New Vacuumtube Voltmeter

• THE TYPE of peak-reading vacuam-t ill>c voltmeter now considered standard in the communications industry was first introduced by the General Radio Company some ten years ago when the Type 726-A Vacuum-Tube Voltmeter was announced.1 The novel design and excellent performance of this voltmeter has maintained its popularity over the intervening years considerably beyond the time when advances in the art would normally necessitate a new design. The development, during this period, of new tubes, circuits, and construction techniques has, however, progressed continuously, and these now make it possible to design a completely new instrument having even better performance.

In the design of the new Type 1800-A Vacuum-Tube Voltmeter, which replaces the Type 726-A, effort has been directed to correcting the minor disadvantages that have been found in the older voltmeter as well as to incorporating desirable new features that make the instrument more flexible and convenient.

1 W. N. Tuttle, "Type 72tS-A Vacuum-Tube Voltmeter/' General Radio Experimenter, Vol. XI. No. 12, May 1937, pp. 1—6.

Figure 1. Panel view of the Type 1800-A Vacuum-Tube Voltmeter.

1 W. N. Tuttle, "Type 72tS-A Vacuum-Tube Voltmeter/' General Radio Experimenter, Vol. XI. No. 12, May 1937, pp. 1—6.

Some of these disadvantages were:

1. The shielding of the probe of the Type 726-A was not complete, and trouble with pickup from strong fields at frequencies over about 50 Mc was sometimes encountered.

2. Individual zero adjustments for each range were used in the Type 726-A, and these sometimes drifted from their correct setting, so that readjustment of the zero when switching from range to range was necessary.

3. The voltage-regulating transformer in the Type 726-A was explicitly designed for one line frequency. In addition, the transformer radiated an appreciable 60-cycle field.

4. The Type 726-A probe was relatively bulky and difficult to get into confined spaces.

5. The Type 726-A meter was somewhat difficult to lead at a distance because of the knife-edge pointer and was confusing because of the arrangement of the scales. Reflections from the glass were often bothersome, and the light from the pilot light distracting.

6. The cabinet of the Type 726-A, while convenient for ordinary bench use, was rather large and could not readily be used in other positions than the normal one in which the panel was at a 15° angle to the horizontal.

These disadvantages, with others of a more minor nature, have all been eliminated in the Type 1800-A, and many new features have been added. These include:

1. The natural frequency of the probe has been increased by nearly 3:1, and the gain in high-frequency performance because of the complete shielding is even greater.

2. The input capacitance has been reduced by a factor of 2, and the parallel resistance component increased fourfold at low frequencies.

3. An additional 0-0.5 volt scale has been added that extends the sensitivity by a factor of 3.

4. A complete set of d-c scales has been added, covering the same ranges as the a-c scales.

5., A set of terminal parts has been provided that makes it possible to assemble, on the end of the probe, connectors that will fit all standard General Radio Type 138, 274, and 774 terminals.

6. The new instrument is smaller, lighter, and easier to use.

7. It can be operated with the panel horizontal, vertical, or inclined.

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