Diode Construction
Part of Vacuum Tubes
The vacuum diode is the simplest tube structure — a heated cathode and a plate sealed in a glass envelope — and its construction illustrates all the fundamental challenges of tube manufacturing.
Why This Matters
Building vacuum tubes from scratch is a demanding but achievable goal for a community with metalworking and glassworking capability. The simplest tube to build is the diode — only two electrodes, no fine grid wires to wind, and relatively tolerant of imperfect vacuum and electrode geometry. Starting with diode construction builds the skills and processes needed for the more complex triodes and pentodes used in amplifiers.
Even if you intend to rely entirely on salvaged tubes for amplification, understanding diode construction illuminates how all tubes work at a physical level. The same electrode assembly techniques, vacuum processing procedures, and glass-sealing methods apply to every tube type. A builder who has made a working diode has mastered the hard parts of tube manufacturing.
Vacuum diodes also have direct applications in power supplies (rectifiers) and detectors (AM radio signal detection). Building diode rectifiers allows construction of the high-voltage DC supplies that power all tube equipment.
Electrode Design and Materials
The vacuum diode contains exactly two electrodes: the cathode (electron source) and the anode or plate (electron collector). The geometry must allow electron flow from cathode to plate with minimal obstruction and maximum efficiency.
For a small-signal diode rectifier, start with a directly-heated cathode made from 0.1mm tungsten wire formed into a hairpin or a small coil. The cathode filament should be 5-15mm in length to provide adequate emission area. Tungsten wire salvaged from broken incandescent light bulbs provides a convenient source — a standard 60W bulb contains approximately 580mm of 0.046mm tungsten wire; larger bulbs have thicker wire. Salvage bulbs carefully to extract the filament intact.
For higher emission capability, coat the tungsten wire with a mixture of barium carbonate, strontium carbonate, and calcium carbonate before assembly. Prepare the coating mixture by dissolving the carbonates in an organic binder (collodion or diluted acetone-soluble lacquer) and painting or dipping the filament. The coated filament is processed at high temperature under vacuum to convert carbonates to active oxides.
The plate should enclose the cathode on all sides to collect as many emitted electrons as possible. A cylindrical plate geometry with the cathode centered inside gives good collection efficiency. Form the plate from thin nickel sheet (0.1-0.3mm) by rolling into a cylinder approximately 10-20mm in diameter and 15-25mm tall. Nickel is preferred because it withstands high temperatures, accepts electron bombardment without sputtering, and can be outgassed cleanly in vacuum.
For the plate seal-in leads, use metal with a thermal expansion coefficient close to the glass being used. Nickel wire works well with soft lead glass. Kovar (iron-nickel-cobalt alloy) is the preferred metal for glass seals in commercial tubes because its expansion matches borosilicate glass precisely, but nickel is available from salvage and provides workable seals with soda-lime glass if the seal geometry is correct.
Glass Envelope and Sealing
The glass envelope provides the vacuum enclosure and serves as the mechanical support for the electrode assembly. For a simple diode, a glass cylinder 25-50mm in diameter and 50-100mm long is sufficient. Pharmaceutical vials, laboratory test tubes, or lamp bulb glass provide suitable material.
The leads from the electrodes must pass through the glass wall in gas-tight seals. Two methods are practical for small-scale construction.
Pinch seals use a flat, thin glass section heated and squeezed around the wire leads. Place the wire leads on a flat glass substrate, bring the seal glass up over the wires, heat to softening with a torch, and press with pliers or a heated steel form. The glass flows around the wires and creates a seal when cooled. Pinch seals work well with soft (soda-lime) glass and nickel or copper wire.
Butt seals press a glass tube wall against a pre-sealed glass bead around each wire. Pre-melt small glass beads around each lead wire, then heat the tube end and bring the beaded wire in contact with the softened glass. Used in commercial tubes for precision seal geometry. Requires more skill but produces stronger seals.
After sealing the electrode assembly into one end of the tube envelope, attach a short glass tube (exhaust tube) to the other end for connection to the vacuum pump. Seal the other end of the main envelope, leaving only the exhaust tube open.
Vacuum Processing
The tube must be evacuated to a pressure below approximately 0.001 Pa (10^-5 mbar) to eliminate gas molecule collisions that would prevent electron flow and cause premature failure. Achieving this vacuum requires a multi-stage pumping process.
Stage 1: Roughing pump. A mechanical vacuum pump (rotary vane pump, piston pump, or water aspirator) reduces pressure from atmospheric (100,000 Pa) to approximately 10 Pa. A simple water aspirator connected to running water achieves about 3000 Pa, insufficient on its own but useful as a roughing stage. A hand-operated piston pump with check valves reaches similar pressures with more effort.
Stage 2: Diffusion pump or sorption pump. Below 10 Pa, mechanical pumps become inefficient. An oil diffusion pump uses heated oil vapor jets to capture gas molecules and carry them to the backing pump. Diffusion pumps can reach 10^-4 Pa or below. Construction requires a metal or glass chamber, an electric heater, and high-vacuum oil (a clean, low-vapor-pressure oil such as refined castor oil or commercial vacuum pump oil).
Stage 3: Bakeout. While pumping, heat the entire glass envelope and electrode assembly to 300-400°C with a tube furnace or electrical heating tape. Baking drives out gas absorbed in the glass walls and metal surfaces — gas that would be released slowly over months of operation if not removed now. Continue baking and pumping for several hours until the pressure stops falling.
Stage 4: Outgassing electrodes. While still hot and under vacuum, pass current through the cathode filament to heat it electrically. The high temperatures reached (1000-2000°C depending on cathode type) drive off absorbed gases from the electrode surfaces. Monitor with a vacuum gauge or discharge tube indicator.
Stage 5: Getter firing. A small piece of getter material (barium metal in a small metal cup) is positioned inside the tube before sealing. After pumping and bakeout, heat the getter by induction (using a radio-frequency coil outside the glass) or by electron bombardment (briefly running the tube at high voltage). The barium evaporates and deposits on the glass walls, where it absorbs any remaining gas molecules. The silvery metallic deposit on the inside of a finished tube is the fired getter.
Stage 6: Tip-off. When the vacuum is adequate (confirmed by measuring the discharge characteristics at low voltage, or by the appearance of a clean getter flash), seal the exhaust tube while the pump is still connected by melting it with a torch until the glass pinches closed and separates.
Testing and Verification
A finished diode should conduct current readily when the plate is positive relative to the cathode and conduct negligible current when the plate is negative (reverse biased). Test by connecting a DC supply through a current-limiting resistor to the tube. With a heated cathode and 50-100V positive plate voltage, current should flow proportional to the applied voltage up to the emission limit.
Poor vacuum manifests as blue or purple glow inside the tube during operation (residual gas ionization), soft vacuum (the tube conducts in reverse direction due to ion currents), or the diode acting as if it has a high series resistance (gas collisions impede electron flow). A properly evacuated tube shows no visible glow in darkness.
Test emission adequacy by measuring the saturation current — the maximum current the cathode can supply regardless of how high the plate voltage is raised. For an oxide-coated cathode diode with a 10mm × 5mm active area, saturation current should reach at least 10-50mA. Insufficient emission indicates the cathode was not properly activated or was contaminated during processing.