What is the KT66 vacuum tube?

The KT66 is a tetrode vacuum tube designed for power (pp/se) applications. Warm British beam tetrode..

What are the key specifications of the KT66?

Transconductance gm ≈ 7 mA/V, plate resistance rp ≈ 22.5 kΩ, max plate voltage 550 V, max plate dissipation 30 W, heater 6.3 V / 1.3 A.

Is the KT66 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the KT66 as Sought-after 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 KT66 sound like?

Warm British beam tetrode.. 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.

KT66 datasheet — pinout & specs

Characteristics

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

HeaterIndirectly heated

Specifications of KT66 — Heater
Heater voltageVh	6.3 V
Heater currentIh	1.3 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of KT66 — Limiting values
Plate voltageVa max	550 V
Plate dissipationPa max	30 W

Mechanical

Specifications of KT66 — Mechanical
Base	Octal
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of KT66 — Small signal
Transconductance	7 mA/V
Plate resistance	22.5 kΩ

Capacitances

Specifications of KT66 — Capacitances
Input	14.5 pF
Grid–plate	1 pF
Output	10.5 pF

Charactereditorial reading, not a measurement

Specifications of KT66 — Character
Gain	0%
Power	75%
Drive	78%
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	B+	Va	Vg2	Vg1 / Rk	Ia	Ig2	Load	Pout	THD	Source
Class A, single-endedpentode connection		250 V	250 V	-15 V	85 mA	6.3 mA	2.2 kΩ	7.25 W	9%	OCR, to be checked Marconi's Wireless Telegraph Company Limited · 1956 · p. 2 · transcript
Class A, single-endedtriode connection · cathode bias	270 V	250 V		-20 V	60 mA		2.8 kΩ	2.2 W	6%	OCR, to be checked GEC / The M-O Valve Company Limited · 1977 · p. 2 · transcript
Class AB1, push-pullpentode connection · cathode bias · values for two tubes		250 V	250 V	200 Ω	165 mA	20 mA	4 kΩ a-a	17 W	4%	OCR, to be checked Marconi's Wireless Telegraph Company Limited · 1956 · p. 2 · transcript
Class AB1, push-pullultra-linear · cathode bias	450 V			-35 V	62.5 mA		7 kΩ a-a	32 W	2%	OCR, to be checked GEC / The M-O Valve Company Limited · 1977 · p. 3 · transcript

Using this tube

Applications

High-power output

Marshall JTM45Leak Stereo 20Quad II

Guitar ampPA/public address

Featured in these amplifiers

Classic circuits

Williamson
Hi-Fi Classic15W
D.T.N. Williamson's landmark 1947 design. Triode-connected push-pull with global NFB. Set the standard for high-fidelity amplification.
PPtriodeVb 450V · Vg -35V · Vg₂ 450V · RL 10kΩWireless World, April 1947
Leak TL12+
Hi-Fi Classic12W
Harold Leak's improved Point One amplifier. Triode-connected KT66 with very low distortion. Refined, transparent British sound.
PPtriodeVb 420V · Vg -33V · Vg₂ 420V · RL 8kΩLeak TL12+ Schematic
Quad II
Hi-Fi Classic15W
Peter Walker's iconic 1953 power amplifier. Cathode-coupled phase splitter with KT66 output. Legendary musical presentation.
PPVb 420V · Vg -22V · Vg₂ 420V · RL 8kΩQuad II Service Manual
Marshall JTM45
Guitar Amp30W
Jim Marshall's first amplifier (1962). Based on the Bassman circuit with KT66 output tubes. Raw, dynamic British crunch. The birth of rock tone.
PPVb 480V · Vg -24V · Vg₂ 480V · RL 8kΩMarshall JTM45 Schematic

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : GEC·Marconi datasheetActive model : Koren v1

Manufacturer datasheets

3 manufacturer datasheets cross-checked for KT663 verified

Triode-connectedMarconi's Wireless Telegraph CJUNE 1956

Va: 400 V

KT66 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: GEC·Marconi datasheet
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Koren v1

Context

The KT66 is the predecessor to the KT88, also developed by GEC/MOV, with a lower plate dissipation (~25W) and a warmer, more romantic sonic character. It was the output tube of the original Marshall JTM45 and the Williamson amplifier, both landmark designs in their respective domains. The KT66 has a sound often described as more triode-like than the KT88, with a lush midrange and gentle top end that flatters acoustic music and vocals. Original GEC KT66s with the clear-glass bottle are extremely rare and expensive; Gold Lion reissues are the most common current production option.

History

Introduced: 1937
Manufacturer: GEC·Marconi
Era: Pre-War

METRIX TESTER

KT66 Tetrode Vacuum Tube