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The Series RF-1OA High Frequency Vacuum Tube Test Probe is specifically designed for A.C. voltage measurements over a wide range of frequencies when used in conjunction with PRECISION Vacuum Tube Voltmeters. Precision Vacuum Tube Voltmeters are basically D.C. voltage measuring instruments. In order to provide facilities for measuring high frequency A.C. voltages, a high impedance rectifier Probe must be used in conjunction with the Vacuum Tube Voltmeter. Series RF-10A is specifically designed to perform this function through the rectification action of the Probe tube circuit. This rectifier network converts the high frequency A.C. voltage under test to a proportionate D.C. value and feeds this D.C. voltage to the V.T.V.M. Circuit. Analysis and Examination of the RP-10A schematic will reveal that the circuit is of the peak indicating type. The D.C. output of the Probe which is applied to the D.C. V.T.V.M. assumes a value substantially equal to the positive peak voltage of the A.C. applied to the tip of the Probe. Basically this relationship between the A.C. Input and the D.C. output of the Probe is true only when the rectified Probe output feeds into an infinitely high D.C. load resistance as would be typified by the grid circuit of a vacuum tube. The design of Precision Vacuum Tube Voltmeters are such that an infinitely high input circuit load cannot be obtained, in that the permanent V.T.V.M. input resistance is 13-1/3 megohms on all D.C. voltage ranges. Accordingly the rectified output of Series RP-10A is never quite truly equal to the full peak A.C. input to the tip of the Probe. However, the difference is small enough to permit calibration on the basis of true positive peak voltage.