What are the key specifications of the GU81?

Amplification factor μ ≈ 32, transconductance gm ≈ 5.5 mA/V, max plate voltage 3000 V, max plate dissipation 450 W, heater 12.6 V / 10.5 A.

Is the GU81 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the GU81 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 GU81 sound like?

Soviet high-power transmitting pentode (450W plate dissipation). Directly-heated thoriated-tungsten filament 12.6V × 11A; anode and screen grid both on top caps; control grid + suppressor + filament on the 5-pin base. RF service / modulator — NOT a small-signal or audio-output tube.. 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.

GU81 GU-81 / ГУ-81 Pentode Vacuum Tube

Q: What is the GU81 vacuum tube?

The GU81 (GU-81 / ГУ-81) is a pentode vacuum tube designed for transmitting (high-power rf / modulator) applications. Soviet high-power transmitting pentode (450W plate dissipation). Directly-heated thoriated-tungsten filament 12.6V × 11A; anode and screen grid both on top caps; control grid + suppressor + filament on the 5-pin base. RF service / modulator — NOT a small-signal or audio-output tube..

Power pentode delivering up to 450W in pentode, UL, or triode mode

PentodeHeater 12.6 V · 10.5 A5-pin Special (transmitting) + Top CapRussian 1960

Plate dissipation: 450W
Max plate voltage: 3000V
Transconductance: 5.5mA/V

GU81

Characteristics

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

HeaterDirectly heated

Specifications of GU81 — Heater
Heater voltageVh	12.6 V
Heater currentIh	10.5 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of GU81 — Limiting values
Plate voltageVa max	3000 V
Plate dissipationPa max	450 W

Mechanical

Specifications of GU81 — Mechanical
Base	5-pin Special (transmitting) + Top Cap
Rarity	Moderate
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of GU81 — Small signal
Transconductance	5.5 mA/V
Amplification factor	32

Capacitances

Specifications of GU81 — Capacitances
Input	28.5 pF
Grid–plate	0.1 pF
Output	23.5 pF

Charactereditorial reading, not a measurement

Specifications of GU81 — Character
Gain	32%
Power	100%
Drive	50%
Linearity	84%

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

Using this tube

Applications

Power output

Generic pentode transmitting (high-power rf / modulator) stage

PA/public address

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : Russian ГУ-81 datasheet · external audit feedbackActive model : Koren pentode

Manufacturer datasheets

1 manufacturer datasheets cross-checked for GU-81 / ГУ-811 verified

GU81 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Russian ГУ-81 datasheet · external audit feedback
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Koren pentode

Context

The GU81 is a directly-heated pentode from Russian, introduced around 1960. Commonly used in transmitting (high-power rf / modulator) applications. Soviet high-power transmitting pentode (450W plate dissipation). Directly-heated thoriated-tungsten filament 12.6V × 11A; anode and screen grid both on top caps; control grid + suppressor + filament on the 5-pin base. RF service / modulator — NOT a small-signal or audio-output tube.

History

Introduced: 1960
Manufacturer: Russian
Era: Late Production

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