A mixer tube used in a superheterodyne radio receiver. A pentagrid converter tube has five grids, a cathode, a plate, and a filament.

The incoming radio-frequency signal is applied to the signal grid, and a signal from a local oscillator is applied to the oscillator grid. The two signals mix and form two additional signals. The frequency of one is the sum of the two, and the frequency of the other is the difference between the two. The other three grids in the pentagrid tube are: two screen grids and one suppressor grid.