The KT33C audio power output valve was introduced by M-OV in 1939 and replaced the KT33. This version had a heater centre tap and was designed for series heater chains. This exhibit is branded GEC for use in professional equipment.
For the 5 Watts output the anode load would be 3,000 Ohms. The distortion at full power is 7.5%.
The GEC brand label.
The lower mica and pinch. The rectangular cathode has four insulated heater wires threaded through it.
The classic envelope is 44 mm in diameter and, excluding the IO base pins, is 109 mm tall.
References: Data-sheet & 1043. Type KT33C was first introduced in 1939.