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Voltage Pentode - Sharp Cutoff (single) ·Pentode

12SJ7

RF pentode with documented pentode and triode-connected Class A operation. Its signal-amplifier characteristics suit RF and general voltage-amplifier stages requiring a sharp-cutoff transfer characteristic.

Directly heatedCold cathodeCommon cathodeDissimilar

Sections: 1
Base: 8N
Socket code: IO
Base pins: 8
Nominal heater: 12.6 V / 150 mA

Description

12SJ7 — Sharp-Cutoff Pentode

The 12SJ7 is a sharp-cutoff pentode designed for use as a radio-frequency amplifier, biased detector, or high-gain audio-frequency amplifier. It is also characterized as a triple-grid detector amplifier. In properly designed circuits, it can also serve as a radio-frequency oscillator or mixer tube. The 12SJ7 is electrically identical to the 6SJ7 and 6SJ7GT, with the exception of its heater characteristics and capacitance ratings. It is available in both a metal shell envelope (12SJ7) and a glass bulb envelope (12SJ7GT).

Construction, Physical Dimensions, and Base Pinout

The tube features a coated unipotential cathode. It is designed with an 8-pin octal base. The metal shell version (12SJ7) utilizes a small wafer octal 8-pin phenolic base (basing designation B6-21 or MT-8), while the glass bulb version (12SJ7GT) utilizes a T-9 glass envelope with a small wafer octal 8-pin base featuring a metal sleeve.

The physical dimensions of the two variants are as follows:

Metal Shell (12SJ7): Maximum overall height of 2-5/8 inches (or up to 2-11/16 inches depending on the manufacturer), maximum seated height of 2-1/16 inches, and a maximum diameter of 1-5/16 inches.
Glass Bulb (12SJ7GT): Maximum overall height of 3-5/16 inches, maximum seated height of 2-3/4 inches, and a maximum diameter of 1-5/16 inches.

The tube can be mounted in any position. The terminal connections, viewed from the bottom of the base (using the 8N or RMA BN basing diagram), are assigned as follows:

Pin 1: Shell and Internal Shield
Pin 2: Heater
Pin 3: Grid Number 3 (Suppressor Grid)
Pin 4: Grid Number 1 (Control Grid)
Pin 5: Cathode
Pin 6: Grid Number 2 (Screen Grid)
Pin 7: Heater
Pin 8: Plate

The heater is designed to operate on either alternating current (AC) or direct current (DC) with a nominal voltage of 12.6 volts and a current of 0.15 amperes (150 mA).

Electrical Characteristics and Operating Points

The 12SJ7 can be operated in either a pentode connection or a triode connection (where the screen grid and suppressor grid are tied directly to the plate).

Class A Pentode Connection

Under typical Class A pentode operating conditions, the suppressor grid (Grid 3) is tied to the cathode at the socket terminal. Typical characteristics at two different plate voltage operating points are:

Parameter	Operating Point A	Operating Point B
Plate Voltage	100 V	250 V
Screen (Grid 2) Voltage	100 V	100 V
Control Grid (Grid 1) Voltage	-3 V	-3 V
Cathode Bias Resistor	—	1,000 Ω
Plate Current	2.9 mA	3.0 mA
Screen Current	0.9 mA	0.8 mA
Plate Resistance (Approx.)	0.7 MΩ	1.0 to 1.5 MΩ
Transconductance (Mutual Conductance)	1,575 µmhos	1,650 µmhos
Control Grid Voltage (for cathode current cut-off)	-9 V	-9 V

Class A Triode Connection

When configured as a triode (with Grid 2 and Grid 3 connected to the plate), the typical operating characteristics are:

Parameter	Operating Point C	Operating Point D
Plate Voltage	180 V	250 V
Control Grid Bias Voltage	-6 V	-8.5 V
Amplification Factor	19	19
Plate Resistance	8,250 Ω	7,600 Ω
Transconductance	2,300 µmhos	2,500 µmhos
Plate Current	6.0 mA	9.2 mA

Maximum Ratings

The maximum ratings of the 12SJ7 depend on the connection type and the rating system applied (Design Center vs. Absolute).

