This ancient circuit (1973) was used to make a common multimeter a high impedance multimeter or tube multimeter as was common at the time when digital multimeters did not exist. If the reader finds a transformer with the required characteristics and a 6CG7 valve, it can mount this circuit and use it with a standard analog multimeter, increasing its input sensitivity. The balance potentiometer can be 4k7. Input resistors must be precision.
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