What is the 2A3-40 JJ vacuum tube?

The 2A3-40 JJ is a triode vacuum tube designed for power (set) applications. Enhanced 2A3..

What is a 2A3-40 JJ substitute or equivalent?

Direct equivalents to the 2A3-40 JJ include 2A3. These share the same pinout and operating envelope, so they can usually be swapped without circuit changes — though batch-to-batch variations between manufacturers (NOS vs current production) may shift the sonic character noticeably.

What are the key specifications of the 2A3-40 JJ?

Amplification factor μ ≈ 4, transconductance gm ≈ 5.25 mA/V, plate resistance rp ≈ 800 Ω, max plate voltage 300 V, max plate dissipation 15 W, heater 2.5 V / 2.5 A.

Is the 2A3-40 JJ still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 2A3-40 JJ as Common availability. NOS (new-old-stock) examples from RCA, Mullard, Tung-Sol, GE, Sylvania and Telefunken trade among collectors; modern production where it exists comes mostly from JJ Electronic, Electro-Harmonix, Tung-Sol (New Sensor), Genalex, and a handful of small-batch makers.

What does the 2A3-40 JJ sound like?

Enhanced 2A3.. Sonic character also depends heavily on the circuit operating point, the surrounding tubes in the stage, and the output transformer when present — use the interactive loadline calculator on this page to explore the trade-offs.

2A3-40 JJ Triode Vacuum Tube

Also known as 2A3

High-power directly-heated triode for single-ended amplification

TriodeHeater 2.5 V · 2.5 AUX-4JJ Electronic 2005

Plate dissipation: 15W
Max plate voltage: 300V
Transconductance: 5.25mA/V

2A3-40 JJ

Characteristics

Datasheet values, once each. Every value cites its sheet.

HeaterDirectly heated

Specifications of 2A3-40 JJ — Heater
Heater voltageVh	2.5 V
Heater currentIh	2.5 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 2A3-40 JJ — Limiting values
Plate voltageVa max	300 V
Plate dissipationPa max	15 W

Mechanical

Specifications of 2A3-40 JJ — Mechanical
Base	UX-4
Rarity	Common
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 2A3-40 JJ — Small signal
Transconductance	5.25 mA/V
Plate resistance	800 Ω
Amplification factor	4

Capacitances

Specifications of 2A3-40 JJ — Capacitances
Grid–cathode	7.5 pF
Grid–plate	16.5 pF
Plate–cathode	5.5 pF

Charactereditorial reading, not a measurement

Specifications of 2A3-40 JJ — Character
Gain	4%
Power	38%
Drive	99%
Linearity	95%

datasheet cited · model or editorial reading · catalogue value, no citation attached yet · nothing invented

Using this tube

Applications

SET output
Push-pull output
Headphone output

Generic triode power (set) stage

Hi-Fi stereoHeadphone amp

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : JJ Electronic 2A3-40 specActive model : Norman Koren

Manufacturer datasheets

2A3-40 JJ DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: JJ Electronic 2A3-40 spec
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Norman Koren

Context

The 2A3-40 JJ is a directly-heated triode from JJ Electronic, introduced around 2005. It provides a gain factor (μ) of 4 with 800Ω plate resistance. Commonly used in power (set) applications. Enhanced 2A3.

History

Introduced: 2005
Manufacturer: JJ Electronic
Era: Modern

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