Can I swap a 5U4G for a 5Y3GT?

They share a base and envelope so they fit the same socket, but the operating point differs — expect to re-set the bias rather than perform a blind drop-in.

What is the difference between the 5U4G and the 5Y3GT?

The 5U4G has μ≈— with about — W maximum plate dissipation; the 5Y3GT has μ≈— with about — W. The higher μ is on the 5U4G, the higher plate dissipation rating on the 5U4G.

Which is better, the 5U4G or the 5Y3GT?

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.

5U4G vs 5Y3GT

5U4G / 5U4GB versus 5Y3GT — two rectifiers compared.

5U4G vs 5Y3GT: what differs

The 5U4G and 5Y3GT share a family but trade gain for headroom: 5Y3GT has the higher amplification factor (μ ≈ —), making it the typical choice for high-gain preamp stages. 5U4G runs lower μ (—) and is the standard cathode follower / phase-splitter / line-stage triode. They share the same envelope and base — wire them in the same socket and only the operating point changes.

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

5U4G

5Y3GT

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.

5U4G

—no μ published (rectifier or value missing from the catalogue)

5Y3GT

—no μ published (rectifier or value missing from the catalogue)

Difference—

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.

5U4G

75 ΩRa ≥ 150 Ω – 225 Ω

5Y3GT

—no rp published (rectifier or value missing from the catalogue)

Difference—

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.

5U4G

—no datasheet typical operating point in the catalogue for this tube

5Y3GT

—no datasheet typical operating point in the catalogue for this tube

Difference—

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.

5U4G

—no maximum plate dissipation published

5Y3GT

—no maximum plate dissipation published

Difference—

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).

5U4G

15 W5 V × 3 A

5Y3GT

10 W5 V × 2 A

Differencedifference −5 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.

5U4G

octalOctal · 8 pins

5Y3GT

octalOctal · 8 pins

Differencesame base, identical pinout

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

Physical compatibility

5U4G

5Y3GT

Base

octal

Pinout

Octal

identical pin by pin (8 pins)

Sections

Bulb diameter (max)

—

— no mechanical data in the catalogue

Heating

5U4G

5Y3GT

Heating

Direct (DHT)

Heater voltage

5 V

same voltage — the same winding can feed both · 5U4G: no center tap · 5Y3GT: no center tap

Heater current

3 A

2 A

current difference −1 A (−33 %)

Heater power

15 W

10 W

Bias

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

5U4G

5Y3GT

Va typical

—no datasheet typical operating point in the catalogue for this tube

Vg typical

—

Ia typical

—

Indicative Rk = |Vg| / Ia

—

Pd typical = Va · Ia

—

Performance for the objective

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

5U4G

5Y3GT

Difference

—

75 Ω

—

Pa max

—

Va max

450 V

350 V

difference −100 V

Substitution possible under conditions

Not listed as equivalents in the catalogue. Same base, same pinout and same heater voltage: a swap is possible only under these conditions:

heater current differs (3 A vs 2 A) — check the winding rating
re-set the bias — the typical operating point is not published for both tubes

Substitutes for 5U4G Substitutes for 5Y3GT

Side-by-side specifications

Side-by-side specifications of 5U4G and 5Y3GT
Spec	5U4G	5Y3GT
Full name	5U4G / 5U4GB	5Y3GT
Type	Rectifier	Rectifier
Primary use	Rectifier	Rectifier
Era	1940	1938
Heating	direct	direct
Heater voltage	5 V	5 V
Heater current	3 A	2 A
Amplification factor (μ)	—	—
Transconductance (gm)	—	—
Plate resistance (rp)	75 Ω	—
Max plate voltage	450 V	350 V
Max plate dissipation	—	—
Base	octal	octal
Manufacturer	RCA·Sylvania	RCA
Rarity	Common	Common

Sonic character

Listening folklore — not electrical data

5U4G

Softer delivery adds warmth.

5Y3GT

Classic sag. Blues tone.

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 5U4G vs 5Y3GT in the interactive comparator →

Related comparisons

GZ34 vs 5U4G →