EQUIPMENT AT MOUNTAIN LAKES 



119 



from the projector current but the circuit is arranged 

 to save the calculation and calibrate the hydrophone 

 in absolute units. With key 2 in one position, the 

 meter is connected to the hydrophone and deflected 

 by its output voltage. With the key reversed, the me- 

 ter is across a resistor carrying the projector current 

 which produces the sound field. The attenuator is 

 then adjusted until the meter reads the same as when 

 connected to the hydrophone. As the position of the 

 attenuator is related to the sound field and its output 

 voltage matched to that of the hydrophone, it may be 

 calibrated to read the sensitivity of the hydrophone 

 directly in units referred to 1 volt per dyne per sc| c m 

 for the normal tank stiffness of 10 s dynes per c in. Cor- 

 rections for other values of the stiffness are taken from 

 a chart. 



6.2.5 



High-Power System 



The high-power amplifiers at the Orlando and 

 Mountain Lakes stations have approximately the 

 same electric characteristics. The overall gain of each 

 amplifier is about 24 db and the response is flat with- 

 in 1.5 db from f to 100 kc. At the extreme ends the 

 response falls off, 4 db at 150 kc, 10 db at 200 c. The 

 amplifiers should not be driven at high powers below 

 200 c because of increased coil losses. 



The output impedance is 100 ohms; the input may 

 be either 135 or 600 ohms. The power available with 

 a minimum of distortion is 1 ,200 watts but 1 ,500 may 

 be obtained with a slight increase in harmonic 

 content. 



Each system is provided with a repeating coil that 

 will match the line of impedance of 100 ohms to a 50-, 

 100-, or 500-ohm load. In addition, there is a 30-db 

 pad capable of dissipating 1,500 watts and having a 

 100-ohm input and a 135-ohm output to match the 

 repeating coil to the transmission measuring set. At 

 both stations the pads and repeating coils are 

 mounted in the transmitting booth and the connec- 

 tions appear on a coaxial jack strip in the booth. 



It is obvious that care has to be exercised in the 

 choice of lines and switching elements for currents 

 that may reach fi amperes and potentials as high as 

 500 volts. It has been found that the lead-covered co- 

 axial pier lines and the coaxial jacks handle these 

 powers satisfactorily but special patch cords had to 

 be constructed to provide connections of adequate 

 capacity. The plan of using patch cords and jack 

 strips at these powers involves the danger of discon- 



necting the high-level side when in use. Large arcs 

 may result and the damage to the jacks and terminals 

 will be minor compared to that which may occur 

 from the overload voltage developed in the final stage 

 of the amplifier. With this in mind, particular atten- 

 tion is given to the location of the jacks, and special 

 lines are run whenever possible. Warning signs are 

 kept at the critical junction points. 



The connections from the intermediate-frequency 

 systems to the low-level input of the power amplifier 

 are made in a jack field associated with the amplifier. 

 No caution is needed for these connections, as break- 

 ing them under load causes no damage. 



In using the amplifiers, power-level measurements 

 are made in two ways. The first involves a 30-db pad 

 and the transmission measuring set to determine the 

 available power for the 100-ohm output. The second 

 requires either a thermocouple wattmeter or a record- 

 ing wattmeter to measure the power delivered to the 

 load. Apart from the precautions needed for the high 

 powers involved, these amplifiers are treated merely 

 as extensions to the existing equipment. 



In their electrical details the amplifiers at Orlando 

 and at Mountain Lakes differ considerably. While 

 both are essentially two-stage, push-pull, transformer- 

 coupled units, the power requirements are quite dif- 

 ferent. 



The Orlando amplifier is a Navy echo-ranging type 

 in which the input, output, and interstage transform- 

 ers are replaced with special transformers designed 

 for the particular frequency and power range. How- 

 ever, the power supply and control circuits are re- 

 tained and these require a 3-phase, 440-voIt, 60-cycle 

 supply. Since this is not available from the power 

 lines, a motor-generator set was used with 6-kva maxi- 

 mum output. 



To compensate for fluctuation of line voltage, in- 

 ternal impedance of the generator, and other sources 

 of instability, an electronic regulator is used which 

 maintains the peak voltage within I per cent. Con- 

 trol of the peak is chosen because the critical voltages 

 in the amplifier are determined by the peak of the 

 supply rather than by the rms value. Because of the 

 change in wave form, the rms value shows a variation 

 of 5 per cent from no load to f idl load for a peak varia- 

 tion of only 1 per cent. Another reason for controlling 

 the peak voltage is the speed of response which in this 

 regulator will compensate for any changes in a few 

 cycles. Adjustments are also made to prevent hunt- 

 ing. Such adjustments are very necessary with rotat- 



