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.

Pentodes valves made by Philips between 1928 and 1935:
a) Type B-433 the first pentode valve in the world
b) Type AF-3 mu variable pentode with socket type      P used in RF stages.

Pentode valves made in the USA between 1928 and 1935:
a) Type 51
b) Type 2A5