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

12AT6

Double-diode high-mu triode widely used for radio detection, AVC generation, and audio voltage amplification. Paired diodes handle detection and control-voltage rectification while the triode supplies subsequent gain.

Directly heatedCold cathodeCommon cathodeDissimilar
Sections
3
Base
7BT
Socket code
B7G
Base pins
7
Nominal heater
12.6 V / 150 mA

Description

12AT6 — Twin-Diode High-Mu Triode

Overview

The 12AT6 is a miniature, seven-pin multi-unit vacuum tube containing a high-mu triode and two independent diodes sharing a common cathode. It is designed for use as a combined detector, amplifier, and automatic volume control (AVC) tube in automobile and AC-operated radio receivers. The triode section is engineered to provide an output voltage swing sufficient to drive most beam power output tubes to full power.

The 12AT6 is electrically identical to the 6AT6 except for its heater ratings, which are configured for 12.6-volt operation rather than 6.3-volt operation.

Mechanical Construction and Pinout

The 12AT6 is constructed in a T-5 1/2 glass envelope with a miniature button 7-pin base (JETEC E7-1, outline 5-2 or 5C). It features a coated unipotential cathode that is common to both the triode and the two diode units. The tube can be mounted in any position.

Physical Dimensions

  • Maximum Diameter: 0.750 inches
  • Maximum Seated Height: 1.875 inches (1-7/8")
  • Maximum Overall Height: 2.125 inches (2-1/8")

Pin Connections (7BT Basing)

The 7BT basing arrangement provides the following connections, viewed from the bottom of the tube:

  • Pin 1: Triode Grid (GT)
  • Pin 2: Cathode (K) (Common to triode and diode units)
  • Pin 3: Heater (H)
  • Pin 4: Heater (H)
  • Pin 5: Diode Plate No. 2 (PD2)
  • Pin 6: Diode Plate No. 1 (PD1)
  • Pin 7: Triode Plate (PT)

Heater and Electrical Ratings

The heater is designed for operation from either AC or DC power supplies. The maximum ratings are interpreted according to the Design-Center System (or Design Maximum Rating System).

Heater Specifications

  • Heater Voltage (AC/DC): 12.6 V (with some early documentation noting 12.5 V)
  • Heater Current: 0.15 A (150 mA)
  • Maximum Heater-Cathode Voltage: 90 V (heater positive or negative with respect to cathode)

Triode Unit Maximum Ratings

  • Maximum Plate Voltage: 300 V
  • Maximum Plate Dissipation: 0.5 W
  • Maximum Positive DC Grid Voltage: 0 V

Diode Units Maximum Ratings

  • Maximum Continuous Diode Current (Each Unit): 1.0 mA

Electrical Characteristics and Class A1 Operation

At typical operating points, the triode unit functions as a high-mu Class A1 amplifier. The amplification factor remains constant at 70, while plate resistance and transconductance vary with the applied plate and grid voltages.

Typical Triode Operating Points

  • Operating Point A:
    • Plate Voltage: 100 V
    • Grid Voltage: -1 V
    • Amplification Factor: 70
    • Plate Resistance: 54,000 ̩
    • Transconductance: 1,300 %mhos
    • Plate Current: 0.8 mA
  • Operating Point B:
    • Plate Voltage: 250 V
    • Grid Voltage: -3 V
    • Amplification Factor: 70
    • Plate Resistance: 58,000 ̩
    • Transconductance: 1,200 %mhos
    • Plate Current: 1.0 mA

Diode Unit Characteristics

The two diode plates are positioned around the common cathode sleeve. Under test conditions with 10 V DC applied, each diode unit exhibits an average plate current of 2.0 mA (with some early specifications defining a minimum limit of 0.8 mA).

