Tutorials Audio

Lee de Forest: The Grid That Made Amplification Possible

By Infinite Creation · Profile

At a Glance

Lee de Forest (1873-1961) added a third electrode. A control grid, between the filament and plate of Fleming's valve in 1906, producing the Audion. A small voltage on the grid controls a much larger current through the tube, which is amplification: the single capability broadcasting could not exist without. He spent much of his life in patent litigation, notably with Edwin Armstrong.

Lived: 1873-1961, United States
Anchor year: 1906, the Audion
Strand: canon, and litigious

Career

The problem as it stood

A detector recovers a signal but cannot make it stronger. Every stage of a radio system, reception, transmission, recording, public address, was limited by having no way to increase a signal's power without distorting it beyond use.

What he actually did

De Forest placed a wire grid between the hot filament and the plate. Varying the grid voltage modulates the electron stream, so a small input controls a large output. He did not initially understand why it worked, and early Audions were poorly evacuated and erratic.

How it worked

The grid sits in the path of the electrons and its charge either helps or hinders them. Because it draws almost no current itself, a tiny input power controls a much larger output power, that ratio is gain. Chain several stages and arbitrary amplification becomes available, which is the foundation of all electronics until the transistor.

What it made possible

Broadcasting, long-distance telephony, sound recording and reproduction at usable levels, sound film, radar, and the first computers. Almost nothing in the rest of this corpus is possible without amplification.

What happened to him

He was involved in continual litigation and several failed businesses, and also developed the Phonofilm sound-on-film system, itself the subject of a credit dispute with Theodore Case, who supplied essential components.

Where the credit landed

De Forest and Armstrong litigated over regeneration for decades. In 1934 the US Supreme Court in RCA v. Radio Engineering Laboratories found for de Forest, but it did so by applying a burden of proof, and the case remains the leading citation for the clear-and-convincing standard for patent invalidity. It did not find that de Forest invented regeneration. The engineering community largely sided with Armstrong. Court rulings are legal outcomes, never historical findings.

Known For

The grid controls a large current with almost no input power, that ratio is gain.
The 1934 ruling turned on burden of proof, not on who invented regeneration.
De Forest did not initially understand why the Audion worked.

Knowledge Base

What You'll Learn

Amplification is the capability that separates the whole modern chain from what came before.

Why the 1934 ruling is misread

RCA v. Radio Engineering Laboratories (1934), written by Cardozo, upheld de Forest's patents by holding that a challenger must prove invalidity by clear and convincing evidence: a procedural standard still cited today. It is a ruling about how much proof is required, not a finding about who first conceived regeneration. This is the mirror image of the Marconi misreading: there a ruling is taken to have awarded priority it declined to address, here a ruling on burden of proof is taken as a verdict on invention.

Where This Fits

The Audion made amplification possible at every link of the chain traced in A History of Broadcast Technology.

FAQ

Q: What did the Audion do that Fleming's valve could not?
A: Amplify. Fleming's diode rectifies. Adding a control grid means a small input voltage controls a much larger output current, so the signal can be made stronger.

Q: Did the courts decide de Forest invented regeneration?
A: No. The 1934 Supreme Court ruling upheld his patents by applying a clear-and-convincing standard of proof for invalidity. It was a decision about burden of proof, not a historical finding, and most engineers credited Armstrong.

Sources

The Audion patent was checked against the record. Checked 20 August 2026.

US Patent 879,532, Space Telegraphy. De Forest's patent for the three-electrode valve, filed 29 January 1907 and granted 18 February 1908. Adding a control grid to a diode is what turned a detector into an amplifier.