The 6C4 triode was specified as both an amplifier and an RF power valve. In the latter role it would deliver 5.5 Watts. With a very small electrode size in a small envelope volume, this would make it ideal for hand portable equipment.
In larger apparatus it could deliver enough drive power for much bigger valves in Class C. Class C drives the control grid positive and when positive the grid draws current and thus the drive valve has to be able to supply this.
The Type designation is etched into the glass as well as being printed on. The RCA octagon contains the 6C4. The control grid has no cooling fins at the top.
The box anode end view and the RCA logo.
The anode is ribbed for dimensional stability and side flanges provide both cooling area and aplace for the stitches that hold the two halves together.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.