How Vacuum Tubes Work
From heated cathode to amplified signal — animated demonstrations of thermionic emission, the diode, the triode, and beyond.
Thermionic Emission
Heat a metal, free the electrons
When a metal is heated to a sufficiently high temperature, electrons gain enough kinetic energy to escape from the surface. This is thermionic emission — the fundamental principle behind every vacuum tube.
In a vacuum tube, the cathode is heated (either directly by passing current through a filament, or indirectly by a separate heater element), releasing a cloud of electrons into the surrounding vacuum. Without an electric field to attract them, these electrons form a space charge — a hovering cloud near the cathode surface.
Current density depends exponentially on temperature and work function
The Simplest Tube
Place a positive plate near the cathode and electrons flow
Add a positively charged plate (anode) near the cathode, and electrons are attracted across the vacuum. Current flows. Reverse the voltage — plate is negative — and no current flows. The tube conducts in one direction only. This is rectification.
Drag the slider to change the plate voltage. Watch how electron flow responds — more voltage, more acceleration, more current. Go negative and the flow stops completely.
Adding Control
A wire grid between cathode and plate — the birth of amplification
Insert a wire mesh — the control grid — between cathode and plate. A small voltage on this grid modulates the much larger electron flow. A few volts on the grid can control hundreds of volts at the plate. This is amplification.
The grid is close to the cathode, so it has a strong electrostatic influence on the electron stream. Make the grid more negative, and it repels electrons back toward the cathode — fewer pass through. Make it less negative (or positive), and more electrons stream toward the plate.
Notice how a small change in grid voltage (the slider) produces a large change in plate current. The ratio — how many volts at the plate one volt on the grid controls — is the amplification factor μ. A 12AX7 has μ = 100: one volt on the grid controls 100 volts at the plate.
The Three Key Parameters
μ, Gm, and rp — connected by the Barkhausen equation
How many volts at the plate one volt on the grid controls. High μ = high gain per stage. 12AX7: μ=100. 12AU7: μ=17. 300B: μ=3.9.
How effectively the grid controls plate current. High Gm = fast, detailed response. Measured as change in plate current per volt of grid change.
The tube's internal impedance. Low rp = better load control and damping. Triodes have lower rp than pentodes, which is why they're prized for bass quality.
From Diode to Pentode
Each added grid trades simplicity for performance
A screen grid between control grid and plate reduces Miller capacitance (Cgp), enabling higher gain at radio frequencies. The trade-off: secondary emission causes the “tetrode kink” — a region of negative resistance.
A suppressor grid (connected to cathode) repels secondary electrons back to the plate, eliminating the tetrode kink. Maximum gain and power, but with more complex harmonics (3rd, 5th).
Uses beam-forming plates instead of a suppressor grid. Focused electron beams create a virtual suppressor through space-charge effects. The 6L6, KT66, KT88 — legendary power tubes.
Screen grid connected to a tap on the output transformer (40-43%). Combines triode linearity with pentode power. The Dynaco ST-70 topology.
Direct vs Indirect Heating
Two approaches to thermionic emission, each with distinct character
| DHT (Direct) | IHT (Indirect) | |
|---|---|---|
| Cathode | Filament IS the cathode | Separate heater warms cathode sleeve |
| Hum | Requires DC heater or hum pot | Inherently low AC hum |
| Warm-up | Fast (seconds) | Slower (30-60 seconds) |
| Sound character | Intimate, pure, direct | Clean, detailed, versatile |
| Examples | 300B, 2A3, 45, 211 | 12AX7, 6SN7, EL34, 6L6 |
| Typical use | SET output, high-end audio | Preamps, push-pull, guitar |
Fundamental Equations
Test Your Knowledge
Review the key concepts of vacuum tube operation covered in this guide.
What is the physical principle that allows a vacuum tube to function?