The 6BL6 is a reflex klystron CW oscillator. It mounts in an external cavity and is a broadband oscillator that can cover the range 550 - 3,800 MHz. The output is 60 mW.
The design use is in signal generators and spectrum analysers.
The Raytheon name and their Type designation RK-6BL6.
At the left of the image and connected to the top cap is the repeller that reflects the electrons back into the cavity that sits between the two external rings.
The cathode is at the top of the bright cylinder near the base.
The decreasing diameter of the rings allows it to be inserted into the external cavity.
The thin Glass Tube envelope is 18 mm in diameter and, excluding the base pins, is 69 mm tall.
