Simpson Model 311-2 VTVM
Overview
Simpson's 311-2 (© 1966, the PCB revision of the © 1958 Model 311) is the bench VTVM counterpart to the B&K Dynamatic 375. Same role — high-Z tube-circuit measurement where a 20 kΩ/V VOM would load the circuit — but Simpson deliberately made the input 22 MΩ on all DC ranges (double the usual VTVM's 11 MΩ). Voltage ranges DCV/ACV 1.5–1500 V (×7), ACV in both RMS and peak-to-peak arcs, ohms ×1…×1M; a 0732 HV probe extends DC to 30 kV and a 0174 RF probe reaches 250 MHz. Fully tube: a 6AL5 peak-to-peak rectifier + a 12AU7 balanced-bridge amplifier (the power-supply rectifier CR-1 is a silicon diode). Custom 200 µA movement, zero-center "DC galvanometer" mode for null/alignment work.
Context
The B&K Dynamatic 375 and Simpson 311-2 are kept side-by-side on the bench as a deliberate redundancy — two independent VTVMs let cross-checks catch a drifting calibration. The Simpson's specific advantages are its 22 MΩ input (vs the B&K's 11 MΩ) and its custom 200 µA movement; the B&K brings the rotating "Dynamatic" drum scale and DC-current ranges. Common service issues: the input 12AU7 drifts (re-zero on every range/function change, esp. AC↔DC; a fresh 12AU7 must be aged ≥48 h powered before recalibration per Simpson); the precision divider resistors (the 22 MΩ/20 MΩ network) drift up; the internal 1.5 V "C" cell battery holder corrodes. Cal pots: R29 ZERO SET / R20 DC CAL / R21 AC CAL / R32 AC contact-potential.
