Input grid resistor?

Discussion in 'Amp Tech Center' started by dougstrum, May 5, 2021.

  1. Lynxtrap

    Lynxtrap Tele-Afflicted

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  2. the fatch

    the fatch Tele-Meister

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    It only cuts frequencies way out of human hearing. As the capacitance is increased, it will eventually cut treble. With a 2.2nF (0.0022uF) capacitor, response would begin to fall off at around 7.2kHz. Guitar amp speakers output very little over 7kHz, so over 2.2nf would begin to cut treble.
     
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  3. LightningPhil

    LightningPhil Tele-Meister

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    A ferrite bead around the cable from the jack to the first resistor could help keep a bit of RF (mobile phone induced hum perhaps) out too. Not seen it in a guitar amp, but quite common on old electrical engineering kit.
     
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  4. Euphonica

    Euphonica Tele-Meister

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    Whoops yes I meant to say the same thing about ferrite beads. I used to hear radio stations until I put one on the main surge protector. Not heard anything since!
     
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  5. LightningPhil

    LightningPhil Tele-Meister

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    The resistor could be replaced with a choke like one of these:
    https://www.mouser.co.uk/ProductDet...EpiMZZMv126LJFLh8y%2B2RKwC9HhBzm%2B//EIaqABA=

    It has a DC resistance of 490 ohms and at 10 kHz, it’s got 6280 ohms of inductance impedance too. However, it’s resonance with a 200pF Miller capacitance gives a huge spike at 35 kHz which is not useful. It would require a 33k resistor in series to have a sensible Q of 0.67. So some caution should be used.

    Interestingly, 10 of the chokes in series with the Miller capacitance increased to 1nf would give a resonance of 5khz and a Q of 6. The ultimate passive top boost? An additional 10k resistor in series would drop the Q to 2 which is much more sensible.

    Here’s a calculator for reactive impedance:
    http://www.learningaboutelectronics.com/Articles/Inductor-impedance-calculator.php#answer

    And for RCL circuits:
    https://www.omnicalculator.com/physics/rlc-circuit
     
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