Resistance-Coupled Amplifier Application

The triode unit of the 12AT6 is commonly employed as a resistance-coupled audio-frequency voltage amplifier. Typical operating parameters for this configuration are detailed below:

  • Plate Supply Voltage: 90 V | 250 V
  • Control Grid Voltage: 0 V | 0 V
  • Plate Load Resistor: 0.22 M̩ (220,000 ̩) | 0.47 M̩ (470,000 ̩)
  • Control Grid Resistor: 10 M̩ | 10 M̩
  • Input Coupling Condenser: 0.01 %F | 0.01 %F
  • Output Coupling Condenser: 0.01 %F | 0.01 %F
  • Grid Resistor of Following Stage: 0.47 M̩ (470,000 ̩) | 0.47 M̩ (470,000 ̩)
  • Maximum Signal Source Impedance: 1,000 ̩ | 1,000 ̩
  • Total Harmonic Distortion: 5% | 5%
  • Peak Output Voltage: 8 V | 34 V
  • Voltage Gain (at 400 Hz): 35 | 46

These operating points demonstrate that higher plate supply voltages combined with larger plate load resistors yield significantly higher peak output voltage swings (increasing from 8 V to 34 V) and higher voltage gains (increasing from 35 to 46) while maintaining distortion at 5%.

Interelectrode Capacitances

The direct interelectrode capacitances of the 12AT6 vary slightly depending on whether an external shield is utilized. When a shield (such as JETEC No. 316) is connected to the cathode (Pin 2), the output capacitance increases due to the shielding effect.

  • Triode Grid to Triode Plate: 2.0 pF (with or without shield; some early sheets list 2.1 pF unshielded)
  • Triode Grid to Cathode and Heater (Input): 2.2 pF (with or without shield; some early sheets list 2.3 pF unshielded)
  • Triode Plate to Cathode and Heater (Output): 1.2 pF (shielded) | 0.8 pF (unshielded; some early sheets list 1.1 pF)
  • Diode Plate No. 2 to Triode Grid (Max): 0.04 pF (with or without shield; some early sheets list 0.025 pF)
  • Diode Plate No. 1 to Triode Grid (Max): 0.07 pF (unshielded)
  • Diode Plate No. 2 to Heater and Cathode: 0.95 pF (unshielded)
  • Diode Plate No. 1 to Heater and Cathode: 0.66 pF (unshielded)

Characteristic Curves and Interpretation

The average plate characteristics of the triode unit show the relationship between plate voltage and plate current across a range of grid bias voltages from 0 V to -7 V. The curves illustrate that the triode operates linearly over a wide voltage range, which is essential for low-distortion audio amplification.

For the diode units, the rectified characteristics show the DC voltage developed across various load resistors (ranging from 0.1 M̩ to 2.0 M̩) as a function of the rectified microamperes and the applied RMS signal input (up to 30 V). These curves assist in designing detector and AVC circuits, allowing engineers to determine the appropriate load resistance to optimize rectified output voltage and linearity.

Related Designations and Comparisons

The 12AT6 is part of a family of double-diode high-mu triodes. It is identical to the 6AT6 except for its heater rating (12.6 V, 0.15 A for the 12AT6 versus 6.3 V, 0.3 A for the 6AT6).

In terms of application and general electrical characteristics (excluding heater ratings and base configurations), the 12AT6 is somewhat similar to older octal and loctal types, including the 6Q7, 6Q7GT, 6SQ7, 6SQ7GT, 7B6, and 7C6. However, these similarities do not imply direct physical or electrical interchangeability due to differences in basing, physical size, and specific operating parameters.

Socket description

Miniature 7-contact

General specifications

Tube and section characteristics

Properties common to every published source and operating point.

Heater and base

Base
7BT
Socket code
B7G
Base pins
7
Nominal heater voltage
12.6 V
Nominal heater current
150 mA
Parallel heater voltage
-
Parallel heater current
-
Series capable
No

7BT - Terminal pinout

Electrical assignments shared by every tube using this base designation.

TerminalSectionConnectionEvidence
PIN11G1AG1 — Control Grid

EIA source label 1G1; independent manufacturer confirmation pending

Unverified

EIA Base Connections, printed page 128, 7BT

PIN2KAK — Cathode

EIA source label K; independent manufacturer confirmation pending

Unverified

EIA Base Connections, printed page 128, 7BT

PIN3H—H — Heater

EIA source label H; independent manufacturer confirmation pending

Unverified

EIA Base Connections, printed page 128, 7BT

PIN4H—H — Heater

EIA source label H; independent manufacturer confirmation pending

Unverified

EIA Base Connections, printed page 128, 7BT

PIN52PBP — Plate (Anode)

EIA source label 2P; independent manufacturer confirmation pending

Unverified

EIA Base Connections, printed page 128, 7BT

PIN63PCP — Plate (Anode)

EIA source label 3P; independent manufacturer confirmation pending

Unverified

EIA Base Connections, printed page 128, 7BT

PIN71PAP — Plate (Anode)

EIA source label 1P; independent manufacturer confirmation pending

Unverified

EIA Base Connections, printed page 128, 7BT

Section ratings

3 physical sections, each with unique specifications.

