Mullard describe the CL33 as a high sensitivity output pentode for use in DC/AC mains operated equipment. The CL4 is electrically identical but is fitted onto a Ct8 base.
This audio power pentode works into an anode load of 4300 Ohms and was designed to have its heater fed as part of a chain from the mains. Such valves were used in universal radio receivers and television sets. The anode voltage of 200 Volts is a clear indication of a design for use on European mains voltages.
The internal metallisation hides the electrode structure.
The coating inside the glass obscures most of the inner detail.
The top mica shows the control grid supports to be copper and fitted with heat radiating fins.
View looking straight at the cathode and grids. The open helix is the suppressor grid.
The classic envelope is 43 mm in diameter and, excluding the IO base pins, is 108 mm tall.
References: Data-sheet, 1040 & 1043. Type CL33 was first introduced in 1940. See also1940 adverts.