Do vacuum tube amplifiers use a combination of grid-leak and cathode bias?
Answer Simon, It is much less sinister. The purpose is to have a shunt path for electron build up on the grid. As you know, the current in a vacuum tube is indeed electrons that are boiled off the cathode. The high voltage on the plate will attract the electrons to it; but as the electrons pass through the grid, some of them will attach to it and must be given an opportunity to ground out or the grid will become voltage polarized and cause the bias of the tube to drift one way or the other, depending on its characteristics. And, yes, there is a grid leak bias concept in tubes, but in the generic case, there is always a grid resistor – for the reasons stated above. If you have a specific circuit or application in mind, let me know and I will give you further comments. In vacuum tube design of old, the engineer always consulted a big book that had the plate characteristics for all tubes. Then, by knowing the grid voltages necessary for the operation anticipated (depending upon the plate
