In a grounded grid amplifier using a triode vacuum tube, each side of the filament is connected to a capacitor whose other end is connected to ground. These are:

Capacitors connected from a power or supply line directly to ground are universally known as by-pass capacitors. In a vacuum tube amplifier, the filament requires low-frequency AC or DC power to heat the tube, but the power supply must be protected from high-frequency RF signals flowing back into it. The by-pass capacitors provide a very low impedance path to ground for stray RF energy, effectively shorting the RF to ground before it can enter the power supply. At the same time, they present a high impedance to the 60 Hz AC or DC filament power, allowing the heater to function normally. Other options describe capacitors used for entirely different purposes. Blocking capacitors are placed in series with a signal path to block DC while passing RF, and tuning capacitors are variable components used to adjust resonant frequencies.