Pentode Connection Ratings

Plate Voltage: 300 V (Design Center / RMA), 330 V (Absolute)
Screen (Grid 2) Voltage: 125 V (Design Center / RMA), 140 V (Absolute)
Screen Supply Voltage: 300 V (Design Center / RMA), 330 V (Absolute)
Grid Bias Voltage: Must never be positive
Plate Dissipation: 2.0 W (RMA) to 2.50 W (Design Center) / 2.75 W (Absolute)
Screen Dissipation: 0.3 W (RMA) to 0.40 W (Design Center) / 0.44 W (Absolute)
D-C Heater-Cathode Voltage: 90 V (Design Center), 100 V (Absolute)

Triode Connection Ratings

Plate Voltage: 250 V (Design Center), 275 V (Absolute)
Grid Bias Voltage: Must never be positive
Plate Dissipation: 2.50 W (Design Center), 2.75 W (Absolute)
D-C Heater-Cathode Voltage: 90 V (Design Center), 100 V (Absolute)

In circuits where the cathode is not directly connected to the heater, the potential difference between the heater and cathode should be kept as low as possible, and must not exceed 100 volts under any conditions. A direct connection between the heater and cathode is recommended.

Interelectrode Capacitances

Capacitances vary depending on whether the tube is a metal shell (12SJ7) or glass bulb (12SJ7GT) type, and whether it is wired in a pentode or triode configuration.

Pentode Connection (with shell and internal shield connected to cathode)

Grid to Plate (Max): 0.005 pF (both 12SJ7 and 12SJ7GT)
Input: 6.0 pF (12SJ7), 6.3 pF (12SJ7GT)
Output: 7.0 pF (12SJ7), 10.0 pF (12SJ7GT)

Triode Connection (with Grid 2 and Grid 3 connected to plate)

Grid to Plate: 2.8 pF
Grid to Cathode: 3.4 pF
Plate to Cathode: 11 pF

Resistance-Coupled Amplifier Operation

The 12SJ7 is highly suited for resistance-coupled audio amplifier service. Under these conditions, the d-c resistance in the grid circuit should not exceed 1.0 megohm under rated maximum conditions. The following table outlines typical operating parameters for a pentode-connected resistance-coupled stage at various plate supply voltages ( $E_{bb}$ ):

Plate Supply ( $E_{bb}$ )	Plate Load ( $R_p$ )	Grid Resistor ( $R_s$ )	Cathode Resistor ( $R_k$ )	Screen Resistor ( $R_{sg}$ )	Voltage Gain	Max Output ( $E_o$ RMS)
90 V	0.10 MΩ	0.10 MΩ	820 Ω	0.27 MΩ	40	19 V
90 V	0.24 MΩ	0.24 MΩ	1,600 Ω	0.68 MΩ	72	20 V
90 V	0.51 MΩ	0.51 MΩ	3,800 Ω	1.50 MΩ	95	21 V
180 V	0.10 MΩ	0.10 MΩ	620 Ω	0.33 MΩ	48	49 V
180 V	0.24 MΩ	0.24 MΩ	910 Ω	0.75 MΩ	95	40 V
180 V	0.51 MΩ	0.51 MΩ	1,800 Ω	1.60 MΩ	153	42 V
300 V	0.10 MΩ	0.10 MΩ	430 Ω	0.43 MΩ	55	69 V
300 V	0.24 MΩ	0.24 MΩ	750 Ω	0.91 MΩ	108	82 V
300 V	0.51 MΩ	0.51 MΩ	1,200 Ω	2.00 MΩ	182	70 V

Note: $E_o$ represents the maximum RMS voltage output for five percent (5%) total harmonic distortion. The value of the grid resistor of the following stage ( $R_{g1}$ ) is non-critical for these operating points.

Characteristic Curves and Interpretation

The plate characteristics of the 12SJ7 in its pentode connection show that above a low plate voltage "knee" (around 30 to 50 volts), the plate current ( $I_b$ ) remains nearly constant as plate voltage increases. This flat response indicates an extremely high plate resistance (ranging from 0.7 to 1.5 megohms depending on operating conditions).

In contrast, the triode-connected plate characteristics display a series of steep, upward-sloping curves. This behavior reflects a much lower plate resistance (7,600 to 8,250 ohms) and a moderate amplification factor of 19.

The transfer characteristics plot transconductance ( $g_m$ ), plate current ( $I_b$ ), and screen current ( $I_{c2}$ ) against the control grid voltage ( $E_{[c1](/tubes/C1)}$ ). At a plate voltage of 250 V and screen voltage of 100 V, the transconductance peaks near 1,650 µmhos at a grid bias of -3 V and falls smoothly toward zero as the grid bias is made more negative, reaching complete plate current cutoff at approximately -9 V.

