The KT33C is described as a beam pentode, virtually another name for the beam tetrode. This is an audio device and the heater has a centre tap, suggesting that 26 Volts at 300 mA is another specification for the heater.
In single ended use the load impedance is given as 3,000 Ohms. For push pull the input voltage peak to peak would be 44 Volts, the load impedance would be 4,000 Ohms, and the ratings are given for Class AB1.
The Osram paper label.
The top mica. The beam plates are connected to the cathode by the S shaped ribbon. The copper control grid supports are topped by L shaped cooling fins. The cathode is of a rectangular box form.
The classic envelope is 45 mm in diameter and, excluding the IO base pins, is 103 mm tall.
References: Datasheet & 1043. Type KT33C was first introduced in 1939.