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In this paper there is developed an approximate treatment of narrow-band triode noise factor, taking into account induced grid noise by a new method. Minimal values of noise factor predicted theoretically are compared with experimental values, and it is found that from a knowledge of circuit losses, shot noise, mutual conductance, space charge input capacitance, and dc grid current measured on the tube, noise factor, and optimum source conductance can be predicted with accuracy over a wide range of operating conditions. It is shown that lead inductance effects and normal induced grid effects (grid noise, space-charge capacitance, transit-time damping) have no influence on noise performance, but merely affect tube admittances. Direct compensation in the grid circuit of components of input admittance to which these effects give rise leads to a deterioration of noise factor usually attributed to grid noise. It is shown that the optimum grid circuit detuning may be provided automatically by the input space-charge capacitance.