Which vacuum tube has five active electrodes: cathode, control grid, screen grid, suspension grid, and plate (anode)?

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

Answer: (D)

This tests how tube types gain more control with extra grids. Five active electrodes means electrons flow from the cathode to the plate while being shaped by grids in between. The list includes the cathode, control grid, screen grid, suppression (suspension) grid, and the plate. That combination is the defining setup of a pentode, which adds the suppression grid on top of the control and screen grids to improve gain, isolation, and reduce unwanted emission effects. Earlier tubes have fewer grids: a diode has only cathode and plate, a triode adds one grid, and a tetrode adds the screen grid. So the configuration with five active electrodes corresponds to a pentode.