guitar pickups. This is the reason why a humbucker plugged into an attenuating jack can sound cleaner, even when the gain is turned up to match the level of a non-attenuating input.
     
  18. NTC

    NTC Tele-Meister

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    Hmm. I wired My " not a 5F11" with 2 input jacks and the standard Hi/Lo inputs like a 5E3 or other Fender. It needs a bit more drive. Maybe emulating the Standard 5F11 method would help? Something similar could be done with two jacks and 47k resistors. The input 1 resistance would be similar.
     
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