Top cap
![6A8 pentagrid converter with grid 4 on the top cap](/Images/godic/202502/16/6A82207.JPG")
![866 mercury rectifier with anode top cap](/Images/godic/202502/16/866_mercury_rectifier_12207.jpeg")
In vacuum tube trechnology, a top cap is a terminal at the top of the tube envelope that connects one of the electrodes, the other electrodes being connected via the tube socket.
Top caps have most commonly been used for:
A few amplifier tubes used two top caps, symmetrically placed, one for anode and the other for grid.