Radio age (Jan 1927-Jan 1928)

RADIO AGE for July-August, 1927 How to Build a Vacuum Tube Voltmeter By KIRK B. MORCROSS A VACUUM tube voltmeter is /A an apparatus consisting of a vacuum tube and associated equipment so arranged and calibrated as to permit the measurement of a wide range of alternating and direct voltages. Alternating voltages may be measured with approximately the same accuracy regardless of their frequencies. A thorough understanding of this type of voltmeter, particularly when it is used in all its various applications, requires detailed study and theoretical considerations. However, one may obtain an excellent idea of its general mode of operation without the use of elaborate equipment and with very little theoretical knowledge. Operation of a tube voltmeter ( See Fig. 1) depends upon the fact that a small increase in voltage in the grid circuit requires a correspondingly larger increase in voltage in the plate circuit to "balance" the tube, that is, to bring the plate current back to its original value. Thus one may determine the value of v o 1 1age added to the battery in the plate circuit by employing a voltmeter of small range and measurring the increase in grid voltage required to restore the balance. The value of the unknown voltage is the product of the increased grid voltage Graph obtained by plotting plate voltage values along vertical axis and values of grid volts horizontally and the "mu" or amplification constant of the tube. Mu is determined for the particular tube used. This rule for voltage measurements works both ways, that is, if a very small voltage is to be measured that voltage is added to the battery in the grid circuit and the increased B battery voltage which is required to restore the balance of the tube is read with a voltmeter. The unknown voltage is then computed by dividing the increased B voltage by mu. The tube voltmeter may also be used to determine the operating condition of tubes and their characteristic curves. A schematic diagram is shown in Fig. 1. Fig. 2 shows the actual arrangement of parts and Fig 3 shows the completed instrument. A 201-A type of tube is employed. The instrument MA is primarily for the purpose of indicating constancy of plate current rather than its actual value in milliamperes. A cheap voltmeter is therefore satisfactory. It should have a range of from five to ten volts. The value of R may be from 400 to 800 ohms which is sufficiently great to prevent an unduly heavy drain upon the C battery. In addition to MA, a direct current voltmeter having a range of about 100 volts is needed. (The exact range depends upon the character of the measurements. ) The voltmeter should have a double scale so that small voltages may be read with fair accuracy. The instrument should be provided with connecting leads terminating in clips so that it may be readily connected to different parts of the circuit. The parts are mounted on a board provided with binding posts assuring convenient connections and changes in B and C. The A battery may consist of dry cells (four connected in series) provided the tube is not burned for long intervals of time. Posts 7 and 8 permit easy addition of C battery as required in the measurement process ; additional C battery can be added at 5 and 6 instead, but if too much voltage be applied here the drain on the battery becomes excessive. In the usual methods of operation of the tube voltmeter the value of mu (amplification constant) for the tube is first determined by a single series of measurements. In lieu of