Variac Rectifier 1260a

mm i iQlixx IN THIS ISSUE

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Type 1261-A Power

Supply 4

Type 1260-A Variac

Rectifier 6

•MOST BRIDGE MEASUREMENTS

and various other laboratory procedures require the use of an amplifier to obtain sufficient sensitivity. The Type 814-A Amplifier has for years been a popular item in the General Radio line and has been widely used with audio-frequency bridges.

Recent developments in tubes and circuits have prompted the design of a new model, retaining all the advantages of the old, but with several distinct improvements.

The Type 1231-A Amplifier and Null Detector consists of a high-gain resistance-coupled amplifier, using the new miniature-type tubes, mounted on a shock-absorbing suspension. The maximum gain in the

Figure 1. Panel view of the Type 1231-A Amplifier and Null Detector.

middle of the audio-frequency range is approximately 90 db, and the instrument is usable over the range from 20 cycles to 100 kc.

A most important feature of the new instrument is the built-in null-indicator circuit, which is essentially a semi-logarithmic vacuum-tube voltmeter, utilizing a single multi-section tube as an a-c amplifier, diode voltmeter, and d-c amplifier, thus allowing a high degree of sensitivity. The voltage developed by the diode is applied as a gain-controlling grid bias to the preceding a-c amplifier section of the tube producing the semi-logarithmic response. No additional indicating devices are necessary, therefore, to use the new instrument as a bridge null indicator, although a pair of phones can be plugged into the output if desired.

The amplifier and null detector may also be made selective with regard to frequency by plugging in the Type 814-P Tuned Circuits. The Type 814-P2 operates at either 400 or 1000 cycles, and the Type 814-P3 at 60 cycles. Other types of tuned circuits or filters can also be used with the instrument.

The new amplifier and null detector is unusually compact and convenient to operate. Normally, the instrument is enclosed in a small walnut case, matching other General Radio equipment, and is operated from internal batteries which will have a long life because of the low current drain. It is also possible where desirable to operate the instrument from the 60-cycle lines by use of the Type 1261-A Power Supply unit. This is the same power supply that is used for operating the General Radio Sound-Level Meters and other battery instruments.

For relay-rack mounting, a panel extension can be provided which mounts the two standard Type 814-P Filters also, thus providing a complete unit assembly.

The main gain control of the amplifier is a high-grade, wire-wound unit, which will last indefinitely with normal use. Push buttons are provided to reduce the input voltage and gain for high-level signals and for selecting operation as a straight amplifier or as a null detector with the semi-logarithmic features. Other push buttons allow checking of the battery voltage and exact setting of the null point for critical bridge measurements.

The input and output connections will take either General Radio Type 774-E Coaxial Connectors or the usual Type 274-M Plugs.

Figure 3 shows the frequency characteristics of the amplifier and complete specifications are appended to this article. The instrument was purposely designed for maximum gain in the

Figure 2. Schematic circuit diagram of the Type 1231-A Amplifier and Null Detetor.

Figure 2. Schematic circuit diagram of the Type 1231-A Amplifier and Null Detetor.

Copyright, 1946, General Radio Company, Cambridge, Mass., U. S. A.

Figure 3. Approximate gain characteristics as a function of frequency.

Figure 3. Approximate gain characteristics as a function of frequency.

important audio-frequency range, but, for operation as a bridge null detector, it will provide substantial gain well beyond this range.

It is expected that the Type 1231-A

Amplifier and Null Detector, because of its small size, high sensitivity, and general convenience, will meet a real need in most communications laboratories.

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