as of such a frequency at a given temperature but the calibration is for the unit as a whole, amplifier included. This in itself is an unusual feature and not in accordance with general practice. The practicability of the arrangement is obvious, however, since the units are in duplicate and in event of failure of one unit due to fracture of the crystal or anything else destroying the accuracy of calibration, the other can be immediately switched in its place and the defective unit shipped back to the laboratory for repair and recalibration. By this practice there is no chance of the frequency of a crystal as specified by the laboratory being affected by association with circuits or loads at variance with those for which it was calibrated. While such precautions as to frequency calibration and maintenance are by no means necessary in amateur radio, they are of prime importance in services requiring close adherence of assigned frequencies.
___The crystal mounting also is unusual, although here again sheer practicality is evidenced. Extreme accurate temperature control has been found difficult in actual service, and methods of minimizing frequency shift with change in temperature have bene found valuable as adjuncts to temperature control. The mounting of

Two of these units are used in the transmitter. The small panel projecting from the front has mounted on it the thermo-regulator adjustment and thermometer for indicating the oven temperature. The oven is immediately behind this panel.

the crystal plays an important part in maintaining constant frequency with slight variations in temperature and the mounting used in this transmitter is such that a comparatively coarse control of temperature is accompanied by negligible frequency drift. The mounting is of the air-gap type, the gap being determined by quartz spacers between the uper and lower plates. The gap is maintained constant by virtue of the identical temperature co-efficient of expansion possessed

by both the oscillating crystal and the quartz spacers. The spacers are thicker than the crystal by the length of the air-gap. Fig. 2 illustrates the principle of the mounting used. In an actual test

extending over a considerable period, the frequency of the transmitter did not vary in excess of 20 cycles from the assigned frequency. The 20-cycle shift occurred during the first hour, after which zero beat was maintained for the remaining hours of the test. The temperature of the oven varied over a range of approximately 1.8 (C.) during the run.
___The oven is of comparatively simple construction and comprises several outer walls of heat- insulating material with an inner compartment of aluminum. Within this compartment are mounted the crystal holder, the element of the thermoregulator whose adjustment is mounted on the front panel, the bulb of the thermometer, and the heater unit. The oven, in turn, is contained in the shield housing the oscillator and its associated amplifer.
___The oscillator tube is a UX-210 with 180 volts on its plate and employs grid-leak bias. The crystal is connected between the grid and filament and the plate tank is tuned to the frequency of the crystal, 1060 kc, by a variable condenser. The output of the crystal oscillator is capacitively coupled to the control grid of the first of the two succeeding stages of screen-grid amplification. These amplifiers each use one UX-865 7-watt screen-grid tube with 500 volts on its plate, and grid- leak bias. Screen-grid voltage is obtained from the plate supply through suitable resistors. The plate power for the whole unit is obtained from a UX-866 mercury-vapor rectifier and filter unit mounted on the panel at the left of that on which the crystal-oscillator-amplifier is mounted. The input to the second amplifier is capacitively coupled to the plate tank of the first amplifier; both plate tanks are tuned by means of variable condensers. The crystal-oscillator-amplifier unit is the only completely shielded section of the whole transmitter; a decidedly interesting feature. The ovens of both units are heated from the house-lighting circuit and are left running continuously to insure constancy of temperature. Immediately below the two units are the switches for throwing either into service.

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