KT77

Beam Tetrode — Octal

GEC

Known Equivalents & Substitutes

6CA7

Similar Tubes (Not Drop-In Compatible)

Same Octal pinout and similar ratings but EL34 is a true pentode (pin 1 = suppressor grid) vs KT77 beam tetrode (pin 1 = NC) — different internal topology

Electrical Specifications

Section 1 — Beam Tetrode

Absolute Maximum

Anode Voltage (max)Anode Voltage (max, design)800V

Screen Voltage (max)Screen Voltage (max, design)600V

Anode Dissipation (max)Anode Dissipation (max, design)25W

Screen Dissipation (max)Screen Dissipation (max)6W

Cathode Current (max)Cathode Current (max, design)180mA

Heater-to-Cathode Voltage (max)Heater-to-Cathode Voltage (max, design)150V

Max Grid ResistorMax Grid Resistor (cathode bias, Pa+g2 < 28 W)1MΩ

Typical Operating

Anode Voltage (typical)Anode Voltage (typical)250V

Screen Voltage (typical)Screen Voltage (typical)250V

Grid 1 Bias VoltageGrid 1 Bias Voltage-13.5V

Anode CurrentAnode Current110mA

Screen CurrentScreen Current

Output

Output PowerOutput Power (PP UL, cathode bias)34W

Load ImpedanceLoad Impedance (PP UL, a-a, cathode bias)6,000Ω

Capacitances

Grid-to-Plate CapacitanceGrid-to-Plate Capacitance1pF

Input CapacitanceInput Capacitance (Cg1-all less a)16.5pF

Output CapacitanceOutput Capacitance (Ca-all less g1)9pF

Section 2 — Triode Strapped

Typical Operating

Anode Voltage (typical)Anode Voltage (g2 connected to anode)250V

Grid 1 Bias VoltageGrid 1 Bias Voltage (PP triode, cathode bias)-30V

Anode CurrentAnode Current (Ia + Ig2)120mA

Anode ResistanceAnode Resistance1,050Ω

Transconductance

Heater

Heater VoltageHeater Voltage6.3V

Heater CurrentHeater Current1.4A

Notes

The KT77 is a beam tetrode (internal beam plates connected to cathode at pin 8), unlike the EL34 which is a true pentode with a suppressor grid on pin 1. Pin 1 on the KT77 is No Connection.
GEC specifies ultra-linear connection as the recommended operating mode. Tetrode connection is possible but the fixed screen supply must not exceed 300 V.
Max grid resistor depends on both bias method and dissipation: cathode bias Pa+g2 < 28 W = 1.0 MΩ, > 28 W = 0.5 MΩ; fixed bias Pa+g2 < 28 W = 0.5 MΩ, > 28 W = 0.25 MΩ.
Triode-strapped data from GEC datasheet measured at Va,g2 = 250 V with g2 connected to anode.

Pin Layout — Octal

1No Connection

2Heater

3Anode

Socket-Compatible Tubes ⚠ Not electrically compatible

350B 6550 6F6 6K6 6M6G 6N3C 6V6 6W6 6Y6 7581 KT90

References