Thermionic and Other Cathode Tubes, Not Elsewhere Specified or Included

Photomultiplier tubes, which detect light via photoemission and electron multiplication, are photocathode devices and would typically fall under the light-sensing tubes line (8540890060) within this heading. However, if a specific photomultiplier design is covered by a more specific heading elsewhere in Chapter 85, that heading takes precedence. Importers should review the full chapter structure and consult a licensed customs broker when the application is ambiguous.

Gas and vapor tubes (8540890020) operate by ionizing a gas or vapor fill to conduct current, as in thyratrons or ignitrons, and are used in high-voltage switching and power-control applications. Diode, triode, and tetrode tubes (8540890040) are high-vacuum devices that control electron flow through electrode geometry rather than gas ionization, covering a wide range of amplifier and rectifier applications. The presence or absence of a gas or vapor fill, confirmed by the manufacturer's specification, is the key distinguishing criterion.