Can I swap a 300B for a 845?

Not blindly. The 300B and 845 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 845?

The 300B has μ≈4 with about 36 W maximum plate dissipation; the 845 has μ≈5 with about 100 W. The higher μ is on the 845, the higher plate dissipation rating on the 845.

Which is better, the 300B or the 845?

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 845

300B versus 845 — two triodes compared.

300B vs 845: what differs

Moving from the 300B to the 845 is one of the classic upgrade paths in tube audio. The 845 has the higher plate dissipation rating (~100 W vs ~36 W), at the cost of a higher running cost (current consumption, transformer requirements, NOS prices). Most builds cannot socket-swap; the chassis layout, output transformer, and power supply must be re-engineered.

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

845

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

845

Differenceratio ×1.3

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

845

1.7 kΩRa ≥ 3.4 kΩ – 5.1 kΩ

Differenceratio ×2.4

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)

845

—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.

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)

845

—no datasheet typical operating point in the catalogue for this tubePa max 100 W

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).

300B

6 W5 V × 1.2 A

845

32.5 W10 V × 3.25 A

Differencedifference +26.5 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.

300B

ux4UX-4 · 4 pins

845

ux4UX-4 · 4 pins

Differencesame base, pins differ

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

Physical compatibility

300B

845

Base

ux4

Pinout

UX-4

differs on 4 pin(s): 1: F- / G, 2: P / F, 3: G / P, 4: F+ / F

Sections

Bulb diameter (max)

—

— no mechanical data in the catalogue

Heating

300B

845

Heating

Direct (DHT)

Heater voltage

5 V

10 V

different voltages — the heater supply changes · 300B: no center tap · 845: no center tap

Heater current

1.2 A

3.25 A

current difference +2.05 A (+171 %)

Heater power

6 W

32.5 W

Bias

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

300B

845

Va typical

300 V

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

Vg typical

−61 V

—

Ia typical

60 mA

—

Indicative Rk = |Vg| / Ia

1 kΩ

—

Pd typical = Va · Ia

18 W

—

300B — Typical point: JJ Electronic (www.jj-electronic.sk) · not dated (modern reissue)

Performance for the objective

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

300B

845

Difference

ratio ×1.3

5.5 mA/V

3.3 mA/V

ratio ×1.7

700 Ω

1.7 kΩ

ratio ×2.4

Pa max

36 W

100 W

difference +64 W

Va max

400 V

1,250 V

difference +850 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: 1: F- / G, 2: P / F, 3: G / P, 4: F+ / F
heater voltage differs (5 V vs 10 V)

Substitutes for 300B Substitutes for 845

Side-by-side specifications

Side-by-side specifications of 300B and 845
Spec	300B	845
Full name	300B	845
Type	Triode	Triode
Primary use	Power (SET)	Transmitting (SET)
Era	1938	1931
Heating	direct	direct
Heater voltage	5 V	10 V
Heater current	1.2 A	3.25 A
Amplification factor (μ)	4	5
Transconductance (gm)	5.5 mA/V	3.3 mA/V
Plate resistance (rp)	700 Ω	1.7 kΩ
Max plate voltage	400 V	1250 V
Max plate dissipation	36 W	100 W
Base	ux4	ux4
Manufacturer	Western Electric	RCA
Rarity	Legendary	Moderate

Sonic character

Listening folklore — not electrical data

300B

Holy grail of SET.

845

King of SET output.

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 845 in the interactive comparator →

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