Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

——rINPUT IMPEDANCE OF PENTODES 309Pig. 2—Circuit for input impedance measurements.The departure at high frequencies, however, is the result of approachingseries resonance of the lead inductance, both internal andexternal, with the input capacitance of the tube. This condition isapproximately indicated in Figure 3.Although the actual input resistancebetween grid and cathode, R{, is inaccessible for direct measurementby the method employed, we can assume for the sake of argumentthat it does not depart from the inverse square law with frequency.The resonant effect then makes the apparent input resistance, R bseem to be lower than the corresponding value of R{. However, inO*~ffi\onps^-Fig. 3a—Series resonance in tube input circuit.oHJW^—Cgk!Fig. 3b—Equivalent tube input circuit.