Can I swap a 300B for a 2A3?

Not blindly. The 300B and 2A3 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 300B and the 2A3?

The 300B has μ≈4 with about 36 W maximum plate dissipation; the 2A3 has μ≈4 with about 15 W. The higher μ is on the 300B, the higher plate dissipation rating on the 300B.

Which is better, the 300B or the 2A3?

Neither is universally better — it depends on the circuit and the tone you want in a hi-fi system. 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.

300B vs 2A3

300B versus 2A3 — two triodes compared.

300B vs 2A3: what differs

The 300B and 2A3 are the two classic rivals of their power class. 300B has the higher plate dissipation rating (~36 W vs ~15 W for the 2A3). 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

300B

2A3

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.

300B

2A3

Differenceidentical

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.

300B

700 ΩRa ≥ 1.4 kΩ – 2.1 kΩ

2A3

800 ΩRa ≥ 1.6 kΩ – 2.4 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.

300B

60 mAat Va 300 V, Vg −61 VTypical point: JJ Electronic (www.jj-electronic.sk) · not dated (modern reissue)

2A3

60 mAat Va 250 V, Vg −45 VTypical point: RCA (RCA Radiotron Division / RCA Manufacturing Co.) — curves also branded Cunningham Radiotron · data October 15, 1947; characteristic curves March 9, 1933 & June 12, 1933

Differenceidentical

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.

300B

18 WPa max 36 W · Pd typical 18 W (50 % of Pa max)Typical point: JJ Electronic (www.jj-electronic.sk) · not dated (modern reissue)

2A3

0 WPa max 15 W · Pd typical 15 W (100 % of Pa max)Typical point: RCA (RCA Radiotron Division / RCA Manufacturing Co.) — curves also branded Cunningham Radiotron · data October 15, 1947; characteristic curves March 9, 1933 & June 12, 1933

Differencedifference −18 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).

300B

6 W5 V × 1.2 A

2A3

6.3 W2.5 V × 2.5 A

Differencedifference +0.25 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.

300B

ux4UX-4 · 4 pins

2A3

ux4UX-4 · 4 pins

Differencesame base, identical pinout

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

Physical compatibility

300B

2A3

Base

ux4

Pinout

UX-4

identical pin by pin (4 pins)

Sections

Bulb diameter (max)

—

— no mechanical data in the catalogue

Heating

300B

2A3

Heating

Direct (DHT)

Heater voltage

5 V

2.5 V

different voltages — the heater supply changes · 300B: no center tap · 2A3: no center tap

Heater current

1.2 A

2.5 A

current difference +1.3 A (+108 %)

Heater power

6 W

6.25 W

Bias

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

300B

2A3

Va typical

300 V

250 V

Vg typical

−61 V

−45 V

Ia typical

60 mA

Indicative Rk = |Vg| / Ia

1 kΩ

750 Ω

Pd typical = Va · Ia

18 W

15 W

300B — Typical point: JJ Electronic (www.jj-electronic.sk) · not dated (modern reissue) · 2A3 — Typical point: RCA (RCA Radiotron Division / RCA Manufacturing Co.) — curves also branded Cunningham Radiotron · data October 15, 1947; characteristic curves March 9, 1933 & June 12, 1933

Performance for the objective

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

300B

2A3

Difference

identical

5.5 mA/V

5.25 mA/V

ratio ×1

700 Ω

800 Ω

ratio ×1.1

Pa max

36 W

15 W

difference −21 W

Va max

400 V

300 V

difference −100 V

Substitution possible under conditions

No direct substitution listed in the catalogue.

What rules it out as a drop-in:

heater voltage differs (5 V vs 2.5 V)

Substitutes for 300B Substitutes for 2A3

Side-by-side specifications

Side-by-side specifications of 300B and 2A3
Spec	300B	2A3
Full name	300B	2A3
Type	Triode	Triode
Primary use	Power (SET)	Power (SET)
Era	1938	1932
Heating	direct	direct
Heater voltage	5 V	2.5 V
Heater current	1.2 A	2.5 A
Amplification factor (μ)	4	4
Transconductance (gm)	5.5 mA/V	5.25 mA/V
Plate resistance (rp)	700 Ω	800 Ω
Max plate voltage	400 V	300 V
Max plate dissipation	36 W	15 W
Base	ux4	ux4
Manufacturer	Western Electric	RCA·WE·Shuguang
Rarity	Legendary	Collector

Sonic character

Listening folklore — not electrical data

300B

Holy grail of SET.

2A3

Crystalline SE power.

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 300B vs 2A3 in the interactive comparator →

Related comparisons

300B vs 845 →