What is the 866A vacuum tube?

The 866A is a rectifier vacuum tube designed for rectifier applications. Heavy duty mercury vapor. Transmitter rectifier..

What is a 866A substitute or equivalent?

Direct equivalents to the 866A include 866. 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.

Is the 866A still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 866A as Moderate 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 866A sound like?

Heavy duty mercury vapor. Transmitter rectifier.. 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

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866A Rectifier Vacuum Tube

Rectifier2.5V / 5AUX-4

Sourcecap audit 2026-05-09Reviewed2026-05-29StatusDataModelCalcModelunder reviewPDF

Full-wave rectifier for vacuum tube power supply designs

Datasheet ↗

866A — RCA·GE — rectifier vacuum tube — (1934)

GAIN0%

POWER0%

DRIVE50%

LINEARITY75%

SPECIFICATIONS

Heater2.5 V · 5 A

Max Plate Voltage2500 V

Pa max—

Gain Factor—

gm—

rp—

PinoutUX-4

Era1934

RarityModerate

HEATER2.5V / 5A

MAX PLATE2500V

BASEux4

Source datasheets

6 manufacturer datasheets cross-checked for 866A

6 verified

EXECUTIVE SUMMARY

The 866A is a directly-heated rectifier from RCA·GE, introduced around 1934. Commonly used in rectifier applications. Heavy duty mercury vapor. Transmitter rectifier.

APPLICATIONS

Power supply rectification

Generic rectifier rectifier stage

HISTORY

Introduced1934ManufacturerRCA·GE

Pre-War

866A Datasheet

Local mirror · PDF

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METRIX TESTER

Test this tube on a Metrix

SPICE model not yet available. Loadline calculator requires calibrated Koren parameters.

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