659 
90 < 
sensitivity of 50 mv d.c. per inch deflection on the cathode-ray tube screen, 
and negligible drift during a day's operation, 
(1) Description of operation, There are five units in the oscillo- 
graph -- 2 camera assembly, an amplifier chassis, a cathode=-ray tube chassis, 
and two power supplics, These can be installed in any*standard relay rack 
of sufficient height, and then interconnected by patch cords. 
Each amplifier (see Fige 8) is built on a small chassis which is then 
plugged into a shock=-mounted relay rack unit which holds 211 four amplifiers. 
This quick-change unit has many advantages, including mass production and 
easy replacement of a faulty unit. The electronic part of the system can 
be understood by referring to Fig. 9. The input is a cathode follower oper- 
ating with reduced plate current and plate voltage. For direct-coupled oper- 
ation, 20 megohms is a reasonable input impedance, If a coupling condenser 
is used, 1000 megonms may be uséd in the grid return if care is taken to 
eliminate leakage currents. The amplifier stage employs 2 conventional 
delta-type attenuator in the cathode circuit. ‘The problem of zero signal 
driit was found to be reduced by the use of 6SH7 tubes rather than 6AC7 tubes, 
which were originally tried. Initial differences in tubes are compensated 
for by the use of a 50-ohm variable resistor (fine gain balance). The next 
stage is used to shift the d-c level of the sighal from +72 volts to -62 volts 
and at the same time to provide positioning control of the cathode-ray spot. 
The output amplifier is conventional, but the use of a cathode-follower out- 
put is necessary when the amplifiers are at a distance from the cathode-ray 
tubes, -This has the further advuntage of lessening the capacitative load on 
the output amplifier. An unexpected phenomenon was observed in the initial 
design when the high-frequency response fell off at large deflections, This 
was because the cathode follower was originally working too close to cutoff 
on the extreme of the signal swing. 
The cathode-ray tube chassis (see Figs. 10 and 11) is extremely compact 
and has nothing unusual in it except for a level shifter which allows a beam- 
brightening signal at ground potential to be applied direct-coupled to the 
grids of the cathode-ray tubes at a level of -1000 volts as described in 
Sec. 2(c). On the front panel there are individual intensity and focus con- 
trols, individual be2m on-off switches, and a master intensity control. A 
Sylvania R1130 crater tube is included to give a time scale on the record. 
