Bypass capacitor for screen grid?

[Deactivated-1]

I don't really know what the screen grid does but I have heard that the resistor value changes the sound a bit so I figured that a bypass cap would also change the tone?
how much would it change it and also can someone explain what exactly the screen grid does?

[David Hess]

The screen grid shields the control grid from the anode (plate).  This has the effect of both raising the anode (plate) resistance and reducing the capacitance between the anode (plate) and control grid so higher gain and higher frequency operation is possible.  It performs the same function as a cascode output transistor in a two transistor amplifier.

https://en.wikipedia.org/wiki/Tetrode#Screen_grid_valve

[TimFox]

To first order, the screening effect of grid 2 (held at a constant voltage with respect to the cathode) makes the electric field at the cathode almost independent of the plate-cathode voltage (extremely high mu, hence the large plate resistance).  In normal pentode operation, the screen bypass capacitor helps maintain the screen-cathode voltage even as the screen current changes with plate voltage.  Note that changing the screen voltage on purpose does change the cathode current:  there is a “screen mu” parameter that describes the triode behavior of the cathode-grid1-grid2 portion of the pentode.  This parameter is generally low (e.g. 4.5 for a 6146 power tube).  Therefore, if the screen is powered through a resistor, with no bypass capacitor, the stage gain is reduced:  see any tube textbook for details.