What is a GU-50 substitute or equivalent?

Direct equivalents to the GU-50 include LS-50 / FU-50 (Telefunken originals); 807 is loose, not equivalent. 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 GU-50?

Transconductance gm ≈ 4 mA/V, max plate voltage 1000 V, max plate dissipation 40 W, heater 6.3 V / 0.705 A.

Is the GU-50 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the GU-50 as Common 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 GU-50 sound like?

Russian LS-50 / FU-50 family transmitting pentode; sometimes compared to 807, but not a direct equivalent.. 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.

GU-50 Pentode Vacuum Tube

Q: What is the GU-50 vacuum tube?

The GU-50 is a pentode vacuum tube designed for transmitting applications. Russian LS-50 / FU-50 family transmitting pentode; sometimes compared to 807, but not a direct equivalent..

Also known as LS-50 · FU-50 (Telefunken originals) · 807 is loose · not equivalent

Power pentode delivering up to 40W in pentode, UL, or triode mode

PentodeHeater 6.3 V · 0.705 ASpecialRussian 1955

Plate dissipation: 40W
Max plate voltage: 1000V
Max screen voltage: 250V
Transconductance: 4mA/V

GU-50

Characteristics

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

HeaterIndirectly heated

Specifications of GU-50 — Heater
Heater voltageVh	6.3 V
Heater currentIh	0.705 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of GU-50 — Limiting values
Plate voltageVa max	1000 V
Screen voltageVg2 max	250 V
Plate dissipationPa max	40 W

Mechanical

Specifications of GU-50 — Mechanical
Base	Special
Rarity	Common
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of GU-50 — Small signal
Transconductance	4 mA/V

Capacitances

Specifications of GU-50 — Capacitances
Input	14 pF
Grid–plate	0.1 pF
Output	9.7 pF

Charactereditorial reading, not a measurement

Specifications of GU-50 — Character
Gain	0%
Power	100%
Drive	50%
Linearity	75%

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

Using this tube

Applications

Power output

Generic pentode transmitting stage

PA/public address

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : Wikipedia GU-50 · OK1RR datasheet · wording corrected: 807 is loose comparison, not direct equivalentActive model : Koren pentode

Manufacturer datasheets

3 manufacturer datasheets cross-checked for GU-503 verified

GU-50 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Wikipedia GU-50 · OK1RR datasheet · wording corrected: 807 is loose comparison, not direct equivalent
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Koren pentode

Context

The GU-50 is a indirectly-heated pentode from Russian, introduced around 1955. Commonly used in transmitting applications. Russian LS-50 / FU-50 family transmitting pentode; sometimes compared to 807, but not a direct equivalent.

History

Introduced: 1955
Manufacturer: Russian
Era: Golden Age

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