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Voltage Radio Detector Converter ·Heptode

1612

Low-microphonics industrial pentagrid amplifier-modulator equivalent to the 6L7, intended for radio mixing, modulation, and controlled RF amplification.

Directly heatedCold cathodeCommon cathodeDissimilar

Sections: 1
Base: 7T
Socket code: -
Base pins: 7
Nominal heater: 6.3 V / 300 mA

Description

1612 — Low-Microphonic Pentagrid Mixer and Amplifier

The 1612 is a specialized metal-shell pentagrid tube designed primarily for applications requiring exceptionally low microphonic response. It is electrically identical to the standard 6L7 metal pentagrid mixer but features ruggedized construction and rigorous testing to minimize microphonics. The tube is highly suited for volume expander-compressor circuits, superheterodyne mixers, and sensitive Class A1 audio and control amplifiers.

Construction, Heater, and Base Pinout

The 1612 is constructed with a metal envelope (MT-8 or 8A-1 bulb) and a coated unipotential cathode. It features a small wafer octal 7-pin base (7-T or 7T designation) and a miniature top cap. The top cap provides a direct connection to Grid #1, which minimizes input capacitance and isolates the primary control grid from the other electrodes.

The heater is designed for either AC or DC operation. In circuits where the cathode is not directly connected to the heater, the potential difference between the heater and cathode must be kept as low as possible to prevent hum and leakage.

Heater Parameter	Value
Heater Voltage (AC or DC)	6.3 V
Heater Current	0.3 A

The physical dimensions of the tube are as follows:

Maximum Overall Length: 3-1/8 inches
Maximum Seated Height: 2-9/16 inches
Maximum Diameter: 1-5/16 inches

The tube can be mounted and operated in any position. The 7-pin octal base connections, viewed from the bottom of the tube, are mapped as follows:

Pin / Cap	Connection
Pin 1	Metal Shell
Pin 2	Heater
Pin 3	Plate
Pin 4	Grids #2 and #4 (Screen Grids)
Pin 5	Grid #3 (Control/Oscillator Grid)
Pin 7	Heater
Pin 8	Cathode
Top Cap	Grid #1 (Control/Signal Grid)

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 1612 are specified with the metal shell connected directly to the cathode. The metal envelope acts as an internal shield, reducing stray capacitances and shielding the internal structure:

Grid #1 to Grid #3: 0.20 pF maximum
Grid #1 to Plate: 0.001 pF maximum
Grid #3 to Plate: 0.10 pF maximum
Grid #1 to All Other Electrodes: 7.5 pF
Grid #3 to All Other Electrodes: 10 pF
Plate to All Other Electrodes: 11 pF

Maximum Ratings

The maximum ratings for the 1612 are established under the Design-Center Rating System. Operating the tube beyond these limits may impair its performance and longevity.

Rating Parameter	Maximum Limit
Plate Voltage	250 V
Screen Voltage (Grids #2 & #4)	100 V
Plate Dissipation	1.5 W
Screen Dissipation (Grid-No. 2 Input)	1.0 W

Typical Operating Conditions and Characteristics

The 1612 can be operated either as a Class A1 amplifier or as a mixer in superheterodyne circuits. The dual-control-grid structure (Grid #1 and Grid #3) allows the tube's gain to be controlled by two independent signal sources.

Class A1 Amplifier Service

In Class A1 amplifier service, both control grids are biased to a typical operating point of -3 V. Under these conditions, the tube exhibits the following typical characteristics:

Plate Voltage: 250 V
Screen Voltage (Grids #2 & #4): 100 V
Control Grid #1 Bias: -3 V
Control Grid #3 Bias: -3 V
Plate Resistance: 0.6 MΩ (600,000 Ω)
Transconductance (Grid #1 to Plate): 1100 µmhos
Plate Current: 5.3 mA
Screen Current: 6.5 mA

When a large negative bias is applied to both control grids, the transconductance is sharply reduced. With both Grid #1 and Grid #3 biased to -15 V, the transconductance from Grid #1 to the plate drops to approximately 5 µmhos, demonstrating the tube's remote-cutoff capability.

Mixer Service in Superheterodyne Receivers

When used as a mixer, Grid #1 serves as the radio-frequency (RF) signal input grid, and Grid #3 serves as the local oscillator input grid. The typical operating parameters for mixer service are:

Plate Supply Voltage: 250 V
Screen Supply Voltage (Grids #2 & #4): 100 V
Control Grid #1 Bias: -3 V
Control Grid #3 (Oscillator Grid) Bias: -10 V
Plate Current: 2.4 mA
Conversion Transconductance: 375 µmhos

Applications and Circuit Design

The primary feature distinguishing the 1612 from the standard 6L7 is its low-microphonic construction. During manufacture, the tube undergoes specialized assembly techniques and testing protocols to ensure that mechanical vibrations do not translate into electrical noise. This makes the 1612 highly recommended for high-gain audio preamplifiers, sensitive laboratory instruments, and volume expander-compressor circuits where microphonic feedback would degrade signal quality.

