12EG6 — Dual-Control Heptode and Cathode-Ray Tube

The designation 12EG6 refers to two entirely distinct electronic devices: a miniature dual-control heptode (or pentagrid amplifier) designed for low-voltage mobile radio applications, and a large electrostatic-focus, electrostatic-deflection cathode-ray oscilloscope tube.

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Part I: The 12EG6 Dual-Control Heptode

Overview and Applications

The heptode variant of the 12EG6 is a miniature, 7-pin receiving tube designed primarily for radio-frequency (RF) amplifier service in "hybrid" automobile receivers. In these receivers, the tube's heater, plate, and screen grid potentials are obtained directly from a nominal 12-volt automotive battery.

The dual-control design features two independent control grids (Grid No. 1 and Grid No. 3). This configuration allows automatic volume control (AVC) voltage to be applied to both grids simultaneously, which significantly reduces back-biasing of the AVC line when the receiver encounters large RF signals.

Construction and Mechanical Specifications

The heptode is housed in a T-5 1/2 glass envelope with a small-button miniature 7-pin base (JETEC No. E7-1, basing diagram EIA 7CH). It utilizes a coated unipotential cathode and is rated for operation in any mounting position.

• Maximum Overall Height: 2 1/8 inches
• Maximum Diameter: 3/4 inch

Pin Connections

Viewed from the bottom of the base, the 7-pin layout (7CH) is connected as follows:

• Pin 1: Grid No. 1 (Oscillator Grid / Control Grid)
• Pin 2: Cathode and Grid No. 5 (Internal Shield)
• Pin 3: Heater
• Pin 4: Heater
• Pin 5: Plate
• Pin 6: Grid No. 2 & Grid No. 4 (Screen Grids)
• Pin 7: Grid No. 3 (Mixer Grid / Control Grid)

Heater Ratings

• Nominal Heater Voltage: 12.6 V (AC or DC)
• Heater Current: 0.15 A
• Automotive Service Range: Designed to operate over a voltage range of 10.0 V to 15.9 V to accommodate the voltage fluctuations of automotive electrical systems. For maximum service life, the recommended operating range is 11 V to 14 V.

Maximum Ratings (Design-Center System)

• Plate Voltage: 16 V to 30 V maximum
• Grids No. 2 & No. 4 (Screen) Voltage: 16 V to 30 V maximum
• Grids No. 2 & No. 4 Supply Voltage: 16 V to 30 V maximum
• Grid No. 3 Voltage (Negative DC): 16 V to 30 V maximum
• Grid No. 3 Voltage (Positive DC): 0 V maximum
• DC Cathode Current: 20 mA maximum
• Peak Heater-Cathode Voltage: 16 V to 30 V (with heater either positive or negative with respect to the cathode)
• Grid No. 3 Circuit Resistance: 10 Megohms maximum

Typical Characteristics and Operating Conditions

When operated as a Class A1 heptode-connected amplifier, the typical operating parameters are:

• Heater Voltage: 12.6 V
• Plate Voltage: 12.6 V
• Grids No. 2 & No. 4 Voltage: 12.6 V
• Grid No. 1 Voltage: -0.6 V to -0.8 V (average bias developed across a 2.2 Megohm grid resistor)
• Grid No. 3 Voltage: -0.8 V (provided by Grid No. 1 voltage through a suitable grid resistor, or with Grid No. 3 connected to Grid No. 1 through a 100,000-ohm resistor)
• Plate Current: 0.4 mA to 0.55 mA (400 µA to 550 µA)
• Screen Current (Grids No. 2 & No. 4): 2.4 mA to 2.8 mA (2400 µA to 2800 µA)
• Transconductance (Grid No. 3 to Plate): 800 µmhos
• Plate Resistance (Approximate): 0.15 Megohms
• Cut-off Characteristics: A negative bias of approximately -3.0 V on Grid No. 1 and Grid No. 3 reduces the Grid No. 3-to-plate transconductance to a range of 10 to 20 µmhos.

Direct Interelectrode Capacitances

Measured with an external shield (EIA/JETEC No. 315 or 316) connected to Pin 2 (Cathode):

• Grid No. 3 to Plate (g3 to p): 0.25 pF maximum
• Grid No. 3 to Grid No. 1 (g3 to g1): 0.15 pF maximum
• Grid No. 1 to Plate (g1 to p): 0.04 pF maximum
• Grid No. 3 Input (g3 to all except p): 6.5 pF
• Grid No. 1 Input (g1 to all except p): 5.7 pF
• Output (p to all except grids): 12 pF
• Grid No. 1 to Cathode (g1 to k+g5): 3.2 pF
• Cathode to All Except Grid No. 1: 23 pF

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Part II: The 12EG6 Cathode-Ray Tube

Overview and Applications

The oscilloscope variant of the 12EG6 is a cathode-ray tube (CRT) designed for visual display applications in oscilloscopes. It utilizes electrostatic focusing and electrostatic deflection to control the electron beam.

Physical and Mechanical Characteristics

• Envelope/Bulb: Glass bulb with a nominal 12-inch diameter (307 mm minimum, 312 mm maximum)
• Neck Diameter: 70 mm
• Overall Length: 635 ± 5 mm
• Minimum Useful Screen Diameter: 280 mm
• Mounting Position: Any
• Base: B12D

Screen Properties

• Fluorescence: Green
• Persistence of Afterglow: Short (10 ms minimum to 100 ms maximum for decay to 1% of initial brightness)

Pin Connections

The B12D base pinout is configured as follows:

• Pin 1: Modulator
• Pin 2: Cathode
• Pin 3: Heater
• Pin 4: Heater
• Pin 5: Anode 1
• Pin 6: Anode 2
• Pin 7: Internal Conductive Coating
• Pin 8: Y2 Deflector Plate
• Pin 9: X2 Deflector Plate
• Pin 10: Anode 3
• Pin 11: X1 Deflector Plate
• Pin 12: Y1 Deflector Plate

Heater Ratings

• Heater Voltage: 4.0 V (AC or DC)
• Heater Current: 1.0 A

Maximum Ratings

• Anode 3 Voltage (Maximum): 5500 V

Typical Operating Conditions

The tube can be operated under different voltage configurations to optimize deflection sensitivity and brightness:

Alignment and Deflection Characteristics

• Trace Alignment: The angle between the X1-X2 trace and the Y1-Y2 trace is 90° ± 3°.
• Spot Centering: The undeflected, focused spot falls within a 15 mm radius circle concentric with the center of the tube face.
• Deflection Polarity: When viewed from the screen with the base spigot uppermost, a positive voltage applied to the X1 plate deflects the spot to the left, while a positive voltage applied to the Y1 plate deflects the spot upwards.

Direct Interelectrode Capacitances

• Modulator to all other electrodes: 15 pF
• Each X Plate to all other electrodes: 15 pF
• Each Y Plate to all other electrodes: 15 pF
• One X to one Y Deflector Plate: 4.0 pF
• Cathode to all other electrodes: 15 pF

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Technical Summary

The 12EG6 designation represents two distinct classes of vacuum devices:

1. A miniature heptode (dual-control pentagrid amplifier) featuring a 12.6 V, 0.15 A heater, designed for low-voltage (12 V battery) automotive RF receiver applications. It provides a transconductance of 800 µmhos and a plate resistance of 0.15 MΩ under typical operating conditions.
2. A 12-inch electrostatic oscilloscope tube featuring a 4.0 V, 1.0 A heater, green fluorescence with short afterglow, and high-voltage operation up to 5500 V on Anode 3. It offers deflection sensitivities of 0.24 to 0.4 mm/V depending on anode voltage configurations.