What is the 421A vacuum tube?

The 421A is a triode vacuum tube designed for power/output applications. WE-branded 5998. Audiophile grail OTL tube..

What is a 421A substitute or equivalent?

Direct equivalents to the 421A include 5998, 7236. 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 421A?

Amplification factor μ ≈ 6, transconductance gm ≈ 14 mA/V, plate resistance rp ≈ 350 Ω, max plate voltage 275 V, max plate dissipation 15 W, heater 6.3 V / 2.4 A.

Is the 421A still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 421A as Legendary 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 421A sound like?

WE-branded 5998. Audiophile grail OTL tube.. 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.

421A Triode Vacuum Tube

Also known as 5998 · 7236

Power triode delivering 15W for push-pull and SET designs

Dual triodeHeater 6.3 V · 2.4 AOctal2 sectionsWestern Electric 1953

Plate dissipation: 15W
Max plate voltage: 275V
Transconductance: 14mA/V

421A

Characteristics

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

HeaterIndirectly heated

Specifications of 421A — Heater
Heater voltageVh	6.3 V
Heater currentIh	2.4 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 421A — Limiting values
Plate voltageVa max	275 V
Plate dissipationPa max	15 W

Mechanical

Specifications of 421A — Mechanical
Base	Octal
Rarity	Legendary
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 421A — Small signal
Transconductance	14 mA/V
Plate resistance	350 Ω
Amplification factor	6

Capacitances

Specifications of 421A — Capacitances
Grid–cathode	7.7 pF
Grid–plate	18.3 pF
Plate–cathode	2.5 pF

Charactereditorial reading, not a measurement

Specifications of 421A — Character
Gain	6%
Power	38%
Drive	100%
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
Phase splitter

Generic triode power/output stage

Hi-Fi stereoHeadphone amp

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Active model : Norman Koren

Manufacturer datasheets

1 manufacturer datasheets cross-checked for 421A1 verified

421A DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Western Electric
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Norman Koren

Context

The 421A is a dual indirectly-heated triode from Western Electric, introduced around 1953. It provides a gain factor (μ) of 6 with 350Ω plate resistance. Commonly used in power/output applications. WE-branded 5998. Audiophile grail OTL tube.

History

Introduced: 1953
Manufacturer: Western Electric
Era: Golden Age

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