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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).
{{electronics-stub
Vacuum tubes
In vacuum tube
A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
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
Tube sockets are Electrical connector, electrical sockets into which vacuum tubes (electronic valves) can be plugged, holding them in place and providing terminals, which can be soldered into the circuit, for each of the pins. Sockets are designe ...
.
Top caps have most commonly been used for:
* Amplifier or similar tube control grid
The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consi ...
connection, to provide greater circuit stability by isolating the low-signal circuit from the rest of the tube connections.
* Anode
An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...
connection, to isolate the high-tension circuit, allowing higher voltages to be used.
* Physical convenience. Grid top caps on frequency converters could be connected directly to an adjacent coil; Anode top caps on output tubes could be connected by ''flying leads'' directly to an output transformer. Shorter leads generally mean greater stability.
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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Vacuum tubes