Abstract: An apparatus measures the operational parameters of a vacuum tube that has an anode, a cathode, a grid and a filament. A source of a bias potential for the anode is included along with a generator that produces a series of pulse at each of a plurality of voltage levels. A resistor couples the pulses to the vacuum tube grid and another resistor couples the cathode to ground potential. A voltmeter sequentially measures the voltages on the grid, the anode, the cathode, and across the resistors. From these voltage measurements, the system computes the levels of the grid current and the filament current. These parameters of the tube are displayed graphically for use in comparing the performance of different tubes.