Can I swap a 12AX7 for a 5751?

Yes — the 12AX7 and 5751 are direct equivalents (same pinout and electrical envelope) and drop into the same socket without rebiasing in nearly every circuit. Manufacturer and batch differences usually matter more than the type number.

What is the difference between the 12AX7 and the 5751?

The 12AX7 has μ≈100 with about 1 W maximum plate dissipation; the 5751 has μ≈70 with about 0.8 W. The higher μ is on the 12AX7, the higher plate dissipation rating on the 12AX7.

Which is better, the 12AX7 or the 5751?

Neither is universally better — it depends on the circuit and the tone you want in a guitar amp. Compare the side-by-side specifications and the comparison by objective on this page (gain, load, current, dissipation margin, heater, base) to choose for your build.

12AX7 vs 5751

12AX7 / ECC83 versus 5751 — two triodes compared.

12AX7 vs 5751: what differs

The 12AX7 and 5751 are directly equivalent in most applications — same pinout, same electrical envelope, same general sonic signature. 12AX7 is the American/RETMA designation; 5751 is the European Mullard-Philips name (or vice versa). You can swap one for the other in nearly every circuit without rebiasing, with the caveat that production-batch differences between manufacturers (Mullard vs Sylvania vs JJ vs current Russian production) often matter more than the type-number itself.

Comparison by objective

Choose what the stage has to do: the quantities below are reordered and weighted for that objective. Every value comes from the catalogue and from the datasheet typical operating point; a missing value shows “—” with its reason. No score, no ranking — only measured differences.

Voltage amplification of a small signal — common cathode with a resistive plate load.

Quantity

12AX7

5751

Difference

Available gain (μ)Weight 3/3 · decisiveμ is the ceiling of the voltage gain of a common-cathode stage.Av ≈ μ · Ra / (Ra + rp) — with Ra = 2–3 × rp the stage reaches about 2/3 to 3/4 of μ. As a follower the gain is close to 1 whatever μ.

12AX7

100

5751

Differenceratio ×1.4

Load to drive (rp)Weight 2/3 · importantPlate resistance: the lower rp, the lower the load the stage can drive at the same gain and linearity.Ra ≥ 2–3 × rp for a common-cathode stage. As a cathode follower the output impedance is about 1 / gm.

12AX7

62.5 kΩRa ≥ 125 kΩ – 187.5 kΩ

5751

58 kΩRa ≥ 116 kΩ – 174 kΩ

Differenceratio ×1.1

Available current (Ia typical)Weight 1/3 · to checkPlate current at the datasheet typical operating point — what the stage can deliver into a capacitance or a low load.Read from the published typical point (Va, Vg). No operating point is derived from a model here.

12AX7

1.2 mAat Va 250 V, Vg −2 VTypical point: RCA RC-30, 12AX7 typical operation

5751

1.1 mAat Va 250 V, Vg −3 VTypical point: GE 5751 datasheet, typical operation

Differenceratio ×1.1

Dissipation margin (Pa max − Pd typical)Weight 1/3 · to checkDifference between the maximum plate dissipation and the dissipation at the typical point (Pd = Va · Ia).Pd at idle < 70 % of Pa max.

12AX7

0.7 WPa max 1 W · Pd typical 0.3 W (30 % of Pa max)Typical point: RCA RC-30, 12AX7 typical operation

5751

0.53 WPa max 0.8 W · Pd typical 0.28 W (34 % of Pa max)Typical point: GE 5751 datasheet, typical operation

Differencedifference −0.17 W

Heater (V × A)Weight 1/3 · to checkHeater voltage and current, hence the power drawn from the heater winding.Same voltage required; the winding must supply the summed current of the tubes in parallel (or the same current in series).

12AX7

1.9 W12.6 V × 0.15 A

5751

2.2 W12.6 V × 0.175 A

Differencedifference +0.31 W

Base compatibilityWeight 1/3 · to checkBase and pin-by-pin function according to the catalogue pinouts.Same base and identical pinout → same socket. Same base, different pins → rewiring. Different base → adapter or new socket.

12AX7

noval9-pin Noval · 9 pins

5751

noval9-pin Noval · 9 pins

Differencesame base, identical pinout

Rules: Amplifier topologies guide, §07 “Design rules of thumb”

Physical compatibility

12AX7

5751

Base

noval

Pinout

9-pin Noval

identical pin by pin (9 pins)

Sections

Bulb diameter (max)

—

— no mechanical data in the catalogue

Heating

12AX7

5751

Heating

Indirect (IHT)

Heater voltage

12.6 V

same voltage — the same winding can feed both · 12AX7: center-tapped heater: 6.3 V parallel / 12.6 V series · 5751: center-tapped heater: 6.3 V parallel / 12.6 V series

Heater current

0.15 A

0.175 A

current difference +0.025 A (+17 %)

Heater power

1.89 W

2.2 W

Bias

Datasheet typical operating point (one per tube, SEQ-02) and the cathode resistor it implies.

12AX7

5751

Va typical

250 V

Vg typical

−2 V

−3 V

Ia typical

1.2 mA

1.1 mA

Indicative Rk = |Vg| / Ia

1.7 kΩ

2.7 kΩ

Pd typical = Va · Ia

0.3 W

0.28 W

12AX7 — Typical point: RCA RC-30, 12AX7 typical operation · 5751 — Typical point: GE 5751 datasheet, typical operation

Performance for the objective

Catalogue values, weighted as in the objective table (Gain input stage). rp is a design parameter, not a merit.

12AX7

5751

Difference

100

ratio ×1.4

1.6 mA/V

1.2 mA/V

ratio ×1.3

62.5 kΩ

58 kΩ

ratio ×1.1

Pa max

1 W

0.8 W

difference −0.2 W

Va max

300 V

330 V

difference +30 V

Substitution possible under conditions

12AX7 and 5751 list each other as equivalents in the catalogue.

heater current differs (0.15 A vs 0.175 A) — check the winding rating

Substitutes for 12AX7 Substitutes for 5751

Side-by-side specifications

Side-by-side specifications of 12AX7 and 5751
Spec	12AX7	5751
Full name	12AX7 / ECC83	5751
Type	Triode	Triode
Primary use	Preamp	Preamp
Era	1947	1954
Heating	indirect	indirect
Heater voltage	12.6 V	12.6 V
Heater current	0.15 A	0.175 A
Amplification factor (μ)	100	70
Transconductance (gm)	1.6 mA/V	1.2 mA/V
Plate resistance (rp)	62.5 kΩ	58 kΩ
Max plate voltage	300 V	330 V
Max plate dissipation	1 W	0.8 W
Base	noval	noval
Manufacturer	Mullard·RCA·Telefunken	GE·Sylvania·RCA
Rarity	Common	Sought-after

Sonic character

Listening folklore — not electrical data

12AX7

Rich warm midrange. The gold standard.

5751

Military 12AX7. More clarity.

Need an interactive deep dive?

The Amperatubes comparator visualises loadlines for both tubes side-by-side, simulates the SPICE response, and lets you adjust the operating point in real time.

Open 12AX7 vs 5751 in the interactive comparator →

12AX7 vs 5751

12AX7 vs 5751: what differs

Comparison by objective

Physical compatibility

Heating

Bias

Performance for the objective

Substitution possible under conditions

Side-by-side specifications

Sonic character

Need an interactive deep dive?

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