A thermionic valve similar to a tetrode with the addition of a third grid, the suppressor grid to reduce the secondary emission effect; was/is used in high quality audio and radio products.
A thermionic valve containing an anode, a cathode, and a control grid; small changes to the charge on the grid control the flow from cathode to anode, which makes amplification possible.
a thermionic tube having five electrodes
a thermionic vacuum tube having three electrodes; fluctuations of the charge on the grid control the flow from cathode to anode which making amplification possible
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube (thermionic valve), which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926.
A triode is an electronic amplifying vacuum tube (or valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode (Fleming valve), the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode.