Vacuum Electron Tubes Electronics Pentode

Picture 1. 6AG5 RF Pentode Vacuum Tube

Figure 1. Pentode

Vacuum tubes were invented, and then perfected to their nearly present form, in the beginning of the 1900's. The pinnacle of the vacuum tube technology was - without a doubt - is the pentode. The pentode was the latest important major vacuum tube development effort. The pentode is possibly the most often used vacuum tube in the world. Often used pentode symbol on schematics and drawings is on Figure 1.

The pentode is the result of the intensive effort to improve the characteristics of the triode and the screen-grid tube or tetrode. Interestingly the total cathode current flow is hardly influenced by the pentode's anode potential resulting in efficient, lower voltage circuits. The pentode is nearly an ideal current source and its plate voltage vs. plate current characteristics closely resembles that of the transistors for flatness.

A pentode is a five-electrode vacuum tube see Picture 1. When the term "vacuum tube" is used - contrary to the vague Lee deForest Audion Patent explanation - it always means high-vacuum. The harder is this high-vacuum, the better is the electron tube. It was found out in the early part of the 1900's when the same type tubes were made by several different manufacturers that the best tube was always the one that was evacuated the best and hardest.

The five pentode electrodes - in order of increasing potential - are the cathode, the control grid - G1, screen grid - G2, suppressor grid - G3 and the anode - A. In our example the cathode is located at the center of a cylindrical assembly and is usually is at zero or reference potential.

The G1 control grid electrode operates at a small negative potential with respect to the cathode. The space-charge electrons are repelled by this negative potential and they avoid the G1 thus no grid current flows, allowing excellent high input impedance for the tube.

The G2 - screen grid electrode is there to shield the anode from the capacitance of the control grid - G1 electrode, and it is at a fairly large positive potential relative to the cathode. A portion of the electron flow settles on this grid and that results in a small additional inefficiency in a form of heat dissipated on the screen grid.

The G3 - suppressor grid electrode usually connected to - often internally and permanently - and operates at cathode potential. The purpose of the G3 electrode is to repel the secondary electrons back to the anode and prevent them to collect on the G2 - screen grid. It is especially important when the anode terminal is less positive then the G2.