The spectrophotometer photomultiplier tube is located in a "light-tight" box whose cover
should not be removed except when it becomes necessary to replace the tube. Also, the position
of the tube within the box must not be disturbed since the overall instrument calibration may be
affected. Replacing the photomultiplier tube should be made as a “last resort” repair. The tube is
a cold cathode type, and does not fail due to open cathode heaters, such as those tube used in
radio amplifier applications