instruments, in 1933, has been the Type 6S0 Impedance Bridge — witness a sale of over 3700 of these instruments to date. By means of this bridge resistors, inductors and capacitors can be measured rapidly and conveniently over a wide range of values.

Oil many occasions, the need arises for using an auxiliary audio amplifier to increase the sensitivity of bridge balance when employing either headphones or an a-c galvanometer as the null detector. For this purpose General Radio has successively offered the Type 514 and Type 814 Amplifiers, and at the present time the Type 1231 Amplifier and Null Detector.

The Type 650 Impedance Bridge is self-energized by four No. 6 dry cells housed in the bridge cabinet. These dry cells supply <l-c power for the Wheatstone bridge measurement of resistance and drive a one kilo-

Figure 1. View of the Type 650-A Impedance Bridge with battery compartment at the

Figure 1. View of the Type 650-A Impedance Bridge with battery compartment at the

Figure 2. Panel view of the Type 650-P1 Oscillator-Amplifier, showing lop of bridge panel. Connections are easily made from either the d-c or 1000-cycle terminals to the EXTERNAL GENERATOR terminals of the bridge.

Figure 2. Panel view of the Type 650-P1 Oscillator-Amplifier, showing lop of bridge panel. Connections are easily made from either the d-c or 1000-cycle terminals to the EXTERNAL GENERATOR terminals of the bridge.

cycle microphone hummer for bridge measurements of either inductors or capacitors. While such an arrangement is essential for field use where power lines are not available, most of these bridges are used exclusively in laboratories where a-c mains are always available, in which case it would be desirable to dispense with the dry cells and operate the bridge directly from the power line.

To meet ihese two desirable requirements, the Genera] Radio Company now offers the Type 650-P1 Oscillator-Amplifier, a useful combination unit which is designed to fit into the cabinet compartment formerly housing the dry cells. This 650-P1 comprises:

(1) A source of d-c voltage for resistor measurements.

(2) A one-kilocycle vacuum-tube oscillator to replace the microphone hummer.

(3) An amplifier of sufficient gain for all uses of the bridge.

This auxiliaiy unit is energized from a single-phase a-c power line at a frequency of 50 to 60 cps, and having a nominal voltage of either 115 or 230. The power consumed is about 10 watts. The unit is assembled in a metallic cabinet with a top control panel which replaces the wooden cover of the battery compartment.

When installed, a short plug-termi-

Copyright, 1946, General Radio C

nated jumper connects the bridge input terminals to adjacent d-c or one-kilocycle terminals on the panel of the auxiliary unit. By means of an internal shielded cable a connection is made between the bridge detector terminals and the input of the amplifier. The phones or a-c galvanometer (neither of which are supplied) that are to be used as the null detector are then attached to terminals on the auxiliary unit. The Type 483-F Output Meter is a useful a-c galvanometer for this purpose. A complete set of instructions for installing and operating the Type 650-P1 is supplied.

The power-supply rectifier is a 6H6 duplex diode functioning as a voltage doublet*, i he maximum permissible value of the filtered d-c output is applied to the bridge. This is considerably in excess of the 6 volts originally available from batteries, so that I be sensitivity of balance when measuring the larger values of resistance (one megohm maximum) is definitely enhanced using the galvanometer already incorporated in the Type 650 Bridge.

The one kilocycle oscillator employs a 6SL7GT duplex triode, one triode functioning as an oscillator and the other as a buffer amplifier. The oscillator is of the R-C feedback type and can be more accurately tuned to one kilocycle than is practical with the microphone hummer, thus minimizing inherent frequency errors in the reading of the Q or D dial of the bridge, since the calibration of these dials is a function of the operating frequency. The oscillator frequency is subject to a 20-cycle warm-up rise over a one-hour interval and is adjusted at thermal equilibrium to within ±5 cycles of one kilocycle. Between the oscillator and the buffer amplifier is inserted an oscillator gain control permitting a full range adjustment of the a-c voltage ipany, Cambridge, Mass., U. S. A.

applied to the bridge. This feature, which is not available in the original Type 650 Bridge unless a separate oscillator is used, is desirable in measuring iron-cored inductors at low flux densities approach initial permeability.

