Gettering

Part of Vacuum Tubes

Gettering is the process of removing residual gas from a sealed vacuum tube by chemically reacting it with a highly reactive metal deposited inside the envelope.

Why This Matters

No mechanical vacuum pump can remove every last gas molecule from a tube. After the exhaust tube is sealed and the pump disconnected, a small quantity of gas remains — both in the free space of the envelope and adsorbed (stuck) to the glass and metal surfaces. As the tube heats up during operation, this adsorbed gas desorbs and enters the vacuum, gradually degrading the vacuum quality and shortening tube life.

The getter is the solution. A small quantity of reactive metal — most commonly barium — is enclosed in a small cup or holder inside the tube before sealing. After the tube is pumped down and sealed, the getter is heated by external induction heating, vaporizing the metal and depositing it on the glass walls. The barium metal reacts with any residual gas (oxygen, nitrogen, water vapor, carbon dioxide) that enters the vacuum. The silvery metallic mirror on the inside of a finished tube is the fired getter deposit.

Building or repairing vacuum tubes requires understanding getters: what materials work, how to fire them, what a properly fired getter looks like, and how to diagnose getter failure. In salvage equipment, a tube with a white or brownish (oxidized) getter deposit instead of a shiny silver mirror indicates air infiltration and confirms the tube is failed.

Getter Materials

Barium is the dominant getter material because it reacts vigorously with all the common gases found in tubes and has a low enough vapor pressure at elevated temperatures to be deposited by evaporation.

The reactions that make barium effective:

• 2Ba + O₂ → 2BaO (reaction with oxygen)
• Ba + H₂O → BaO + H₂ (reaction with water vapor)
• 3Ba + N₂ → Ba₃N₂ (reaction with nitrogen, slower)
• Ba + CO₂ → BaO + CO (reaction with carbon dioxide)

Barium metal is not stored or handled in free metallic form — it oxidizes instantly on contact with air. Instead, it is stored as barium azide (Ba(N₃)₂) or as a mixture with nickel powder in a sealed cup. During getter firing, the barium is released from this compound and evaporated. Commercial getters use barium mixed with aluminum and nickel in a ring or pill form.

Calcium can substitute for barium in less demanding applications. Calcium reacts more slowly and has lower gettering capacity, but is easier to handle and can be obtained from naturally occurring calcium metal (produced by electrolytic reduction of calcium chloride — a practical if demanding process). Calcium is a useful getter for tubes with modest vacuum requirements.

Titanium and zirconium are non-evaporable getters — they absorb gases without evaporating. Pieces of titanium or zirconium inside the tube absorb residual gas without being fired. They must be activated at high temperature (500-900°C) during bakeout to clean their surfaces and maximize absorption capacity. Non-evaporable getters don’t produce the characteristic mirror deposit and don’t degrade when exposed to air in the same obvious way as evaporated barium.

Getter Holder Design

The getter must be positioned and fired in a way that deposits the evaporated metal on the glass walls rather than on the electrodes. Metal deposited on the electrodes could short them together or interfere with electron emission.

A simple getter holder is a small cylinder (5-8mm diameter, 8-10mm long) of nickel sheet with the getter material inside and the open end facing toward the glass wall. Position the holder so the evaporated barium travels toward the top dome of the glass envelope where it forms a mirror without reaching the electrodes.

The getter holder must be supported from a dedicated lead wire (the getter tab) that passes through the glass seal. This wire is used only to position the getter — it carries no electrical current during operation. In commercial tubes, the getter is typically supported on a spring-loaded tab that positions it precisely after glass sealing.

Some designs use a glass cup attached to the inner wall of the envelope, positioned above the electrode assembly. The getter material is placed in the cup before sealing, and the entire assembly is fired by external induction after pumping. This approach avoids needing a dedicated getter lead through the glass.

Firing Procedure

The getter must be fired after the tube has been pumped down and baked out but before the exhaust tube is sealed. If fired before pumping, the barium deposits on the glass walls and reacts with atmospheric air, becoming useless.

The standard method is induction heating. A radio-frequency coil surrounds the getter cup from outside the glass envelope. When RF current flows through the coil, it induces eddy currents in the nickel getter cup, heating it rapidly. The nickel cup reaches 900-1000°C in seconds, firing the getter compound and evaporating barium onto the glass walls.

For a community tube-building workshop without RF induction equipment, alternative firing methods include:

Electron bombardment: connect the getter tab to a high voltage (500-1000V) positive supply while the cathode is cold. Electrons emitted by field emission from sharp edges of the getter cup heat it through bombardment. This method is slow and imprecise but requires only a DC power supply.

Resistive heating: if the getter tab is made of high-resistance material (nichrome wire), passing sufficient current through it heats the getter by resistive heating. This approach requires electrical isolation of the getter circuit from the main tube electrodes.

Optical focusing: using a lens or mirror to focus sunlight onto the getter cup through the glass. With sufficient concentration (equivalent to a 100x lens), glass doesn’t significantly absorb near-infrared and visible wavelengths, and the nickel cup can be heated to firing temperature. Impractical for production but interesting as an emergency method.

Diagnosing Getter Condition

A properly fired and active getter appears as a shiny, silver-gray metallic mirror on the glass inside the tube, typically concentrated near the top dome. This mirror is the deposited barium metal, and it remains active as long as it retains this bright metallic appearance.

A white or cream-colored deposit where the silver mirror should be indicates that barium has reacted with oxygen (usually water vapor — BaO + CO₂ → BaCO₃ is also possible). The tube has been exposed to air, either through a crack in the glass, a failed seal, or during manufacture. This tube will have poor vacuum and likely performs poorly or fails immediately.

A getter mirror that is only slightly tarnished around the edges but still mostly silver indicates mild atmospheric contamination — perhaps a microleak that has been present for years. The tube may still function but will have shortened remaining life.

No mirror at all indicates either that the getter was never fired (a manufacturing defect), or that the mirror evaporated — both possibilities suggest the tube is suspect. Check the tube’s performance against specifications.

For salvage equipment being restored, use getter condition as a primary screening criterion before testing tubes electrically. Only tubes with intact, bright getter mirrors are worth testing. Tubes with oxidized getters go directly to the parts bin for electrode material recovery.