Skip to main content
Tube Tinker — Electron Tube Database
Alternatives & variationsDownload PDF datasheet

Voltage - Triode (with diode, heptode or hexode) ·Hexode-Triode

12K8GT

GT-envelope triode-hexode converter for superheterodyne receivers. The triode generates the local oscillator signal while the hexode performs frequency conversion to the receiver intermediate frequency.

Directly heatedCold cathodeCommon cathodeDissimilar
Sections
2
Base
8KV
Socket code
IO
Base pins
8
Nominal heater
12.6 V / 150 mA

Description

12K8GT — Triode-Hexode Converter Vacuum Tube

The 12K8GT is an international octal-based glass vacuum tube designed for use as a frequency changer (converter) in superheterodyne radio receivers. It is particularly suited for AC/DC receiver designs utilizing 150 mA heater series-strings. The device combines a triode oscillator and a hexode mixer within a single envelope. Its physical design is optimized to minimize interaction between the oscillator and mixer sections, ensuring stable operation at high frequencies as well as across the standard broadcast band.

Except for its heater ratings, the electrical characteristics, physical dimensions, and maximum ratings of the 12K8GT are identical to those of the 6.3 V type 6K8GT.

Construction, Heater, and Pinout

The 12K8GT features an indirectly heated, coated unipotential cathode. The heater is designed for either AC or DC operation.

Heater ParameterValue
Heater Voltage12.6 V
Heater Current0.15 A
Heater SupplyAC or DC

Physical Dimensions

The tube is housed in a T-9 glass bulb with a small wafer 8-pin octal base featuring a metal shell.

  • Maximum Overall Length: 3-1/8 inches
  • Maximum Diameter: 1-5/16 inches

Pin Connections

The 12K8GT uses an 8-pin octal base (basing arrangement 8KV) with an external top cap. The pin connections, viewed from the bottom of the base, are as follows:

  • Pin 1: Metal Shell / Shield (internally connected to the metal shell)
  • Pin 2: Heater
  • Pin 3: Hexode Plate (Anode) [P]
  • Pin 4: Hexode Screen (Grids 2 and 4) [Gs]
  • Pin 5: Oscillator Grid / Hexode Grid 1 [Go] (internally connected)
  • Pin 6: Oscillator Anode (Triode Anode) [Po]
  • Pin 7: Heater
  • Pin 8: Cathode [K]
  • Top Cap: Hexode Control Grid (Signal Input Grid) [g]

Electrical Characteristics and Operating Points

The 12K8GT is typically operated in converter service. The table below outlines the typical operating conditions and characteristics under two different hexode plate voltage conditions:

ParameterOperating Point AOperating Point B
Hexode Plate (P) Voltage100 V250 V
Hexode Screen (Gs) Voltage100 V100 V
Hexode Control Grid (G) Voltage-3 V-3 V
Oscillator Anode (Po) Voltage100 V100 V
Hexode Plate Current2.3 mA2.5 mA
Hexode Screen Current6.2 mA6.0 mA
Oscillator Anode Current3.8 mA3.8 mA
Oscillator Grid (Go) Current0.15 mA0.15 mA
Total Cathode Current12.5 mA12.5 mA
Oscillator Grid Resistor50,000 Ω50,000 Ω
Conversion Transconductance325 µmhos350 µmhos
Hexode Plate Resistance (Approx.)0.4 MΩ0.6 MΩ
Hexode Control Grid Voltage (Approx.)*-30 V-30 V

When the oscillator section is not oscillating, its transconductance is approximately 3,000 µmhos under the condition where the triode plate voltage is 100 V and the triode grid voltage is 0 V.

