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421A

Low-mu double power triode with separate cathodes, intended primarily as a series-pass tube in regulated rectifier supplies, providing high-current control capability in voltage-regulator circuits.

Directly heatedCold cathodeCommon cathodeDissimilar

Sections: 2
Base: B8-11
Socket code: UX4
Base pins: 8
Nominal heater: 6.3 V / 2.4 A

Description

The 5998/421A is a double triode with separate indirectly heated cathodes, intended for use as a series tube in regulated rectifier circuits.

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: B8-11
Socket code: UX4
Base pins: 8
Nominal heater voltage: 6.3 V
Nominal heater current: 2.4 A
Parallel heater voltage: -
Parallel heater current: -
Series capable: No

B8-11 - Terminal pinout

Electrical assignments shared by every tube using this base designation.

No verified pinout is recorded for this base designation.

Section ratings

2 physical sections, each with unique specifications.

Section 1

Triode

X-ray radiationInterelectrode capacitances

Plate resistance: 65 Ω
Transconductance: 0.02 S
Amplification factor: 1.3
Max plate voltage: 250 V
Max plate dissipation: 13 W
Max cathode current: 125 mA

Representative row for two electrically identical triode sections; maximum ratings apply to each triode.

Section 2

Triode

X-ray radiationInterelectrode capacitances

Plate resistance: 65 Ω
Transconductance: 0.02 S
Amplification factor: 1.3
Max plate voltage: 250 V
Max plate dissipation: 13 W
Max cathode current: 125 mA

Identical second physical section; duplicated from representative section per physical-section-row policy.

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 Western Electric 400s

Published operating points and test conditions from this source.

SECTION 1

Triode

Other

Maximum heater-cathode voltage
150 V
EachTriode
Maximum grid circuit resistance, cathode bias
0.1 Mohm
EachTriode

Characteristic

Plate current at representative condition
125 mA
PlateSupplyVoltage=110V

Data set 02

2026 Western Electric Other

Published operating points and test conditions from this source.

SECTION 1

Triode

Other

Heater Voltage
6.3 V
General characteristics
Heater Current
2.4 A
General characteristics
Plate Voltage Maximum
250 V
Each triode; design-center value
Plate Dissipation Maximum
13 W
Each triode; design-center value
Cathode Current Maximum
125 mA
Each triode; design-center value
Heater-Cathode Voltage Maximum
150 V
Each triode; design-center value
Bulb Temperature Maximum
200 degC
Design-center value
Grid Circuit Resistance Maximum
0.1 megohm
Cathode bias
Plate Supply Voltage
110 V
Each triode; operating column 1
Cathode Bias Resistance
90 ohm
Each triode; operating column 1
Plate Supply Voltage
110 V
Each triode; operating column 2
Cathode Bias Resistance
65 ohm
Each triode; operating column 2

Characteristic

Plate Current
100 mA
Each triode; operating column 1
Amplification Factor
5.9
Each triode; operating column 1
Plate Resistance
330 ohm
Each triode; operating column 1
Transconductance
18000 uS
Each triode; operating column 1
Plate Current
125 mA
Each triode; operating column 2
Amplification Factor
6.1
Each triode; operating column 2
Plate Resistance
305 ohm
Each triode; operating column 2
Transconductance
20000 uS
Each triode; operating column 2

Application

Recommended Heater-Cathode Voltage for Optimum Life
90 V
Should not exceed for optimum life expectancy
Recommended Bulb Temperature for Optimum Life
150 degC
Forced-air cooling should keep bulb below this temperature