12EQ7 — Diode-Remote-Cutoff Pentode

The 12EQ7 is a multi-unit miniature vacuum tube containing a diode and a remote-cutoff pentode sharing a common unipotential cathode. Housed in a 9-pin miniature envelope, this device is specifically engineered for service in AM and AM/FM broadcast receivers. Its primary applications include functioning as a combined intermediate-frequency (IF) amplifier and AM detector. The pentode section is also highly suitable for radio-frequency (RF) or IF amplification, while the diode section is designed for detection or automatic volume control (AVC) duties.

Physical Construction and Basing

The 12EQ7 is constructed using a T-6 1/2 glass bulb with a small button 9-pin noval base (designated JEDEC E9-1). The tube conforms to the JEDEC 6-3 outline drawing and utilizes the JEDEC 9LQ basing diagram. It is designed to operate in any mounting position.

The physical dimensions of the tube are as follows:

• Maximum Diameter: 0.875 inches
• Maximum Seated Height: 2.375 inches
• Maximum Overall Height: 2.625 inches

The 9LQ basing configuration provides the following pin connections, viewed from the bottom of the tube:

• Pin 1: Grid No. 3 (G3 / Suppressor Grid)
• Pin 2: Grid No. 1 (G1 / Control Grid)
• Pin 3: Cathode (K)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: Grid No. 2 (G2 / Screen Grid)
• Pin 7: Pentode Plate (1P)
• Pin 8: Diode Plate (2P)
• Pin 9: Internal Shield (IS)

Heater Characteristics and Ratings

The 12EQ7 features an indirectly heated, coated unipotential cathode. The heater is designed for series or parallel operation with the following electrical specifications:

Maximum Heater-Cathode Ratings

To ensure reliable operation and prevent insulation breakdown, the voltage difference between the heater and the cathode must be kept within specified limits:

• Heater Negative with Respect to Cathode: 200 V (Peak or Total DC and Peak)
• Heater Positive with Respect to Cathode: 200 V (Peak or Total DC and Peak), with the DC component restricted to a maximum of 100 V.

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 12EQ7 are measured without an external shield. These values define the capacitive coupling between elements, which is critical for stable high-frequency RF and IF amplification:

Maximum Ratings

The maximum ratings for the 12EQ7 are established under the Design-Maximum System. Operating the tube beyond these limits may impair performance or shorten its operating life.

Pentode Unit (Class A1 Amplifier)

• Plate Voltage: 300 V
• Grid No. 3 (Suppressor Grid) Voltage (Positive or Negative): 300 V
• Grid No. 2 (Screen Grid) Supply Voltage: 300 V
• Grid No. 2 Voltage: See Rating Chart (dependent on operating conditions)
• Grid No. 1 (Control Grid) Positive Bias Value: 0 V
• Grid No. 1 (Control Grid) Negative Bias Value: 50 V
• Plate Dissipation: 3.0 Watts
• Grid No. 3 Input: 0.2 Watt
• Grid No. 2 Input (for Grid No. 2 voltages up to 150 V): 0.6 Watt
• Grid No. 2 Input (for Grid No. 2 voltages between 150 V and 300 V): See Rating Chart
• Bulb Temperature (at hottest point on bulb surface): 150 °C

Diode Unit

• Plate Current: 1.0 mA (Design-Maximum continuous value; Sylvania lists a maximum rating of 3.0 mA)

Typical Operating Characteristics

The typical operating parameters of the pentode unit under Class A1 amplifier conditions demonstrate its performance as a high-gain, remote-cutoff stage:

The remote-cutoff characteristic of the control grid is illustrated by the bias voltage required to reduce the transconductance. A negative bias of approximately -20 V reduces the transconductance from its nominal 3800 µmhos down to 40 µmhos, allowing effective automatic volume control (AVC) without introducing severe distortion on strong signals.

Diode Unit Characteristics

The diode unit is characterized by its instantaneous voltage drop under typical detection or AVC conditions:

• Tube Voltage Drop (for plate current of 2.0 mA): 10 V
• Average Diode Current (with 10 V DC applied): 2.0 mA

Related Designations

The 12EQ7 belongs to a family of identical multi-unit tubes designed with different heater configurations to accommodate various receiver power supply topologies:

• 6EQ7: Identical to the 12EQ7 except for its heater ratings, which require a 6.3 V supply at a current of 300 mA (0.3 A).
• 20EQ7: Identical to the 12EQ7 except for its heater ratings, which require a 20 V supply at a current of 100 mA (0.1 A).

These tubes share the same physical dimensions, 9LQ basing diagram, capacitances, and electrical characteristics for both the pentode and diode units, but they are not directly interchangeable without modifying the heater supply circuit.

Technical Summary

The 12EQ7 is a versatile, 9-pin miniature diode-remote-cutoff pentode designed for AM and AM/FM broadcast receiver applications. Combining an IF/RF amplifier pentode and a detection/AVC diode in a single T-6 1/2 envelope, it optimizes space and component count in consumer electronics. With a 12.6 V, 150 mA heater, the tube is well-suited for series-string heater arrangements. The pentode unit provides a transconductance of 3800 µmhos and a plate resistance of 0.25 MΩ at a typical plate and screen potential of 100 V, while its remote-cutoff grid design allows for smooth gain control over a wide range of signal strengths.