What factors determine the anode plate-to-grid capacitance? I know that the capacitance of a regular capacitor is determined by the surface area of the plates and is inversely proportional to the distance between them. Does this hold for vacuum tube grids and anode plates? The reason I'm asking is what are the ways to vary the anode-grid capacitance and in particular, how would this factor into designing a Thyratron that has sufficient capacitance to self-oscillate when there is a cathode-grid feedback loop. The idea of this device is to use it as a relaxation oscillator that can oscillate at audio frequencies(so sufficient large capacitance is required).