What is a GU81 substitute or equivalent?

Direct equivalents to the GU81 include None — Soviet high-power transmitting pentode class. 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 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.

By Jean-Michel Falligan Devergne · Reviewed May 29, 2026

←GU81

← GU-50 GZ34 →Compare…

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..

Pentode12.6V / 10.5A5-pin Special (transmitting) + Top CapSPICE

SourceRussian ГУ-81 datasheet · external audit feedbackReviewed2026-05-29StatusDataModelCalcModelKoren pentodePDF

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

Datasheet ↗SPICE

GU-81 / ГУ-81 — Russian — pentode vacuum tube — (1960)

5.5mA/V

28.5pF

0.1pF

23.50pF

GAIN32%

POWER100%

DRIVE50%

LINEARITY84%

SPECIFICATIONS

Heater12.6 V · 10.5 A

Max Plate Voltage3000 V

Pa max450 W

Gain Factor32

gm5.5 mA/V

rp—

Inter-electrode capacitances

Cgk28.5 pF

Cgp0.1 pF

Cpk23.5 pF

Pinout5-pin Special (transmitting) + Top Cap

Era1960

RarityModerate

HEATER12.6V / 10.5A

MAX PLATE3000V / 450W

BASEspecial-5

Source datasheets

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

1 verified

EXECUTIVE SUMMARY

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.

APPLICATIONS

Power output

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

• PA/public address

HISTORY

Introduced1960ManufacturerRussian

Late Production

GU81 Datasheet

Local mirror · PDF

↗

METRIX TESTER

Test this tube on a Metrix

SIMILAR PENTODES

View all →

12SJ7Preamp 307APower/Output 310APreamp 35LR6Sweep / Power 47Power/Output 4P1LPower (SET output) — DHT pentode 5639Video / wideband (power)5654RF / Preamp