IN many voice amplifier circuits a high-impedance gain-control potentiometer in the grid circuit is essential. The Type 642-D Volume Control is primarily designed as a high quality unit for the various types of amplifiers of this sort. It may be used in any standard voice amplifier circuit, connected across the secondary of the input or interstage coupling units.

The total resistance is 200,000 ohms, divided into ten equal steps of 3 decibels, providing a total attenuation of 30 decibels. Each resistance unit is adjusted to an accuracy of +1%, which gives attenuation ratios to within approximately 0.1 decibel. Because of its high impedance, accuracy of calibration, and compactness, its use as a multiplier for vacuum-tube voltmeters is also recommended. Although normal-


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The extremely high impedance of this volume control makes it ideal for use across the grid circuit of an amplifier

The extremely high impedance of this volume control makes it ideal for use across the grid circuit of an amplifier


Each step on the Type 642-D Volume Control corresponds to an attenuation of 3 decibels

ly used in circuits drawing no current, a current of 4 milliamperes will not cause a temperature rise sufficient to change the rated accuracy of calibration.

The frequency characteristic of any reasonably non-inductive potentiometer connected to the grid of a vacuum tube depends almost entirely upon the capacitance of this circuit. The Type 642 Volume Control can be worked into a capacitance of 40 inicromicrofarads with an error of not more than 0.1 decibel at 10,000 cycles per second. Since the capacity of the usual vacuum tube and its socket at the grid terminal is less than 20 inicromicrofarads, a negligible error will result at 10,000 cycles per second and the rated accuracy of 0.1 decibel will hold to approximately 20,000 cycles per second.

The amount of this error is also largely dependent upon the position of the contact arm and the figures mentioned above are for the worst conditions; that is, for attenuations of 3 or 6 decibels. The error is very much less at the higher values of attenuation.

The switch arm is constructed of four-leaf spring phosphor bronze, which provides for long wear and exceptionally low electrical contact noise. The construction is similar to that employed in the precision Type 602 Decade -Resistance Boxes. A cam-type detent is provided. This may be removed easily if smootli action is required.

The over-all dimensions are inches diameter and ?>}/§ inches depth behind the panel.

The price is $25.00.

Modifications of the standard unit for different impedances and attenuations per step can be arranged on special order.

—Arthur E. Thiesseiv

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