The second effect I studied is called secondary emission. Unlike saturation, secondary emission often occurs under normal tube operation. When the electrons travel across the tube toward the plate, they are accelerated by the electric field between the elements as described in the general tube information page. In the tetrode, the flowing electrons often gain enough kinetic energy to splash other electrons off of the plate where they strike. Recall that the net field between the screen grid and the plate is composed of the addition of the field from the plate and the field from the screen grid. The control grid does not significantly affect the net field in this region because it is so far away. If the net field here is influenced more by the plate than the screen grid, the splashed out electrons will be pushed back toward the plate as they should be. Alternately, if the screen grid holds more influence over this net field than the plate, the resultant field will pull the splashed out electrons away from the plate and back toward the screen grid. The result is that the beginning current increases proportionally with the potential across the plate as it should. When the kinetic energy of the electrons reaches the threshold they start to knock out electrons from the plate. If the screen grid is controlling the field, the splashed out electrons take away from the net current flowing to the plate. Then, as the plate potential increases, the number of electrons knocked out increases, and the net current decreases. Finally, when the plate potential gets high enough, it overpowers the screen grid and the net field begins to pull the splashed out electrons back to the plate and the current goes back to increasing proportionally to the plate potential. (Eastman)
The electrons pulled from the cathode knocking other electrons off the plate is called secondary emission. This effect can adversely affect the output of the tube. In order to get rid of this effect, another grid, the suppressor grid, is inserted between the screen grid and the plate, forming a pentode. The suppressor grid applies another electric field next to the plate that overpowers the field from the screen grid to push the emitted electrons back to the plate, even if the plate potential is low. By the conservation of current in the circuit, the total current must remain constant as long as the system is closed. The falling plate current is compensated by the current on the screen. This creates a sort of mirror effect between the two currents and I have included graphs of the two plotted together both with respect to the plate voltage.
Secondary emission on the 6AU6 tube
Secondary emission on the 6CL6 tube
Secondary emission on the 6146 tube
Secondary emission on the 5763 tube
In both the 6CL6 and the 6AU6 the third screen offers a method of suppressing secondary emission. By connecting the third grid to the cathode, effectively grounding the third grid, the kinetic energy of the electrons is reduced by the electric field that the grid creates. In these two tubes, grounding the third grid eliminates secondary emission.
Eliminating secondary emission in the 6AU6 and 6CL6
Interestingly, in the 5763 tube I was not able to turn the secondary emission effect off. Grounding the third grid only succeeds in reducing the effect, not eliminating it.
Attempting to eliminate secondary emission in the 5763
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