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2. Basic Building Blocks > The Tetrode - Pg. 86

86 Valve Amplifiers FIGURE 2.14 Dissected 417A: Note the size and shape of the remaining anode section relative to the active area of the grid. but the term had already been used for tetrodes, so the rather less satisfactory terms guided-grid triode or beam triode were used (see Figure 2.14). Valves such as the PC97, PC900 and 6GK5 have internal support rod screens and bath-tub anodes, causing C ag to fall to ,0.5 pF À a very worth- while improvement. Sadly, most of these Ultra High Frequency (UHF) valves were designed to be variable- valves, to allow Automatic Gain Control (AGC), and we will see later that this causes distortion (see Figure 2.15). THE TETRODE As described by the patent, the tetrode [3] was invented to overcome the reduction in gain caused by the electric field of the anode interacting with the electric field of the grid. An auxiliary, or screen grid, g 2 , is placed between the anode and the grid to screen the changing anode potential from the grid. To maintain electron flow to the anode, g 2 is connected to a positive potential slightly lower than that of the anode so that electrons are attracted to g 2 , but most pass through the (coarse) mesh to be captured by the anode as anode current (see Figure 2.16). Although originally devised to increase voltage gain, the far more impor- tant effect of adding g 2 is that it screens the anode from the grid at AC and greatly reduces Miller capacitance, allowing useful amplification at much