Same Octal pinout but ~double the mu and gm — changes bias points, not a drop-in in regulator or amplifier circuits
Electrical Specifications
Absolute Maximum
Anode Voltage (max)Anode Voltage (max)250V
Anode Dissipation (max)Anode Dissipation (max, per plate)13W
Cathode Current (max)Plate Current (max, per section)125mA
Typical Operating
Anode Voltage (typical)Anode Voltage110V
Notes
Only Tung-Sol manufactured the 5998. Western Electric rebranded it as the 421A, and it also appeared under Chatham and IBM labels.
The 5998A is a later version with higher ratings (Va_max 275V, Pa_max 15W per section) in a different envelope (GT shape vs ST shape).
Not a direct drop-in replacement for 6AS7/6080 in all circuits: the 5998 has approximately double the mu and transconductance, which changes bias points and may affect stability in regulator and amplifier circuits.
Grid circuit resistance limit for the 6AS7 family is typically 1.0 MΩ for cathode bias, 0.1 MΩ for fixed bias — the 5998 datasheet likely specifies similar limits but the exact value could not be confirmed from accessible sources.
Pin Layout — Octal
1Grid (triode 2)
2Anode (triode 2)
3Cathode (triode 2)
4
Socket-Compatible Tubes ⚠ Not electrically compatible