The KT33 audio power output valve was introduced in 1937 and was replaced by the KT33C in 1939 and that had a heater centre tap. Electrically the two were identical.
For the 5 Watts output the anode load would be 3,000 Ohms. The distortion at full power was not quoted.
The mutual conductance is given as 10, this suggests a very small gap between the cathode and the control grid, in turn implying very tight manufacturing tolerances.
The classic envelope is 48 mm in diameter and, excluding the IO base pins, is 97 mm tall.
References: Datasheet & 1043. Type KT33 was first introduced in 1937.
Absolute Maximum Operating Conditions¶
Updated October 01, 2013.