The 17BF11 is part of the Compactron range of television valves and this example features a pair of dissimilar pentodes.
Pentode two is a dual control type for FM TV sound demodulation, electrically this pentode is similar to the 6DT6. Pentode one is the audio output valve with up to 2.4 Watts of output.
The main picture shows the side view of both pentodes. The audio output pentode is on the left and the anode plate is extended to a bulge that slides past the second pentode. In this way extra surface area is provided for the audio device. Also the audio pentode has heat radiating fins fitted to the control grid.
The working face of the audio pentode.
The audio pentode is on the right and mounted at right angles to the second pentode.
The Type designation on the dome.
The wide glass tube envelope is 28 mm in diameter, and excluding the B12C base pins is 58 mm tall.
Reference: Data-sheet. Type 17BF11 was first introduced in 1964.