A tube caddy was a very large carrying case sometimes with hundreds of compartments for vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s. They were carried by repair technicians who did home service calls in the days when radios

Receiver (radio)

A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...

 and television sets were too large and heavy for the average homeowner to bring to the repair shop. The tube caddy had to be large enough to hold several of each type of tube likely to be needed on a typical day of service calls so that the technician did not need to return to the shop during his rounds. Because tubes are quite light the caddy did not have to accommodate a large payload and could also be constructed with very light materials so the overall package weighed far less than it appeared to. Most repairs could be effected with the replacement of tubes since there were few other active components in electronic

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

equipment at that time so this was a very efficient way to accomplish fixes.