The 6JS6C is a purpose designed colour television line output valve, or sweep tube. Like many other line output valves they also could be used for RF power amplifiers. The 6JS6C was used in television designs from the early 1970s.
The Yaesu FT-101 used these valves for the final power output stage operating up to 30 MHz and far outside the manufacturers original specification.
The anode is substantial as this valve has an anode dissipation of 30 Watts. One of the two heaters can be seen at the top of the rectangular cathode. The insulated heater is straight wire and passes through the cathode four times.
The 12 pin glass base. The heater connections are easy to see.
The wide glass tube envelope is 38 mm in diameter, and excluding the base pins is 92 mm tall.
References: Datasheet & 3002. Type 6JS6C was first introduced in 1968.