The 8608 is described as a premium quality power pentode. It was designed for wideband video power amplification and has a very high mutual conductance. This suggests the use of frame grid construction and polished cathode coating as the clearance between control grid and cathode will be very small.
The general construction has a central cathode and inner grids, beam plates to catch secondary electrons and finally a split anode that brings only small areas of metal close to the electron stream.
Looking down the length of one anode.
The cathode under the microscope. The control grid is wound with very thin wires. In frame grid construction the wire will be wound to half the breaking strain of the wire to ensure that it does not kink.
With the screen grid in focus, the control grid below is not seen.
The screen grid seen against the edge of the beam plate.
The wide glass tube envelope is 28 mm in diameter and, excluding the B9A base pins, is 61 mm tall.
Reference: Data-sheet. Type 8608 was first introduced in 1965. See also1965 adverts.
