The 6BM8 is an audio triode plus output pentode of seven Watt anode dissipation. The output section is a beam tetrode rather than a pentode.
The triode section has a amplification factor of 70, and this valve was much used in record players with crystal pickups. The latter had a relatively high output and when coupled with the single ended pentode output stage made effectively a single valve design. Only a rectifier was required to complete the line-up.
The triode on the right with the screen separating it from the beam tetrode. The cathode is rectangular in cross section.
The stitched anode with the beam plate extending above the top mica.
The thin glass tube envelope is 20 mm in diameter and excluding the B9A base pins the valve is 70 mm long.
References: Data-sheet & 1040. Type 6BM8 was first introduced in 1956. See also1956 adverts.
