The PM12M is an variable μ RF tetrode known at the time as a screened grid valve because the screen grid took the form of an enclosing cage.
The first screened grid valve was the S625. Soon the cage arrangement had replaced the double ended original M-OV design.
The top cap
The complete valve. The screen begins above the pinch as a skirt and then the grid cage can be seen with the anode connection being taken to the screw top cap
A closer look at the plastic moulding and the screw connection. If the adhesive fails the wire to the anode is easily broken. The screw top is not original, it should be an insert in a black plastic cap.
The screen cage with the twin corrugated anode sheets either side.
The horizontal separation of central filament and control grid to the screen grid cage and finally the anode sheets held rigid in glass insulators connected to the screen skirt.
The glass beads holding the anode sheets. The inner surface of the lower anode can be seen.
The anode is just slightly narrower than the control grid. The screen is much wider.
The glass bead can be seen to consist of the bead and two lengths of glass tube.
Here the twin hairpin filaments can be seen. The filaments are held in tension by two springs fitted to the top mica.
The balloon envelope is 49 mm in diameter, and excluding the B4 base pins is 121 mm tall.
References: Data-sheet & 1043 Type PM12M was first introduced in 1933. See also1933 adverts.
