Suppressor grid
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A grid used in a thermionic valve (also called vacuum tube) to suppress secondary emission.
In its simplest form, a suppressor grid is interposed between the screen grid and the anode (plate) of a tetrode, turning it into a pentode.
The suppressor grid is usually connected to the cathode and more often than not, this connection is made within the glass envelope.
Because the grid is negatively charged relative to both the anode and the screen grid, it repels any secondary electrons back to the anode preventing them from contributing to the screen grid current, and to any negative resistance characteristic.