Pentagrid

PentagridThe pentagrid is a receiving valve that has 5 control grids besides a cathode and an anode. The question arises what the difference is between the pentagrid and the heptode as they both have a seven-electrode tube with 5 grids besides a cathode. They both have the same number of grids. Still it's easy to see that their functions are different. The first grid from the cathode in both valves is a control grid in a heterodyne which is a transmitting triode. The pentagrid's second grid functions as anode in the same triode while the heptode does not have this grid. The next step was the development of two screen grids instead of one. The reason was that tetrodes and pentodes had only one control grid that was screened off the anode by the screen grid. A new lamp, the pentagrid, retained this development: it is the "lowest" of the two screen grids. But then the second control grid became closer to the anode. So, the mixing section of the valve was turned into a simple triode the main disadvantage of which is a high anode-grid capacitance. In order to eliminate this disadvantage, an additional second screen grid (internally connected to the first screen grid as they both performed the same function) was placed between the second control grid and the anode. The use of pentagrids solved two problems in superheterodyne receivers, that is to decrease the input-to-output capacitance in a valve (and, consequently, to increase the device's frequency range), and increase its efficiency due to fewer valves used.