This RW3 travelling wave tube is new and has serial number 3757. It is rated for service at 4.2 GHz and to deliver 5 Watts of RF power. The gain of this tube is 37.8 dB.
In operation the tube would housed to provide a magnet lined cavity and waveguide input and output ports.
In this second picture the electron gun can be seen. The cathode current would be 38 mA
Something that is not seen with integrated TWT Amplifiers is the helix, that critical guide for the slow wave. Whilst only slow compared with the speed of light, the transfer of energy from the electron beam to the travelling wave takes place when both are travelling at the same speed along the tube. See Travelling wave tube amplifiers.
The thin glass tube envelope is 18 mm in diameter, and excluding the base pins, is 285 mm tall. The fixing ring at the base is 52 mm diameter.