What is the KT77 vacuum tube?

The KT77 is a tetrode vacuum tube designed for power (pp) applications. GEC EL34 replacement..

What is a KT77 substitute or equivalent?

Direct equivalents to the KT77 include EL34. 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 KT77?

Amplification factor μ ≈ 12, transconductance gm ≈ 10.5 mA/V, plate resistance rp ≈ 23.0 kΩ, max plate voltage 800 V, max plate dissipation 25 W, heater 6.3 V / 1.4 A.

Is the KT77 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the KT77 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 KT77 sound like?

GEC EL34 replacement.. 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.

KT77 Tetrode Vacuum Tube

Also known as EL34

Beam power tetrode rated at 25W for high-power output stages

TetrodeHeater 6.3 V · 1.4 AOctalGEC 1957

Plate dissipation: 25W
Max plate voltage: 800V
Max screen voltage: 800V
Max cathode current: 180mA
Transconductance: 10.5mA/V

KT77

Characteristics

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

HeaterIndirectly heated

Specifications of KT77 — Heater
Heater voltageVh	6.3 V
Heater currentIh	1.4 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of KT77 — Limiting values
Plate voltageVa max	800 V
Screen voltageVg2 max	800 V
Plate dissipationPa max	25 W
Cathode currentIk max	180 mA

Mechanical

Specifications of KT77 — Mechanical
Base	Octal
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of KT77 — Small signal
Transconductance	10.5 mA/V
Plate resistance	23 kΩ
Amplification factor	12

Capacitances

Specifications of KT77 — Capacitances
Input	16.5 pF
Grid–plate	1 pF
Output	9 pF

Charactereditorial reading, not a measurement

Specifications of KT77 — Character
Gain	12%
Power	63%
Drive	77%
Linearity	94%

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	Ia	Load	Pout	THD	Source
Class AB1, push-pullultra-linear · cathode bias	430 V	390 V	390 V	66 mA	6 kΩ a-a	34 W	2.5%	OCR, to be checked Genalex / The M-O Valve Company Limited · 1977 · p. 2 · transcript
Class AB1, push-pulltriode connection · cathode bias	430 V	396 V		69 mA	5 kΩ a-a	18 W	1.2%	OCR, to be checked Genalex / The M-O Valve Company Limited · 1977 · p. 3 · transcript

Using this tube

Applications

High-power output

Marshall JCM800Hiwatt DR103

Guitar amp

Featured in these amplifiers

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : JJ KT77 datasheet (true pentode w/ g3)Active model : Koren v1

Manufacturer datasheets

2 manufacturer datasheets cross-checked for KT772 verified

KT77 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: JJ KT77 datasheet (true pentode w/ g3)
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Koren v1

Context

The KT77 is the GEC/MOV true pentode equivalent to the EL34, sharing the same pinout and similar operating characteristics but with subtly different sonic properties. It was developed as a higher-reliability, higher-linearity alternative to the EL34 for professional and industrial applications. The KT77 tends to offer slightly tighter bass and more extended treble than a typical EL34 while retaining the midrange richness of the pentode topology. Original GEC production was limited and commands high prices; the Gold Lion reissue by New Sensor is the primary current production source and is well-regarded.

History

Introduced: 1957
Manufacturer: GEC
Era: Golden Age

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