The 4C28 is a pulse modulator triode with a peak output of 6.5 kW and a maximum anode dissipation of 150 Watts. The image above shows the similar CV92 on the left and the 4C28 on the right. Both feature oxide coated cathodes.
The design purpose was VHF navigation transmitters such as the LORAN and Shoran systems. The 4C28 was introduced in 1943 by RCA but withdrawn at the end of 1945 there after it was made by Central.
The Type designation on the brass cooling fins. Part of the grid can just be seen.
The makers name and Made in USA.
The filament connections with the evacuation pip to the right. The covered leads are welded to the pins emerging from the valve.
The envelope is folded back on itself with the threaded grid connection within the recess.
The lettering on the original box showing the date as July 1954 and the serial number as J5933.
The wide glass tube envelope is 33 mm in diameter, and excluding the base pins is 94 mm tall.
References: Data-sheet & 3002. Type 4C28 was first introduced in 1943.