What is the 5751 vacuum tube?

The 5751 is a triode vacuum tube designed for preamp applications. Military 12AX7. More clarity..

What is a 5751 substitute or equivalent?

Direct equivalents to the 5751 include 12AX7 (70%). 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 5751?

Amplification factor μ ≈ 70, transconductance gm ≈ 1.2 mA/V, plate resistance rp ≈ 58.0 kΩ, max plate voltage 330 V, max plate dissipation 0.8 W, heater 12.6 V / 0.175 A.

Is the 5751 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 5751 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 5751 sound like?

Military 12AX7. More clarity.. 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.

5751 Triode Vacuum Tube

Also known as 12AX7 (70%)

Medium-gain triode for voltage amplification applications

Dual triodeHeater 12.6 V · 0.175 A9-pin Noval2 sectionsGE·Sylvania·RCA 1954

Plate dissipation: 0.8W
Max plate voltage: 330V
Amplification factor: 70
Transconductance: 1.2mA/V
Plate resistance: 58kΩ

5751

Characteristics

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

HeaterIndirectly heated

Specifications of 5751 — Heater
Heater voltageVh	12.6 V
Heater currentIh	0.175 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 5751 — Limiting values
Plate voltageVa max	330 V
Plate dissipationPa max	0.8 W
Cathode currentIk max	6 mA
Peak plate voltageVao	600 V

Mechanical

Specifications of 5751 — Mechanical
Base	9-pin Noval
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 5751 — Small signal
Transconductance	1.2 mA/V
Plate resistance	58 kΩ
Amplification factor	70

Capacitances

Specifications of 5751 — Capacitances
Grid–cathode	1.4 pF
Grid–plate	1.4 pF
Plate–cathode	0.46 pF

Charactereditorial reading, not a measurement

Specifications of 5751 — Character
Gain	70%
Power	2%
Drive	42%
Linearity	65%

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

Typical operation

Canonical operating pointSPICE model2 availableModel

B+: 300 V
Load: 220 kΩ
Va: 180 V
Ia: 0.82 mA
Vg: -2.2 V
Pd: 0.15 W
Gain: 36.4 dB

At Q: Ayumi Nakabayashi · 2 engines
μ: 68.7
gm: 1.2 mA/V
rp: 57.1 kΩ

Design a stage →

Using this tube

Applications

Voltage amplifier
High-gain preamp
Phase splitter

Military-spec preamp stagesHi-Fi low-noise preamp

Hi-Fi stereoStudio/recording

Featured in these amplifiers

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : GE·Sylvania·RCA datasheetActive model : Ayumi Nakabayashi · 2 engines

Manufacturer datasheets

6 manufacturer datasheets cross-checked for 57516 verified

5751 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: GE·Sylvania·RCA datasheet
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Ayumi Nakabayashi · 2 engines

Context

The 5751 is a military-grade dual triode with a mu of approximately 70, designed as a high-reliability substitute for the 12AX7 in critical applications. Its 30% lower gain compared to a 12AX7 provides more headroom and a tighter, more controlled sound that many find preferable in both hi-fi and guitar amplifier service. The ruggedized construction with closer element spacing typically results in lower microphonics and longer lifespan. GE five-star, Sylvania Gold Brand, and Raytheon windmill-getter versions are the most sought-after variants, and it remains a popular 12AX7 substitute in demanding audio circuits.

History

Introduced: 1954
Manufacturer: GE·Sylvania·RCA
Era: Golden Age

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