Ms. Kristen Lane
Trade Compliance Manager
Richardson Electronics, Ltd.
40W267 Keslinger Rd.
LaFox, IL 60147

RE: The tariff classification of receiver tubes (triode, tetrode, and pentode) from an unspecified country

Dear Ms. Lane:

In your letter dated July 24, 2009 you requested a tariff classification ruling.

The merchandise subject to this ruling is a triode, tetrode, and pentode. The triode, tetrode, and pentode are types of receiver tubes. The primary use of these receiver tubes is for signal amplification, such as an audio amplifier or in two-way mobile communications equipment.

The triode is identified as 12AX7A twin triode. The 12AX7A twin triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements; the filament or cathode, the grid and the plate or anode. The principle of its operation is that, as with a thermionic diode, the heated cathode (either directly or indirectly by means of a filament) causes a space charge of electrons that may be attracted to the positively charged plate (anode) and create a current. Applying a negative charge to the control grid will tend to repel some of the (also negatively charged) electrons back towards the cathode: the larger the charge on the grid, the smaller the current to the plate. If an AC signal is superimposed on the DC bias of the grid, an amplified version of the AC signal appears in the plate circuit. Although triodes are now largely obsolete in consumer electronics, having been replaced by the transistor, triodes continue to be used in certain high-end and professional audio applications, as well as in microphone preamplifiers and electric guitar amplifiers. The tetrode is identified as QQE03-12 tetrode. The QQE03-12 tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode (a filament or cathode, a grid and a plate or anode) and an additional screen grid, which significantly changes its behavior. The grid, which is nearest to the cathode, is the “control grid”; the voltage applied to it causes the anode current to vary. In normal operation, with a resistive load, this varying current will result in varying (AC) voltage measured at the anode. With proper biasing, this voltage will be an amplified, but inverted, version of the AC voltage applied to the control grid, thus the tetrode can provide voltage gain. The screen grid, sometimes called a “shield grid”, provides a screening effect, isolating the control grid from the anode. This helps to suppress unwanted oscillation, and to reduce an undesirable effect in triodes called the “Miller effect”, where the gain of the tube causes a feedback effect increasing the apparent capacitance of the tube’s grid, limiting the tube’s high-frequency gain. In normal operation, the screen grid is connected to a positive voltage, and bypassed to the cathode with a capacitor. This shields the grid from the anode, reducing Miller capacitance between those two electrodes to a very low level and improving the tube’s gain at high frequencies. Power tetrodes are commonly used in radio transmitting equipment, because the need for neutralization is less than with triodes. A tetrode can be converted to act as a triode by connecting the screen grid to the anode.

The pentode is identified as 6L6GC pentode. The 6L6GC pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube. Under certain operating conditions, a tetrode exhibits negative resistance due to secondary emission of electrons from the anode to the screen grid, which is called a “tetrode kink.” The undesirable nature of the “tetrode kink” led tube designers to add a third grid, called the suppressor grid, resulting in a vacuum tube called a pentode. As such, the 6L6GC pentode has the three electrodes of a triode and a tetrode (a filament or cathode, a grid and a plate or anode), an additional screen grid, like the tetrode, and a third grid called a suppressor grid. The suppressor grid is held at a low potential, usually grounded or connected to the cathode. The secondary emission (“tetrode kink”) still occurs, but the electrons can no longer reach the screen grid, since they have less energy than the primary electrons and, hence, cannot pass the grounded suppressor grid. Therefore, these secondary electrons are re-collected by the anode. Pentode valves were first used in consumer-type radio receivers. These types of tubes (vacuum) were replaced by transistors, however they continue to be used in certain applications, including high-power radio transmitter and in high-end and professional audio applications, microphone preamplifiers and electric guitar amplifiers.

The applicable subheading for the triode (12AX7A twin), tetrode (QQE03-12), pentode (6L6GC) receiver tubes will be 8540.81.0000, Harmonized Tariff Schedule of the United States (HTSUS), which provides for “Other tubes: Receiver or amplifier tubes.” The rate of duty will be 4.2 percent ad valorem.

Duty rates are provided for your convenience and are subject to change. The text of the most recent HTSUS and the accompanying duty rates are provided on World Wide Web at

This ruling is being issued under the provisions of Part 177 of the Customs Regulations (19 C.F.R. 177).

A copy of the ruling or the control number indicated above should be provided with the entry documents filed at the time this merchandise is imported. If you have any questions regarding the ruling, contact National Import Specialist Linda M. Hackett at (646) 733-3015.


Robert B. Swierupski
National Commodity Specialist Division