Related Designations

The 12SJ7 belongs to a family of 12.6-volt octal-base tubes. It is electrically identical to the 6.3-volt 6SJ7 and 6SJ7GT except for its heater requirements (which require 12.6 V at 0.15 A instead of 6.3 V at 0.3 A) and minor capacitance variations. Other related 12.6-volt octal tubes designed for complementary functions in receiver circuits include:

12SK7 / 12SK7-GT: A remote-cutoff (vari-mu) RF pentode.
12SQ7 / 12SQ7-GT: A double-diode high-mu triode.
12SA7 / 12SA7-GT: A pentagrid converter.
12SL7GT: A high-mu twin triode.
12SN7GT / 12SN7GTA: A medium-mu twin triode.

Socket description

International Octal

General specifications

Tube and section characteristics

Properties common to every published source and operating point.

Heater and base

Base: 8N
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

8N - Terminal pinout

Electrical assignments shared by every tube using this base designation.

Terminal	Section	Connection	Evidence
PIN1SH	A	SH — Internal Shield EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N
PIN2H	—	H — Heater EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N
PIN3G3	A	G3 — Suppressor Grid EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N
PIN4G1	A	G1 — Control Grid EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N
PIN5K	A	K — Cathode EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N
PIN6G2	A	G2 — Screen Grid EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N
PIN7H	—	H — Heater EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N
PIN8P	A	P — Plate (Anode) EIA transcription; independent manufacturer confirmation pending	Unverified EIA Base Connections, printed page 129, 8N

Section ratings

1 physical section, each with unique specifications.

Section 1

Pentode

X-ray radiationInterelectrode capacitances

Plate resistance: 700000 Ω
Transconductance: 0.001575 S
Max plate voltage: 330 V
Max screen voltage: 140 V
Max plate dissipation: 2.75 W
Max screen dissipation: 0.44 W
Max cathode current: 6 mA

Promoted from printed maximum/characteristic columns; additional published values retained in TUBE_SECTION_PARAMETER.

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

Pentode

Characteristics

Anode Voltage
250 V
Anode Current
3 mA
Screen Voltage
100 V
Screen Current
0.8 mA
Control Grid Voltage
-3 V
Cathode Bias Resistor
1000 ohm
Control Grid Cutoff Voltage
-9 V

Data set 02

195X General Electric Essential Characteristics

Published operating points and test conditions from this source.

SECTION 1

Pentode

Other

Plate voltage
250 V
ClassA;PentodeConnection
Screen voltage
100 V
ClassA;PentodeConnection
Control-grid voltage
-3 V
ClassA;PentodeConnection
Plate current
3 mA
ClassA;PentodeConnection
Screen current
0.8 mA
ClassA;PentodeConnection
Plate voltage
250 V
ClassA;TriodeConnection
Control-grid voltage
-8.5 V
ClassA;TriodeConnection
Plate current
9.2 mA
ClassA;TriodeConnection

Characteristic

Plate resistance
1000000 Ω
PentodeConnection
Transconductance
1650 µS
PentodeConnection
Plate resistance
7600 Ω
TriodeConnection
Transconductance
2500 µS
TriodeConnection
Amplification factor
19
TriodeConnection

Data set 03

1968 General Electric Co

Published operating points and test conditions from this source.

SECTION 1

Pentode

Other

Typical Plate Voltage
100 V
PentodeConnection
Typical Screen Voltage
100 V
PentodeConnection

Characteristic

Triode-Connection Plate Resistance
7600 Ohm
TriodeConnection
Triode-Connection Amplification Factor
19
TriodeConnection

Data set 04

1971 RCA HB3 Binder 5 of 7

Published operating points and test conditions from this source.

SECTION 1

Pentode

Other

Heater voltage
12.6 V
Heater current
150 mA
Plate voltage
250 V
ClassA
Screen voltage
100 V
ClassA
Grid voltage
-3 V
ClassA
Plate current
3 mA
ClassA
Screen current
1 mA
ClassA
Maximum plate voltage
300 V
Maximum screen voltage
100 V

Characteristic

Plate resistance
1000000 ohm
ClassA
Transconductance
1650 uS
ClassA

Capacitance

Grid-to-plate capacitance
0.015 pF