Maximum Ratings

The maximum ratings for the 12K8GT must be observed to ensure reliable operation. These limits are defined as follows:

  • Maximum Hexode Plate (P) Voltage: 300 V
  • Maximum Hexode Screen (Gs) Supply Voltage: 300 V
  • Maximum Hexode Screen (Gs) Voltage: 150 V
  • Maximum Hexode Plate Dissipation: 0.75 W
  • Maximum Hexode Screen Dissipation: 0.7 W
  • Maximum Total Cathode Current: 16 mA
  • Minimum External Signal Grid (g) Bias Voltage: 0 V
  • Maximum Oscillator Anode (Po) Voltage: 125 V
  • Maximum Oscillator Anode Dissipation: 0.75 W

Interelectrode Capacitances

Direct interelectrode capacitances are measured with an external shield connected to the cathode. The capacitance values are:

  • Signal Grid to Mixer Plate (g to P): 0.08 pF (Maximum)
  • Signal Grid to Oscillator Plate (g to Po): 0.05 pF (Maximum)
  • Signal Grid to Oscillator Grid (g to Go): 0.2 pF (Maximum)
  • Oscillator Grid to Oscillator Plate (Go to Po): 1.8 pF
  • Signal Input (G to all other electrodes): 4.6 pF
  • Oscillator Input (Go to all other electrodes except Po): 6.5 pF
  • Oscillator Output (Po to all other electrodes except Go): 3.4 pF
  • Mixer Output (P to all other electrodes): 4.8 pF
  • Oscillator Grid to Mixer Plate (Go to P): 0.15 pF (Maximum)

Technical Summary

The 12K8GT is an octal-based, indirectly heated triode-hexode converter tube designed for AC/DC superheterodyne radio receivers. Operating with a 12.6 V, 0.15 A heater, it integrates a triode oscillator and a hexode mixer in a single envelope. The tube features a conversion transconductance of 325 to 350 µmhos and a hexode plate resistance of 0.4 to 0.6 MΩ under typical operating conditions. By shielding the oscillator and mixer sections internally, the 12K8GT achieves stable frequency conversion with minimal interaction, making it highly effective for both broadcast and high-frequency applications.

Socket description

International Octal

General specifications

Tube and section characteristics

Properties common to every published source and operating point.

Heater and base

Base
8KV
Socket code
IO
Base pins
8
Nominal heater voltage
12.6 V
Nominal heater current
150 mA
Parallel heater voltage
-
Parallel heater current
-
Series capable
No

8KV - 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

No fixed ratings are recorded for this section.

Published ratings/characteristics promoted where semantically applicable.

Section 2

Hexode

X-ray radiationInterelectrode capacitances
Plate resistance
600000 Ω
Transconductance
0.00035 S
Max plate voltage
300 V
Max screen voltage
150 V

Hexode converter section uses SectionType=Other because Hexode is not present in the locked SectionType vocabulary.

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

1948 Brimar Radio Valve Manual

Published operating points and test conditions from this source.

SECTION 1

Triode

Characteristics

  • Anode Supply Voltage

    250 V

  • Anode Voltage

    100 V

  • Anode Resistor

    40000 ohm

  • Anode Current

    3.8 mA

  • Grid Resistor

    50000 ohm

  • Grid Current

    0.15 mA

SECTION 2

Hexode

Characteristics

  • Anode Voltage

    250 V

  • Anode Current

    2.5 mA

  • Screen Voltage

    100 V

    G2,G4

  • Screen Current

    6 mA

  • Control Grid Voltage

    -3 V

    G3

  • Cathode Bias Resistor

    300 ohm

  • Conversion Conductance

    360 uS

  • Control Grid Voltage for Low Conversion Conductance

    -30 V

    ConversionConductance=0.002mA/V

Data set 02

195X General Electric Essential Characteristics

Published operating points and test conditions from this source.

SECTION 1

Triode

Other

  • Triode oscillator plate voltage

    100 V

    Converter

  • Triode oscillator plate current

    3.8 mA

    Converter

SECTION 2

Hexode

Other

  • Plate voltage

    250 V

    Converter

  • Screen voltage

    100 V

    Converter

  • Grid voltage

    -3 V

    Converter

  • Plate current

    2.5 mA

    Converter

  • Screen current

    6 mA

    Converter

Characteristic

  • Oscillator grid current

    0.15 mA

    Converter

Data set 03

1961 Ferranti Technical Handbook

Published operating points and test conditions from this source.

SECTION 1

Triode

Application

  • Application

    Oscillator

SECTION 2

Hexode

Application

  • Application

    Frequency changer / mixer