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How a vacuum tube wears out

Typically, preamp tubes last a very long time. A 12AX7 will typically last years in an amplifier before it wears out.

To understand why a 12AX7 can last a very long time, let's see how vacuum tubes wear out.

Vacuum Tube Cathodes

Vacuum tubes lose performance when their cathodes wear out or start to lose the vacuum.

The Vacuum tube cathode is a piece of metal coated with metal oxides. When metal oxides are heated, they emit electrons. The electrons shoot up to the plate, causing the vacuum tube to conduct.

The metal oxide is not permanent, and it slowly wears out with continuous use. Metal oxide wears quicker when it emits higher amounts of electrons under higher current conditions.

Preamp tubes do not require high currents. Most 12AX7 amplifier circuit, for example, uses less than 1mA. 12AX7 often last years for this reason.

Getter and Outgassing

Another reason tubes wear out is the out-gassing of internal metal pieces. The alloy used inside vacuum tubes contains absorbed gas. When the alloy is heated, it emits a minute amount of the absorbed gas. This gas will reduce the vacuum and degrade the cathode metal oxide material.

Vacuum tubes have an internal coating to eliminate this gas. The coating is on the internal surface of the glass and has a shiny metallic look. This coating is called the getter.

The getter material is sacrificial and disappears as it reacts with gas. Once the getter is gone, the gas will accumulate inside the tube, degrading the cathode and reducing the conductance.

How long it takes to get to this terminal state depends on how hard the tube is used. If a vacuum tube puts out a lot of power and runs hot for a long time, it will reach this state sooner.

Power tubes don't last very long for this reason. Power tubes, by default, puts out a lot of power and dissipate a lot of heat. If you use your amp regularly, you may need to change the power tubes in months.

Reverb Driver Tubes

While most preamp circuit requires less power from preamp tubes, some applications require more power. Specifically, a reverb driver is an application that requires lots of juice from the tube, and a reverb driver tube will wear out fairly quickly. A worn-out reverb driver tube can cause noise and other issues with the reverb.

In most amps, the power tube driver (also called phase inverter or PI) is not under that much stress, and you don't have to replace the driver tube when replacing the power tubes.