A type of vacuum tube used in high-end audio preamplifiers, ham radios and a variety of other electronic circuits. A pentode is like a tetrode with the addition of a "suppressor grid" between the screen grid and the plate. Typically biased at or near the cathode voltage, the suppressor grid provides additional isolation between the control grid and plate. See tetrode
The Pentode Uses Three Grids
In a pentode, the addition of a screen grid and suppressor grid between the control grid and plate controls feedback and oscillation.