Top cap

In vacuum tube technology, 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.

In audio amplifier tube application, the top cap was originally used for the grid connection, and a serviceman could apply a moist finger to the terminal to confirm that the stage and subsequent circuits were working by listening for the hum this produced in the loudspeaker. This practice led to some nasty accidents when anode top caps were first introduced to amplifier stages (they had been used on rectifiers for some time).

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Wikipedia