The 6F15 is an early all glass variable μ pentode. It would have been designed for AGC controlled RF and IF amplifiers.
The mutual conductance is a function of the winding pitch of the control grid. In variable μ types the grid would be wound with a graded pitch from fine to course.
The perforated screen is held by tabs in the mica sheets. The fingers of the top mica are bent down showing how the mica disc fits tightly into the envelope.
Just above the base skirt the suppressor grid wire can be seen through the holes in the screen. The anode is bright and the grids extend outside the anode area.
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 57 mm tall.
References: Data-sheet & 1040. Type 6F15 was first introduced in 1948. See also1948 adverts.
