What is the KT63 vacuum tube?

The KT63 is a tetrode vacuum tube designed for power (se) applications. British vintage pentode. Warm midrange..

What is a KT63 substitute or equivalent?

Direct equivalents to the KT63 include 6F6. 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 KT63?

Transconductance gm ≈ 2.5 mA/V, plate resistance rp ≈ 80.0 kΩ, max plate voltage 250 V, max plate dissipation 11 W, heater 6.3 V / 0.7 A.

Is the KT63 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the KT63 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 KT63 sound like?

British vintage pentode. Warm midrange.. 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.

KT63 Tetrode Vacuum Tube

Also known as 6F6

Beam power tetrode rated at 11W for high-power output stages

TetrodeHeater 6.3 V · 0.7 AOctalGEC·MOV 1940

Plate dissipation: 11W
Max plate voltage: 250V
Transconductance: 2.5mA/V

KT63

Characteristics

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

HeaterIndirectly heated

Specifications of KT63 — Heater
Heater voltageVh	6.3 V
Heater currentIh	0.7 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of KT63 — Limiting values
Plate voltageVa max	250 V
Plate dissipationPa max	11 W

Mechanical

Specifications of KT63 — Mechanical
Base	Octal
Rarity	Moderate
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of KT63 — Small signal
Transconductance	2.5 mA/V
Plate resistance	80 kΩ

Capacitances

Specifications of KT63 — Capacitances
Input	9.8 pF
Grid–plate	0.8 pF
Output	9 pF

Charactereditorial reading, not a measurement

Specifications of KT63 — Character
Gain	0%
Power	28%
Drive	20%
Linearity	75%

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

Typical operation

Conditions published by the manufacturers, one row per condition. Nothing is computed.

Typical operation
Setup	Va	Vg2	Vg1 / Rk	Ia	Ig2	Load	Pout	Source
Class A, single-endedpentode connection · cathode bias	250 V	250 V	420 Ω	34 mA	5.5 mA	7 kΩ	3 W	OCR, to be checked Marconi · non datée (catalogue Marconi, prix en £/shillings "10/6") · p. 1 · transcript

Using this tube

Interactive loadline calculator not available for this tube yet — it needs calibrated Koren or Ayumi parameters.

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026

Manufacturer datasheets

1 manufacturer datasheets cross-checked for KT631 verified

KT63 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: GEC·MOV
Reviewed: 2026-05-29
Status: DataModelCalc
Model: under review

Context

The KT63 is a indirectly-heated beam tetrode from GEC·MOV, introduced around 1940. Commonly used in power (se) applications. British vintage pentode. Warm midrange.

History

Introduced: 1940
Manufacturer: GEC·MOV
Era: Pre-War

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