The W61 is a post war variable mu RF pentode for broadcast receivers. The grid bias range is given as -3.0 to -25.0 Volts.
The Osram paper label.
Here the grid structure can be seen. The control grid has copper supports and like the screen grid passes flat across the cathode. The outer suppressor grid is an open wound helix.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 94 mm tall.