In volume expander-compressor circuits, the dual-grid control capability is utilized by applying the primary audio signal to Grid #1 while applying a variable DC control voltage (derived from the signal envelope) to Grid #3. This varies the gain of the stage dynamically. Because the characteristic curves of the 6L7 apply directly to the 1612, designers can utilize standard 6L7 load lines and transfer curves to calculate circuit performance, distortion, and gain margins.

General specifications

Tube and section characteristics

Properties common to every published source and operating point.

Heater and base

Base: 7T
Socket code: -
Base pins: 7
Nominal heater voltage: 6.3 V
Nominal heater current: 300 mA
Parallel heater voltage: -
Parallel heater current: -
Series capable: No

7T - Terminal pinout

Electrical assignments shared by every tube using this base designation.

No verified pinout is recorded for this base designation.

Section ratings

1 physical section, each with unique specifications.

Section 1

Heptode

X-ray radiationInterelectrode capacitances

Plate resistance: 600000 Ω
Transconductance: 0.0011 S
Max plate voltage: 250 V
Max screen voltage: 100 V
Max plate dissipation: 1.5 W
Max screen dissipation: 1 W

Published promoted values applied where semantically applicable; additional table values retained in section parameters.

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

1950 Sylvania

Published operating points and test conditions from this source.

SECTION 1

Heptode

Characteristic

Equivalent Characteristics Reference
6L7
Source states characteristics and basing are identical to 6L7

Data set 02

195X General Electric Essential Characteristics

Published operating points and test conditions from this source.

SECTION 1

Heptode

Other

Plate voltage
250 V
ClassA;Mixer
Screen voltage
100 V
ClassA;Mixer
Control-grid voltage
-3 V
ClassA;Mixer
Plate current
5.3 mA
ClassA;Mixer
Screen current
6.5 mA
ClassA;Mixer

Characteristic

Plate resistance
600000 Ω
ClassA;Mixer
Conversion transconductance
1100 µS
Grid3=-3V

Data set 03

1960 RCA Industrial and Communications Types

Published operating points and test conditions from this source.

SECTION 1

Heptode

Characteristic

HeaterOrFilamentVoltage
6.3 V
HeaterOrFilamentCurrent
300 mA
Conversion Transconductance
375 uS
SuperheterodyneMixer

Other

OscillatorGridBias
-10 V
Converter

Data set 04

1971 RCA HB3 Binder 3 of 7

Published operating points and test conditions from this source.

SECTION 1

Heptode

Characteristics

Voltage
6.3 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Current
0.3
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #1 to All Other Electrodes — published value 1
1
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #1 to All Other Electrodes — published value 2
7.5
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #3 to All Other Electrodes — published value 1
3
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #3 to All Other Electrodes — published value 2
10
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Plate to All Other Electrodes
11
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 1 -Shell — published value 1
1
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 1 -Shell — published value 2
0
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 1 -Shell — published value 3
5
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 1 -Shell — published value 4
3
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 2 -Heater — published value 1
2
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 2 -Heater — published value 2
7
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 3 -Plate — published value 1
3
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 3 -Plate — published value 2
8
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 4 -Grids #2d #4 — published value 1
4
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 4 -Grids #2d #4 — published value 2
2
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 4 -Grids #2d #4 — published value 3
4
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Pin 4 -Grids #2d #4 — published value 4
1
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Plate
250 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Screen (Grids #2 & #4) — published value 1
2 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Screen (Grids #2 & #4) — published value 2
4 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Screen (Grids #2 & #4) — published value 3
100 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Control Grid (Grid #1) — published value 1
1 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Control Grid (Grid #1) — published value 2
-3 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Control Grid (Grid #3) — published value 1
3 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Control Grid (Grid #3) — published value 2
-3 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Plate Res
0.6 Mohm
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Transcond. (Grid #1- Plate) — published value 1
1
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Transcond. (Grid #1- Plate) — published value 2
1100
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Transcond. (Grid #1 - Plate)' — published value 1
1
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Transcond. (Grid #1 - Plate)' — published value 2
5
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Plate Cur
5.3 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Screen Cur
6.5 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
' Mith Grid I1 Dias = —15 volts; Grid /3 — published value 1
15 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
' Mith Grid I1 Dias = —15 volts; Grid /3 — published value 2
3 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.

Other

Grid #1 to Grid #3 — published value 1
1 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #1 to Grid #3 — published value 2
3 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #1 to Grid #3 — published value 3
0.2 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #1 to Plate — published value 1
1 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #1 to Plate — published value 2
0.001 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #3 to Plate — published value 1
3 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Grid #3 to Plate — published value 2
0.1 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Plate Voltage
250 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Screen Voltage
100 V
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Plate Dissipation
1.5 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.
Screen Dissipation
1 mA
Published datasheet value; multiple numbers on one source row represent separate limits or operating conditions.