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.

By Jean-Michel Falligan Devergne · Reviewed May 29, 2026

←421A

← 417A 45 →Compare → 5998

421A Triode Vacuum Tube

Dual triode6.3V / 2.4AOctalSPICE

SourceWestern ElectricReviewed2026-05-29StatusDataModelCalcModelNorman KorenPDF

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

Datasheet ↗SPICE

421A — Western Electric — triode vacuum tube — (1953)

350Ω

14.0mA/V

7.7pF

18.3pF

2.50pF

GAIN6%

POWER38%

DRIVE100%

LINEARITY95%

SPECIFICATIONS

Heater6.3 V · 2.4 A

Max Plate Voltage275 V

Pa max15 W

Gain Factor6

gm14 mA/V

rp350 Ω

Inter-electrode capacitances

Cgk7.7 pF

Cgp18.3 pF

Cpk2.5 pF

PinoutOctal

Era1953

RarityLegendary

HEATER6.3V / 2.4A

MAX PLATE275V / 15W

BASEoctal

SECTIONS2 (dual)

Source datasheets

1 manufacturer datasheets cross-checked for 421A

1 verified

TYPICAL OPERATING POINT

Va110V

Ia115.46mA

Vg-10V

Pd12.7W

RL600Ω

Av11.9dB

EXECUTIVE SUMMARY

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.

APPLICATIONS

SET outputPush-pull outputHeadphone outputPhase splitter

Generic triode power/output stage

• Hi-Fi stereo• Headphone amp

VS 5998

μ 0%Rp +0%Pd 0%Full comparison →

HISTORY

Introduced1953ManufacturerWestern Electric

Golden Age

421A Datasheet

Local mirror · PDF

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METRIX TESTER

Test this tube on a Metrix

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