726B (RK 726B as made by Raytheon) - This tube was developed in the Bell Laboratories in WWII based on the earlier X band 723A reflex klystron. It was manufactured during the war by WE and Raytheon. The 726 uses a larger
cavity inside the metal envelope and covers a frequency range from 2.883 - 3.173GHz with a typical output of 150mW. Typical operation is with a resonator voltage of 300V, ~30mA. The tube was used in the local oscillator
section of early S band radars, superceding the earlier 707. The cavity is inside the vacuum envelope, and is tuned by the small rectangular 'knob' on the tuning strut. the tuning screw has opposing direction of threads at each end, and thus causes the bow springs to flex, changing the length and
deforming the upper diaphragm of the cavity. A nut on the other strut is used in the factory to set the operating range.
The microwave energy iscoupled out the coaxial line in the modified octal socket - The basic design of this tube made it cheap, lightweight and easy to replace in the field.
The design was used for many other tubes covering quite a range of frequencies, and various tubes of this design were manufactured for several decades.
On the reverse side of the tube in the picture is the serial number A0001 -This was the first tube of this type to roll off the Raytheon production line!
ENGINEERING STANDARDS is also printed on the reverse side in red ink (too dark to be seen in the photo). The tube appears unused, so this may have been used as a design reference by a research institute. It was recovered from a cleanout of the University of Miami radar lab.