Section 1

Diode

X-ray radiationInterelectrode capacitances

No fixed ratings are recorded for this section.

Published promoted fields populated semantically; identical repeated sections compressed.

Section 2

Triode

X-ray radiationInterelectrode capacitances
Plate resistance
54000 Ω
Transconductance
0.0013 S
Amplification factor
70
Max plate voltage
300 V
Max plate dissipation
0.5 W

Published promoted fields populated semantically; identical repeated sections compressed.

Section 3

Diode

X-ray radiationInterelectrode capacitances

No fixed ratings are recorded for this section.

Second physical detector diode; duplicated from identical diode section.

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

1975 RCA RC30

Published operating points and test conditions from this source.

SECTION 1

Diode

Other

  • Heater Voltage (ac/dc) 6.3

    12.6 V

    Published in approved 6AT6 target entry; Section2

  • Heater Current 0.3

    150 mA

    Published in approved 6AT6 target entry; Section2

  • Triode Grid to Triode Plate

    2 pF

    Published in approved 6AT6 target entry; Section2

  • Triode Grid to Cathode and Heater

    2.2 pF

    Published in approved 6AT6 target entry; Section2

  • Triode Plate to Cathode and Heater

    0.8 pF

    Published in approved 6AT6 target entry; Section2

  • Plate Voltage

    300 V

    Published in approved 6AT6 target entry; Section2

  • Plate Dissipation

    0.5 W

    Published in approved 6AT6 target entry; Section2

  • Grid Voltage, Positive-bias value

    0 V

    Published in approved 6AT6 target entry; Section2

  • Plate Voltage 100

    250 V

    Published in approved 6AT6 target entry; Section2

  • Grid Voltage —1

    -3 V

    Published in approved 6AT6 target entry; Section2

  • Plate Resistance 54000

    58000 Ω

    Published in approved 6AT6 target entry; Section2

  • Transconductance 1300

    1200 µS

    Published in approved 6AT6 target entry; Section2

  • Plate Current 0.8

    1 mA

    Published in approved 6AT6 target entry; Section2

  • frequency in the order of

    40 MHz

    Published in approved 6AT6 target entry; Section2

SECTION 2

Triode

Other

  • Heater Voltage (ac/dc) 6.3

    12.6 V

    Published in approved 6AT6 target entry; Section1

  • Heater Current 0.3

    150 mA

    Published in approved 6AT6 target entry; Section1

  • Triode Grid to Triode Plate

    2 pF

    Published in approved 6AT6 target entry; Section1

  • Triode Grid to Cathode and Heater

    2.2 pF

    Published in approved 6AT6 target entry; Section1

  • Triode Plate to Cathode and Heater

    0.8 pF

    Published in approved 6AT6 target entry; Section1

  • Plate Voltage

    300 V

    Published in approved 6AT6 target entry; Section1

  • Plate Dissipation

    0.5 W

    Published in approved 6AT6 target entry; Section1

  • Grid Voltage, Positive-bias value

    0 V

    Published in approved 6AT6 target entry; Section1

  • Plate Voltage 100

    250 V

    Published in approved 6AT6 target entry; Section1

  • Grid Voltage —1

    -3 V

    Published in approved 6AT6 target entry; Section1

  • Plate Resistance 54000

    58000 Ω

    Published in approved 6AT6 target entry; Section1

  • Transconductance 1300

    1200 µS

    Published in approved 6AT6 target entry; Section1

  • Plate Current 0.8

    1 mA

    Published in approved 6AT6 target entry; Section1

  • frequency in the order of

    40 MHz

    Published in approved 6AT6 target entry; Section1