Finding the Plate Resistance

Author: 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 - 5-volt curve, showing possible combinations at this grid voltage, crosses the 20,000-ohm load line, showing the possible combinations with this resistor connected in series with the plate from a 250-volt supply.

Applying the load line

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 - 10-volt 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 5-volt input swing gives a 50-volt (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).

Finding the plate resistance from the same curves

By laying a straight-edge (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 10-milliampere 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 20,000-ohm load line crosses it, the line drawn as tangent goes through the zero current line at 100 volts and through the 10-milliamp 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 a-c 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.