Can I swap a GZ34 for a 5U4G?

Not blindly. The GZ34 and 5U4G differ in bias point, dissipation and often circuit requirements; a swap needs rebiasing and sometimes output-transformer or power-supply changes.

What is the difference between the GZ34 and the 5U4G?

The GZ34 has μ≈— with about — W maximum plate dissipation; the 5U4G has μ≈— with about — W. The higher μ is on the GZ34, the higher plate dissipation rating on the GZ34.

Which is better, the GZ34 or the 5U4G?

Neither is universally better — it depends on the circuit and the tone you want. 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.

GZ34 vs 5U4G

GZ34 / 5AR4 versus 5U4G / 5U4GB — two rectifiers compared.

GZ34 vs 5U4G: what differs

The GZ34 and 5U4G are the two classic rivals of their power class. 5U4G has the higher plate dissipation rating (~— W vs ~— W for the GZ34). Bias points and output transformers differ — never swap blindly; the listening notes stay in their own labelled block below.

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

GZ34

5U4G

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.

GZ34

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

5U4G

—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.

GZ34

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

5U4G

75 ΩRa ≥ 150 Ω – 225 Ω

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.

GZ34

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

5U4G

—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.

GZ34

—no maximum plate dissipation published

5U4G

—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).

GZ34

9.5 W5 V × 1.9 A

5U4G

15 W5 V × 3 A

Differencedifference +5.5 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.

GZ34

octalOctal · 8 pins

5U4G

octalOctal · 8 pins

Differencesame base, pins differ

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

Physical compatibility

GZ34

5U4G

Base

octal

Pinout

Octal

differs on 1 pin(s): 8: K+F / F

Sections

Bulb diameter (max)

—

— no mechanical data in the catalogue

Heating

GZ34

5U4G

Heating

Direct (DHT)

Heater voltage

5 V

same voltage — the same winding can feed both · GZ34: no center tap · 5U4G: no center tap

Heater current

1.9 A

3 A

current difference +1.1 A (+58 %)

Heater power

9.5 W

15 W

Bias

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

GZ34

5U4G

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.

GZ34

5U4G

Difference

—

75 Ω

—

Pa max

—

Va max

550 V

450 V

difference −100 V

Substitution possible under conditions

No direct substitution listed in the catalogue.

What rules it out as a drop-in:

same base but pins differ: 8: K+F / F

Substitutes for GZ34 Substitutes for 5U4G

Side-by-side specifications

Side-by-side specifications of GZ34 and 5U4G
Spec	GZ34	5U4G
Full name	GZ34 / 5AR4	5U4G / 5U4GB
Type	Rectifier	Rectifier
Primary use	Rectifier	Rectifier
Era	1954	1940
Heating	direct	direct
Heater voltage	5 V	5 V
Heater current	1.9 A	3 A
Amplification factor (μ)	—	—
Transconductance (gm)	—	—
Plate resistance (rp)	—	75 Ω
Max plate voltage	550 V	450 V
Max plate dissipation	—	—
Base	octal	octal
Manufacturer	Mullard·Philips	RCA·Sylvania
Rarity	Sought-after	Common

Sonic character

Listening folklore — not electrical data

GZ34

The premier audio rectifier.

5U4G

Softer delivery adds warmth.

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 GZ34 vs 5U4G in the interactive comparator →

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