724-A
The General Radio 724-A Precision Wavemeter was introduced in Catalog J (1936) and remained available through Catalog K4 (1945).
The Type 724-A is an absorption-type wavemeter with a range from 17 kHz to 50 MHz using seven coils with an accuracy of ±0.25%. The primary application of the 724-A is oscillator and transmitter frequency measurement.
The capacitor used is similar to the Type 722-N Precision Capacitor. It is connected to the inductor through a Type 774 coaxial connector. Resonance is achieved using one of three methods – absorption, reaction, or beat method. Using the panel galvanometer, the absorption method is employed. Regardless of the method, once resonance is achieved, it is a matter of using the correct chart for the coil in use to determine the frequency.
Type 724-A was supplied as a collection of calibrated components sharing the same serial number, i.e. the capacitor, inductors, and calibration charts must have the same serial number to achieve the stated accuracy.
The 724-A replaced the 224-A Wavemeter with improved coils and a galvanometer replacing the thermocouple type indicator. The wavemeter was furnished with a wooden storage case, fitted with lock and carrying handle. This case has compartments for holding the capacitor, inductors, and calibration charts. The 724-A uses a 1G4GT/G triode vacuum tube, resulting in the need for a 1.5 V battery for the heater.
Note: The instrument shown in the photos below uses a 1LE3 loctal battery triode. It appears to be a production change.
Specifications
- Frequency Range: 16 kHz to 50 MHz using seven inductors
- Accuracy: ±0.25% between 50 kHz and 50 MHz, 1% between 16 and 50 kHz
- Inductors: 7 coils wound on isolantite forms and enclosed in molded bakelite cases
- Capacitor: Precision worm-drive type similar to Type 722
- Resonance Indicator: galvanometer and vacuum tube rectifier combination
