HTS code 8540.99.40.00 covers "Thermionic, cold cathode or photocathode tubes (for example, vacuum or vapor or gas filled tubes, mercury arc rectifying tubes, cathode-ray tubes, television camera tubes); parts thereof › Other cathode-ray tubes › Other › Electron guns; radio frequency (RF) interaction structures for microwave tubes of subheadings 8540.71 through 8540.79, inclusive" under the US Harmonized Tariff Schedule. This is the official duty rate charged by US Customs when this product is imported into the United States, sourced directly from the U.S. International Trade Commission and refreshed daily.