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Voltage - Triode (under 100V) ·Diode-Diode-Triode

12FT6

Double-diode triode for second-detector and audio-driver service in hybrid automobile radios. The diodes recover and rectify the signal while the low-voltage triode drives subsequent transistorized circuitry.

Directly heatedCold cathodeCommon cathodeDissimilar

Sections: 3
Base: 7BT
Socket code: B7G
Base pins: 7
Nominal heater: 12.6 V / 150 mA

Description

12FT6 — Duo-Diode-Triode Miniature Vacuum Tube

An indirectly heated, multi-unit miniature vacuum tube designed for low-voltage detection and audio driver applications in automotive receiver circuits.

Overview

The 12FT6 is a multi-unit miniature vacuum tube containing two diodes and one high-mu triode in a single envelope. It is specifically designed for service in hybrid (tube and transistor) automobile radio receivers that operate directly from a standard 12-volt storage battery. In these systems, the 12FT6 performs the dual functions of second detector and audio driver amplifier, operating its heater and plate circuits directly from the vehicle's battery supply without the need for a high-voltage step-up power supply.

Physical Construction and Basing

The 12FT6 is constructed in a T-5 1/2 glass bulb utilizing a miniature 7-pin base. The tube features a coated unipotential cathode and is designed to operate in any mounting position.

The physical dimensions of the device are:

Maximum overall length: 2-1/8 inches
Maximum seated height: 1-7/8 inches
Maximum diameter: 3/4 inch

The bottom view of the miniature 7-pin base (7BT designation) provides the following pin connections:

Pin 1: Grid
Pin 2: Cathode
Pin 3: Heater
Pin 4: Heater
Pin 5: Diode Plate 2
Pin 6: Diode Plate 1
Pin 7: Triode Plate

Heater Characteristics and Automotive Design

The heater of the 12FT6 is engineered specifically for the demanding electrical environment of automotive applications. While the nominal heater rating is 12.6 V at 0.15 A (suitable for either AC or DC operation), the heater is designed to operate over a wide voltage range of 10.0 to 15.9 V. This wide operating range ensures that the tube continues to function reliably and safely despite the voltage fluctuations commonly encountered in automotive electrical systems during battery charging or discharging.

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 12FT6, measured without an external shield, are:

Grid to Plate: 2.0 µµF
Input: 1.8 µµF
Output: 1.1 µµF
Diode 1 to Diode 2: 0.9 µµF

Maximum Ratings

The maximum ratings for the 12FT6 are established under the Design Center System. These limits must be respected to ensure reliable operation and longevity:

Parameter	Rating
Heater Voltage	12.6 V
Maximum Plate Voltage	30 V
Maximum Cathode Current	20 mA
Maximum Average Diode Current	1 mA
Maximum Grid #1 Resistance	10 MΩ
Maximum Peak Heater-to-Cathode Voltage	±30 V

The maximum ratings are designed with an adequate safety factor to withstand the wide variations in supply voltages typical of automotive battery service.

Typical Operating Conditions and Characteristics

The triode section of the 12FT6 operates as a Class A amplifier. Because it is designed for low-voltage hybrid systems, its typical operating points are specified at a plate voltage of 12.6 V:

Parameter	Operating Point A	Operating Point B
Heater Voltage	12.6 V	12.6 V
Heater Current	0.15 A	0.15 A
Plate Voltage	12.6 V	12.6 V
Grid #1 Voltage	0 V	—
Grid #1 Circuit Resistance	0 Ω	2.2 MΩ
Plate Resistance (Approx.)	7,600 Ω	13,000 Ω
Transconductance	1,900 µmhos	1,000 µmhos
Amplification Factor	15.0	14.0
Plate Current	2.0 mA	0.6 mA

For the diode units, the average diode current is 3.0 mA per diode when a potential of 10 volts is applied to each diode.

Technical Summary

The 12FT6 is a miniature, 7-pin duo-diode-triode designed for low-voltage automotive applications. By combining a triode amplifier with two independent diode detectors, it serves as an efficient second detector and audio driver in hybrid automobile receivers. Operating directly from a nominal 12-volt battery supply, the tube features a rugged heater designed to tolerate a 10.0 to 15.9 V range, a maximum plate voltage rating of 30 V, and typical triode amplification factors of 14 to 15.

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.

