Valve amp volume control implementation.

[ppppenguin]

Unless the valve is drawing grid current the resistance from grid to 0V is pretty much irrelevant. Whether it's zero or 100K won't make much difference. The grid will be at 0V as intended in any such amplifier stage. The bias is set by the cathode resistor which ensures that the cathode is +ve wrt the grid. In other words the grid is -ve wrt the cathode which is what you want.

If you really don't want the resistance of the pst in parallel with the grid resistor then just AC couple from the slider to grid.