In 1926, B. Tellegen, working at Philips laboratories, invented
the pentode. Basically, the inner elements of this valve consist
of an additional grid mounted between the screen grid and
the anode. This third grid, of spaced winding shape, was later
denoted the supressor. It is normally connected to the cathode.
This termionic device was a practical solution to overcome
the drawback found in the early tetrodes or screened valves
operating as audio amplifiers, of negative resistance caused
by secondary emission form the anode, with electrons being
attracted to the positive charded screen.