The KT8 was a not very successful British attempt to emulate the American type 807. The 807 was based on the RCA type audio 6L6 beam tetrode, but with a carefully-aligned grid structure and smaller cathode to anode spacing for short electron transit times and thus a good RF capability.
The KT8 was based on the audio KT66 but the longer electron path, compared to the 807, was a disadvantage at the high radio frequencies. Not a problem at AF but the KT8 was pretty useless above 15 MHz whereas the 807 was still doing great things at 60 MHz.
In the end the British Services abandoned the KT8 in favour of the American 807. Attempts to make 807s in the UK were largely unsuccessful. At this point enter the KT807 - an 807 on a British B5 base rather than the UX5 of the 807.
The working face of the anode and below the lower mica the characteristic oval shape of the lower screen.
The end view of the anode. The bright beam plate can be seen reaching above and below the anode to the mica sheets.
The B5 base cap.
The classic envelope is 49 mm in diameter and, excluding the B5 base pins, is 127 mm tall.