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 The 4C28 was introduced in 1943 by RCA but withdrawn at the end of 1945 there after it was made by Central.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 heater (top) and cathode leads are insulated with braided fibre glass sleeving. The central brass fins have to have forced air cooling in operation.
The grid connection is within the re-entrant glass envelope.
Thanks to Stig Comstedt for sending the excellent images.
The wide glass tube envelope is 32 mm in diameter, and excluding the base pins is 98 mm tall.
References: Datasheet & 3002. Type 4C28 was first introduced in 1943.