Audion Triode

An evacuated envelope implodes if it fails, and plate supplies run at high voltage. Shield the tube, keep the high-voltage side enclosed, and discharge everything before touching it.

It is called "Space Telegraphy". De Forest was working on radio detection, not amplification — the amplifying property was found later, by others.

Put a heated filament and a cold plate in an evacuated envelope. Current flows one way only — this is the Edison effect, and Fleming's 1904 valve.

The hot filament boils off electrons; the cold plate does not. Reverse the plate voltage and nothing crosses the gap. Rectification, but no gain.

Make an open zigzag or spiral of fine wire. Open, because electrons must pass THROUGH it — a solid electrode here would simply block the stream.

Spacing sets the gain. The grid is near the cathode so its voltage dominates the field where electrons are slowest and most easily steered.

Fuse three wires through the envelope without cracking it. This is the hardest fabrication step and the usual cause of failure.

Pump down and seal off under vacuum. De Forest's early Audions were deliberately left slightly gassy, and the residual gas made them unstable and unpredictable.

Bring the cathode up to emission and put the plate positive. Measure the plate current with the grid connected to the cathode — that is your quiescent point.

Apply a small negative grid bias. Plate current drops sharply. A fraction of a volt on the grid moves milliamps at the plate — that ratio is the gain.

Measure grid current with the grid negative: essentially zero. The controlling electrode consumes no power, which is what makes the device an amplifier rather than a divider.

Sweep the grid and record plate current. The slope of that curve is transconductance, and its steepness is the whole figure of merit.

Choose an operating point mid-curve. Sit too near either end and the output clips — the same linearity problem as every amplifier since.

Apply a small AC signal to the grid, load the plate through a resistor, and compare output swing to input. Gain greater than one is the moment electronics begins.

The patent. US 879,532, "Space Telegraphy", granted 18 February 1908 to Lee De Forest, covering the three-electrode Audion he had devised in 1906-07. The lineage is direct: Edison noticed current flowing from a lamp filament to a stray electrode in 1883 — the Edison effect, which he patented and did not use. John Ambrose Fleming turned it into a two-electrode rectifying valve in 1904. De Forest added a third electrode between them. Each step is small; the third one changed what electricity could be made to do.

He genuinely did not understand it. De Forest believed the residual gas in the envelope was essential and deliberately left his tubes soft, which made them noisy, drifty and short-lived. He is on record as not knowing why the device worked. Irving Langmuir at General Electric established that the effect is pure thermionic emission and that a HARD vacuum makes it far better — and the reliable, high-vacuum tube that followed is what actually carried radio and long-distance telephony. Crediting De Forest with the working triode overstates it; crediting him with the third electrode is exactly right.

Why a control electrode that draws no power is the whole point. With the grid held negative it repels electrons without collecting them, so it consumes essentially no current while strongly modulating the space charge near the cathode. Input power approaches zero, output power does not — the device delivers gain, taking its energy from the plate supply. Every amplifying device since works on the same bargain, and the field-effect transistor's insulated gate is the same idea in silicon.

What it made possible. Transcontinental telephony in 1915 depended on tube repeaters; so did broadcast radio, radar, television, and the first electronic computers — ENIAC ran on about 18,000 tubes. The transistor of 1947 replaced the tube because it is smaller, cooler and more reliable, but it does the identical job: a low-power control terminal governing a larger current. The Audion is where that idea starts, and it started with a patent about telegraphy written by someone with the wrong explanation.