Saturation is an interesting consequence of running a tube outside of its range of intended use. Under normal operation saturation would not occur in the tube. The equation governing the relationship between voltage and current is V=I*R, voltage is equal to the current multiplied by the resistance of the circuit. That means the voltage and current are directly proportional. When the voltage increases, the current should increase as well. When saturation occurs, the voltage increases but the current levels off asymptotically. This effect can damage the tube by overheating the plate. The electron emission of the heated cathode is limited by its physical properties such as its surface area, temperature, and the material it is constructed of. In order to induce saturation current, I ran the tube with the heater temperature lower than normal so that the cathode could not emit as many electrons as it normally would. Then I slowly increased the plate voltage. As the voltage on the plate increases, the current, being proportional, increases as well. The electron emission is limited by the low heat, so the current will be limited as well. The current expected by the potential difference across the plate soon becomes higher than the flow of electrons available from the under-heated cathode. Then, as the plate voltage increases, the current approaches the total electron emission. The result is that the increase in the current is no longer directly proportional to the increase in the plate voltage as it ought to be. (Eastman) I was able to observe saturation in the 6X4 and 5AM8 tubes.

Saturation in the 6X4 tube

Saturation in the 5AM8 tube

General Tube Information Page

Secondary Emission Page

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