Explanation: When the control grid of a vacuum tube is held at an excessively negative voltage, its strong negative potential repels the electrons emitted by the cathode, preventing them from reaching the plate (anode). This effectively "blocks" the flow of electrons, thereby cutting off the plate current. This condition is known as a **blocked grid**. It can be an intended operational state (e.g., in Class C amplifiers for high efficiency) or an unintended fault condition. * **Blocked plate** is not a standard term describing this specific phenomenon. * **Motorboating** refers to a low-frequency parasitic oscillation in an amplifier, often sounding like a motorboat, typically caused by inadequate power supply decoupling, not a static cutoff of plate current due to grid voltage. * **Tuned grid** describes a circuit configuration where the grid circuit of a tube incorporates a resonant (tuned) circuit, usually to select a specific frequency, and has no direct relation to cutting off plate current due to excessive negative grid bias.