Diodes
   Consisting of just a cathode and a plate (anode), the simplest tube is diode. The diode, like the resistor, will conduct more current with more voltage. Unlike the resistor, the line defined by its increase in current in the face of increased voltage is not ruler straight, but instead curves. In fact, it seldom begins at 0 volts, but usually a little to the right, as space charge defines a slightly negatively charged region that must be overcome with more positive plate voltage to permit conduction from cathode to plate. At the other end, the does not continue infinitely as the theoretical resistance does, but instead flatten to some maximum current saturation point where the cathode simplify cannot yield any further current density. Still this line is close to straight and it helps us easily the voltage drop across the diode at a given current.

   If we place a resistor in series with the diode, the current through the resistor will equal the current through the diode. This current will then force a voltage across the resistor, which will subtract from the potential voltage across the diode and limit the maximum current through the diode. Plotting the resistor is easy enough. We start by placing the first point at the B+ voltage and then finding the point on the Y axis that equals the maximum current that could flow through the resistor with the full B+ voltage developed across it: I = V / R, once again. We finish by connecting these points.

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pg. 9