The Type 18 is an audio output pentode. The anode dissipation is 11 Watts, the load impedance should be 7,000 Ω and the cathode bias resistor 410 Ω. Under these conditions an audio power output of 3.2 Watts is to be expected.
The base cap has the Arcturus name printed on it in red. Originally Arcturus were noted for their blue glass envelopes.
The electrodes: the copper supports of the control grid have additional radiation cooling provided by the top fin.
Here the wire grids can be seen as can the coated cathode and the concentric circles of the electrodes.
The classic envelope is 44 mm in diameter, and excluding the UX6 base pins is 103 mm tall.
References: Data-sheet & 1040. Type 18 was first introduced in 1937. See also1937 adverts.