What is the KT61 vacuum tube?

The KT61 is a tetrode vacuum tube designed for power (se) applications. British beam tetrode. Smooth, refined..

What is a KT61 substitute or equivalent?

Direct equivalents to the KT61 include 6P25, CV1853. 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 KT61?

Amplification factor μ ≈ 27, transconductance gm ≈ 10.5 mA/V, plate resistance rp ≈ 2.8 kΩ, max plate voltage 275 V, max plate dissipation 10 W, heater 6.3 V / 0.95 A.

Is the KT61 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the KT61 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 KT61 sound like?

British beam tetrode. Smooth, refined.. 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.

KT61 Tetrode Vacuum Tube

Also known as 6P25 · CV1853

Beam power tetrode rated at 10W for high-power output stages

TetrodeHeater 6.3 V · 0.95 AOctalGEC·MOV 1940

Plate dissipation: 10W
Max plate voltage: 275V
Transconductance: 10.5mA/V

KT61

Characteristics

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

HeaterIndirectly heated

Specifications of KT61 — Heater
Heater voltageVh	6.3 V
Heater currentIh	0.95 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of KT61 — Limiting values
Plate voltageVa max	275 V
Plate dissipationPa max	10 W

Mechanical

Specifications of KT61 — Mechanical
Base	Octal
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of KT61 — Small signal
Transconductance	10.5 mA/V
Plate resistance	2.8 kΩ
Amplification factor	27

Capacitances

Specifications of KT61 — Capacitances
Input	9 pF
Grid–plate	0.3 pF
Output	8.5 pF

Charactereditorial reading, not a measurement

Specifications of KT61 — Character
Gain	27%
Power	25%
Drive	97%
Linearity	87%

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

Using this tube

No interactive loadline model for this tube yet — download a SPICE model below to simulate it in LTspice or ngspice.

Replace or compare

Sources

By Jean-Michel Falligan Devergne

Manufacturer datasheets

KT61 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: GEC·MOV
Reviewed: —
Status: DataModelCalc
Model: under review

Context

The KT61 is a indirectly-heated beam tetrode from GEC·MOV, introduced around 1940. It provides a gain factor (μ) of 27 with 2750Ω plate resistance. Commonly used in power (se) applications. British beam tetrode. Smooth, refined.

History

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

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