Miller & Grid Stopper

Every triode has a small capacitance between grid and plate (Cga), a couple of picofarads. Because the plate swings A times more than the grid, and in opposite phase, the charging current through Cga is (1 + A) times larger than it would otherwise be. Seen from the grid, Cga behaves like a capacitor multiplied by the gain: the Miller effect.

Combined with everything resistive in front of the grid, Cin forms a first-order low-pass filter:

A grid stopper resistor soldered right at the socket is mandatory in high-gain designs to suppress RF oscillation and parasitic ringing. In guitar amplifiers, values of 33–68 kΩ (or more) are chosen deliberately to roll off fizzy harmonics from clipping stages. In hi-fi you want the corner well above 20 kHz. Check the "Max Rgs" readout.

Pentodes such as the EF86 have a far smaller Cga (≈0.05 pF) thanks to the screen grid, which is why they make excellent wide-band, high-gain stages.