The E280F is a special quality amplifier pentode. It features a very high mutual conductance and this is typical of high frequency wideband amplifier valves. This example is marked as Foreign made.
Wideband amplifiers are found in oscilloscopes, spectrum analysers, radar systems, television and specialist measuring systems.
The valve can be seen to be a beam tetrode and not a true pentode. The anode is just two plates either side of the cathode and inner grid assembly. The screen grid is clearly visible but the control grid is too fine and too close to the cathode to be easily seen. The gold plated pins are an indication of the special quality valves from Mullard.
At higher magnification the frame structure of the control grid can be detected but not the individual strands of wire.
The anode plate face on.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 53 mm tall.
Reference: Datasheet. Type E280F was first introduced in 1958.