Between the output of the buffer amplifier and the bridge input terminals is interposed a 5/1 step-down transformer which is doubly shielded and designed to give maximum over-all efficiency with the Type 650 Bridge and to minimize certain residual capacitance errors. This transformer contains separate electrostatic shields around the primary and secondary windings with a substantial separation between them. The primary shield is grounded, while the secondary shield is connected to that secondary terminal which leads to the junction of the A and N arms of the bridge. This arrangement places a negligible capacitance of 9/u/uf across the standard capacitor (10,000 jjl/u.f) bridge arm, and introduces a capacitance of less than 36 ¿tyuf (4.4 megohms reactance) across the CRL rheostat arm (10 kilohms maximum), so that the error introduced is also negligible.

The full output of the oscillator somewhat exceeds that of the microphone hummer and has a distinctly purer wave form. Twenty milliwatts into a matching load of 2 kilohms may be obtained with a harmonic content of less than 2% at full output. The open circuit voltage is in excess of 12 volts, and the hum level is less than 15 millivolts.

I he amplifier comprises two stages utilizing a second 6SL7GT tube. The first stage is tuned by means of a degenerative R-C network to afford peak gain and maximum selectivity at one kilocycle, thus facilitating bridge balances by minimizing harmonics, especially when measuring nonlinear circuit elements such as iron-cored inductors. An open circuit voltage-ratio gain in excess of 50 db is available with a discrimination in excess of 10 db at 0.5 and 2 kilocycles. With average phones a gain of about 45 db is realized. This amplifier is preceded by a gain control affording full range adjustment of detector sensitivity. The hum level is less than 10 millivolts. A blocking capacitor removes any d-c component from the output to the phones or a-c galvanometer and par

Figure 3. Elementary schematic circuit diagram of the Type 650-P1 Oscillator-Amplifier.

Figure 3. Elementary schematic circuit diagram of the Type 650-P1 Oscillator-Amplifier.

General Radio Microphone Hummer

tially resonates the average headphones to one kilocycle. Crosstalk between the oscillator and amplifier components is reduced to an imperceptible minimum by the use of isolation filters and compartment shielding.

In order that this same amplifier may conveniently be used when making a bridge measurement at some frequency other than one kilocycle (using, of course, some other oscillator) a switch is provided whereby the tuning network may be removed, thus giving the amplifier a flat gain characteristic. This flat amplifier has a gain of about 60 db at one kilocycle, dropping by 0.6 db at 100 cycles and 3.5 db at 5 kilocycles. The hum level is less than 400 millivolts.

The hum levels for the two amplifiers quoted above are obtained when employing a resistive load such as the Type 483 Output Meter. Due to resonance selectivity, when using head phones these hum levels are reduced about 20 db and become audibly imperceptible.

On the control panel of the Type 650-P1 are the following items: an input socket for attachment to an a-c power line together with a line switch and pilot light; d-c, 1-kc and phone terminals; a switch for selecting either d-c or 1-kc excitation of the bridge; a switch for rendering the amplifier flat or tuned to 1 kc; and separate control knobs (un-calibrated) for adjusting the oscillator output level and the amplifier gain.

While the Type 650 Impedance Bridge will continue to be sold equipped with the microphone hummer and dry batteries, the addition of this new auxiliary unit should prove beneficial to many old and new users of this bridge.

The Type 650-P1 Oscillator-Amplifier is specifically designed to be used with the Type 650 Bridge. However, as an individual item, it should prove useful with many other bridge systems, providing, in compact form, both a one kilocycle oscillator for exciting such a bridge and an amplifier for the bridge detector. The unit can also serve as a convenient source of well-filtered d-c power (but cannot be used simultaneously as an oscillator-amplifier). The open-circuit d-c voltage is 190 volts with a hum level less than 0.1 volt, and the regulation is closely linear and represented by the equation:

Terminal Voltage - 190 - 23/ wherein / is the load current in milli-amperes. No harm is done by short-circuiting the d-c terminals which affords a maximum current of 8 milliamperes.

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