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, an electric field is created between the third grid and the plate. This suppressive field pushes emitted electrons back to the plate. In these two tubes, grounding the third grid eliminates secondary emission.
Attempting to eliminate secondary emission in the 5763
Secondary Emission Page
Back to Front