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Tube Tinker — Electron Tube Database
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Power - Triode ·Triode

300BM

Mesh-plate directly heated 300B-family audio power triode. Its distinctive mesh anode accompanies traditional low-mu, low-plate-resistance characteristics and plate dissipation in the mid-30-watt range for single-ended Class A output stages.

Directly heatedCold cathodeCommon cathodeDissimilar
Sections
1
Base
UX4
Socket code
UX4
Base pins
4
Nominal heater
5 V / 1.3 A

Description

300BM+ mesh-plate directly heated 300B-family audio triode. The supplied Emission Labs sheet was read for filament data, maximum ratings, representative operating points, gm/rp/mu, output conditions, capacitances, and rectifier filter conditions as applicable.

Socket description

Medium-shell UX 4-contact

General specifications

Tube and section characteristics

Properties common to every published source and operating point.

Heater and base

Base
UX4
Socket code
UX4
Base pins
4
Nominal heater voltage
5 V
Nominal heater current
1.3 A
Parallel heater voltage
-
Parallel heater current
-
Series capable
No

UX4 - Terminal pinout

Electrical assignments shared by every tube using this base designation.

No verified pinout is recorded for this base designation.

Section ratings

1 physical section, each with unique specifications.

Section 1

Triode

X-ray radiationInterelectrode capacitances
Plate resistance
700 Ω
Transconductance
0.0056 S
Amplification factor
3.9
Max plate voltage
450 V
Max plate dissipation
36 W
Max cathode current
100 mA

Source-dependent data

Operating conditions by source

Each block preserves the test conditions and published values from one reference. Values from different sources are intentionally kept separate.

Data set 01

2026 Emission Labs

Published operating points and test conditions from this source.

SECTION 1

Triode

Characteristics

  • Filament voltage

    5 V

  • Filament current

    1.3 A

  • Plate resistance

    700 ohm

    ClassA

  • Amplification factor

    3.9

    ClassA

  • Transconductance

    5600 uS

    ClassA

Other

  • Plate voltage

    300 V

    ClassA

  • Plate current

    56 mA

    ClassA

  • Grid voltage

    -58.5 V

    ClassA

Capacitance

  • Grid-plate capacitance

    15 pF

  • Grid-filament capacitance

    8.5 pF

  • Plate-filament capacitance

    4 pF