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Home Circuit Values Finding the Plate Resistance  
See also: Constructing The New Load Line, The SplitLoad Circuit, The Effect of Changing the Load, LoadLine Construction, Plate Resistance  
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Finding the Plate ResistanceAuthor: N.H. Crowhurst Now suppose the grid potential varies between  5 volts and  10 volts. These curves show that, when the grid potential is  5 volts, the plate current is 5 milliamps and the plate potential is 150 volts. This is where the  5volt curve, showing possible combinations at this grid voltage, crosses the 20,000ohm load line, showing the possible combinations with this resistor connected in series with the plate from a 250volt supply.
When the grid potential is  10 volts, the plate current is 2.5 milliamperes, with a plate potential of 200 volts, where the load line crosses the  10volt curve. Thus with this resistor, we can see quite easily that there is a swing between 150 and 200 volts on the plate. If a 5volt input swing gives a 50volt (200  150) swing at the output, the tube is working at a gain of 10, because the output swing is 10 times the input swing. These curves can also be used to find the plate of the tube at different operating conditions (combinations of grid potential plate current and plate potential).
By laying a straightedge (ruler) along the curve at the point where the two lines cross, its slope at this particular point can be found. Extending the line drawn as tangent to the curve down to the zero current line and up to the 10milliampere line, the corresponding voltages may be read off and the plate resistance calculated. For example, using the 12AU7 curves and laying the ruler along the curve for  5 volts, where the 20,000ohm load line crosses it, the line drawn as tangent goes through the zero current line at 100 volts and through the 10milliamp line at 190 volts. This means, if the slope at the point we chose is extended, that a change of 10 milliamps causes a change of voltage of 90 volts. This is why the relationship is sometimes called an ac resistance  because it deals with changes of fluctuations in current. The value of the resistance, in ohms, is given by Ohm's law. In this example, the plate resistance is 90/.01 or 9000 ohms.


Home Circuit Values Finding the Plate Resistance 