Which vacuum tube has four active electrodes: a cathode, first grid, second grid, and a plate (anode)?

• A. Diode
• B. Triode
• C. Tetrode
• D. Pentode

Answer: (C)

A tetrode is defined by having four active electrodes: the cathode, the first grid (control grid), the second grid (screen grid), and the plate. The control grid modulates the flow of electrons from the cathode to the plate, providing amplification. The screen grid sits between the control grid and plate and is held at a positive voltage to accelerate electrons while shielding the control grid from the plate’s voltage changes. This shielding reduces feedback and capacitance effects, giving better gain and higher-frequency performance. The plate collects the electrons to deliver the amplified current. Diodes have only two electrodes (cathode and plate), triodes add one grid, and pentodes add another grid, so only the tetrode has exactly four active electrodes.