Terminal	Section	Connection	Evidence
PIN11G1	A	G1 — Control Grid EIA source label 1G1; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 128, 7BT
PIN2K	A	K — 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
PIN52P	B	P — Plate (Anode) EIA source label 2P; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 128, 7BT
PIN63P	C	P — Plate (Anode) EIA source label 3P; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 128, 7BT
PIN71P	A	P — 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

Triode

X-ray radiationInterelectrode capacitances

Plate resistance: 7600 Ω
Transconductance: 0.0019 S
Amplification factor: 15
Max plate voltage: 30 V
Max cathode current: 20 mA

Triode audio-driver section; published ratings/characteristics promoted where applicable.

Section 2

Diode

X-ray radiationInterelectrode capacitances

Rectifier nominal output current: 1 mA

Representative of two identical diodes.

Section 3

Diode

X-ray radiationInterelectrode capacitances

No fixed ratings are recorded for this section.

Added approved missing physical diode section row.

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

1961 Tung Sol

Published operating points and test conditions from this source.

SECTION 1

Triode

Other

Heater voltage
12.6 V
Heater current
150 mA

Capacitance

Grid-to-plate capacitance
2 pF

SECTION 2

Diode

Other

Maximum average diode current
1 mA
EachDiode

Capacitance

Diode-to-diode capacitance
0.9 pF

Data set 02

1966 Tung Sol Electronic Tubes 2 of 4

Published operating points and test conditions from this source.

SECTION 1

Triode

Other

WITH "A" AND "B" SUPPLY DIRECTLY FROM A
12 V
Published datasheet
HEATER VOLTAGE
12.6 V
Maximum ratings
MAXIMUM PLATE VOLTAGE
3 V
Maximum ratings; published column 1
MAXIMUM PLATE VOLTAGE
0 V
Maximum ratings; published column 2
MAXIMUM GRID #1 RESISTANCE
10000000 ohm
Maximum ratings
MAXIMUM PEAK HEATER TO CATHODE VOLTAGE ±
3 V
Maximum ratings; published column 1
MAXIMUM PEAK HEATER TO CATHODE VOLTAGE ±
0 V
Maximum ratings; published column 2
i HEATER CURRENT
150 mA
Typical operating conditions
i PLATE VOLTAGE
12.6 V
Typical operating conditions; published column 1
GRID #1 VOLTAGE
0 V
Typical operating conditions
GRID #1 CIRCUIT RESISTANCE
0 ohm
Typical operating conditions; published column 1
GRID #1 CIRCUIT RESISTANCE
22000000 ohm
Typical operating conditions; published column 2
PLATE RESISTANCE (APPROX.)
7600 ohm
Typical operating conditions; published column 1
PLATE RESISTANCE (APPROX.)
13 ohm
Typical operating conditions; published column 2
PLATE RESISTANCE (APPROX.)
0 ohm
Typical operating conditions; published column 3
TRANSCONDUCTANCE
1900 uS
Typical operating conditions; published column 1
TRANSCONDUCTANCE
1 uS
Typical operating conditions; published column 2
TRANSCONDUCTANCE
0 uS
Typical operating conditions; published column 3
PLATE CURRENT
2 mA
Typical operating conditions; published column 1
PLATE CURRENT
0.6 mA
Typical operating conditions; published column 2
THIS TUBE
1 V
Typical operating conditions; published column 1
THIS TUBE
8 V
Typical operating conditions; published column 3
Maximum plate voltage
30 V
Published datasheet; promoted section value
Maximum cathode current
20 mA
Published datasheet; promoted section value

Capacitance

GRID TO PLATE
2 pF
Interelectrode capacitances

Characteristic

Plate resistance
7600 ohm
Published datasheet; promoted section value
Transconductance
0.0019 A/V
Published datasheet; promoted section value
Amplification factor
15
Published datasheet; promoted section value

SECTION 2

Diode

Capacitance

DIODE
1 pF
Interelectrode capacitances; published column 1
DIODE
2 pF
Interelectrode capacitances; published column 2
DIODE
0.9 pF
Interelectrode capacitances; published column 3

Other

MAXIMUM AVERAGE DIODE CURRENT
1 mA
Maximum ratings
WITH
10 V
Typical operating conditions; published column 1
WITH
3 V
Typical operating conditions; published column 2
Maximum/average diode output current
1 mA
Published datasheet; promoted section value

Characteristic

Average diode current at 10 V applied
3 mA
Each diode