7581 — Beam Tetrode, Octal | TubeofileKnown Equivalents & Substitutes
7581A
Similar Tubes (Not Drop-In Compatible)
5881
6L6 family member with same pinout — ruggedised military variant with lower ratings than 75816L6GC
Direct replacement — 7581 is a drop-in for 6L6GC with same pinout and improved zero-bias characteristicsSams lists KT66 as substitute — GEC beam tetrode with similar application and ratings Sams lists KT77 as substitute — GEC beam tetrode designed as 6L6GC/EL34 replacement Electrical Specifications
Section 1 — Beam Tetrode
Absolute Maximum
Anode Voltage (max)Plate Voltage (max, pentode)500V
Screen Voltage (max)Screen Voltage (max)450V
Anode Dissipation (max)Plate Dissipation (max)30W
Screen Dissipation (max)Screen Dissipation (max)5W
Notes
- The 7581 is a direct replacement for the 6L6GC with same pinout (JEDEC 7AC). Brans Vademecum cross-lists it in the 6L6/5932 family.
- Maximum ratings (pentode): Va=500V, Vs=450V (Vs_max = 500V with screen tied to plate in push-pull), Pa=30W, Ps=5W.
- Triode-strapped characteristics at Va=Vg2=250V, Vg1=-20V: mu=8, gm=4700 µmhos, ra=1700 Ω, Ia=40mA. Max Pa (triode) not specified separately.
- Maximum ratings (triode connection): Va=450V.
- Pentode Class A1 at Va=250V, Vs=250V: gm=6000 µmhos, ra=22500 Ω, Ia=72mA, Pout=6.5W into 2500 Ω (same as 6L6).
- Pentode Class A1 at Va=350V, Vs=250V: gm=5200 µmhos, ra=33000 Ω, Ia=54mA, Pout=10.8W into 4200 Ω.
- Rg_max: 0.1 MΩ (fixed bias), 0.5 MΩ (cathode bias) — applies to both pentode and triode connections.
- Basing is 6-pin Octal with short medium shell, low-loss material (JEDEC 7AC, outline drawing 17-19). Pins 1 and 6 have no physical pin.
Pin Layout — Octal
1No Connection
2Heater
3Plate
4
Socket-Compatible Tubes ⚠ Not electrically compatible
Heater-to-Cathode Voltage (max)Heater-to-Cathode Voltage (max)
200V
Max Grid ResistorMax Grid Resistor (fixed bias)0.1MΩ
Typical Operating
Anode Voltage (typical)Plate Voltage250V
Screen Voltage (typical)Screen Voltage250V
Grid 1 Bias VoltageGrid 1 Bias Voltage-14V
Anode CurrentPlate Current72mA
Screen CurrentScreen Current5mA
Anode ResistancePlate Resistance (approx.)22,500Ω
TransconductanceTransconductance6mA/V
Output
Output PowerPower Output (SE Class A1, Va=350V, Vs=250V)10.8W
Load ImpedanceLoad Resistance (SE Class A1, Va=350V)4,200Ω
Capacitances
Grid-to-Plate CapacitanceGrid-to-Plate Capacitance0.6pF
Input CapacitanceInput Capacitance10pF
Output CapacitanceOutput Capacitance6.5pF
Section 2 — Triode Strapped
Absolute Maximum
Anode Voltage (max)Plate Voltage (max, triode)450V
Max Grid ResistorMax Grid Resistor (fixed bias)0.1MΩ
Typical Operating
Anode Voltage (typical)Plate Voltage250V
Grid 1 Bias VoltageGrid 1 Bias Voltage-20V
Anode CurrentPlate Current40mA
Anode ResistancePlate Resistance (approx.)1,700Ω
TransconductanceTransconductance4.7mA/V
Amplification Factor (μ)Amplification Factor8—
Output
Output PowerPower Output (SE Class A1)1.4W
Load ImpedanceLoad Resistance (SE Class A1)5,000Ω
Heater
Heater VoltageHeater Voltage6.3V
Heater CurrentHeater Current0.9A
Screen Grid