PL81 — Power Pentode, Noval | TubeofileKnown Equivalents & Substitutes
21A6
Similar Tubes (Not Drop-In Compatible)
Listed as equivalent in Brans — B8A base, 6.3V heater, not pin-compatible Same electrical characteristics but 6.3V/1.05A parallel heater — not a drop-in in 300mA series-string circuits Electrical Specifications
Section 1 — Pentode
Absolute Maximum
Anode Voltage (max)Anode Voltage (max)250V
Screen Voltage (max)Screen Voltage (max)250V
Anode Dissipation (max)Anode Dissipation (max)8W
Screen Dissipation (max)Screen Dissipation (max)4.5W
Cathode Current (max)Cathode Current (max)180mA
Typical Operating
Anode Voltage (typical)Anode Voltage (typical)250V
Screen Voltage (typical)Screen Voltage (typical)250V
Grid 1 Bias VoltageGrid 1 Bias Voltage-38.5V
Anode CurrentAnode Current32mA
Screen CurrentScreen Current
Output
Output PowerOutput Power3.3W
Load ImpedanceLoad Impedance7,800Ω
Capacitances
Grid-to-Plate CapacitanceAnode to Grid 1 Capacitance0.8pF
Anode-to-Cathode CapacitanceAnode-to-Cathode Capacitance0.1pF
Input CapacitanceInput Capacitance (g1 to all except anode)14pF
Output CapacitanceOutput Capacitance (anode to all except g1)6pF
Section 2 — Triode Strapped
Absolute Maximum
Anode Voltage (max)Anode Voltage (max)250V
Anode Dissipation (max)Anode Dissipation (max)8W
Cathode Current (max)Cathode Current (max)180mA
Typical Operating
Anode Voltage (typical)Anode Voltage250V
Heater
Heater VoltageHeater Voltage22V
Heater CurrentHeater Current0.3A
Notes
- 300mA series-string heater (21.5V/0.3A) for AC/DC TV receivers. The 'P' prefix in Mullard/Philips nomenclature denotes a 300mA heater string design.
- Philips interchange lists 21A6 as the American equivalent designation. CV5077.
- Brans Vademecum lists EL44 as equivalent — this is a B8A-base tube, not pin-compatible with PL81's Noval base.
- Electrically identical to EL81 in all electrode characteristics; only the heater differs.
- Not interchangeable with EL81 without heater circuit modification.
Pin Layout — Noval
1Cathode
2Control Grid (g1)
3Heater
Socket-Compatible Tubes ⚠ Not electrically compatible
Heater-to-Cathode Voltage (max)Heater-to-Cathode Voltage (max)100V
Max Grid ResistorMax Grid 1 Resistor0.5MΩ
2.4mA
Anode ResistanceAnode Resistance15,000Ω
TransconductanceTransconductance4.6mA/V
Grid 1 Bias VoltageGrid 1 Bias Voltage
-38V
Anode CurrentAnode Current40mA
Anode ResistanceAnode Resistance1,000Ω
TransconductanceTransconductance5.5mA/V
Amplification Factor (μ)Amplification Factor (μ)5.5—