The 6HF8 is an audio voltage amplifier triode pentode. The triode has a high mutual conductance and the pentode is 'straight' with a sharp cut-off. The pentode is actually a beam tetrode. All the grids are wound on copper rods.
The pentode anode strap and within the beam plate and the screen grid copper support rod.
The triode section.
The triode on the left with the pentode at right angles on the right.
A closer look at the pentode.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 59 mm tall.
Reference: Data-sheet. Type 6HF8 was first introduced in 1961.