12AZ7 — High-Mu Twin Triode

An indirectly heated, dual-section miniature receiving valve designed for high-gain voltage amplification, frequency conversion, and cathode-drive amplifier service in high-frequency and audio-frequency applications.

Overview

The 12AZ7 is a general-purpose, high-mu twin triode of the 9-pin miniature (Noval) construction. It contains two independent, high-gain triode units within a single T-6 1/2 glass envelope. Each section is equipped with its own separate cathode and control grid, while sharing a common, mid-tapped heater. This physical separation of the cathodes provides excellent circuit flexibility, allowing the two sections to be operated in different configurations or at different bias levels.

The 12AZ7 is designed for a wide range of applications in VHF television receivers and audio systems. Its documented uses include service as a cathode-drive amplifier, a frequency converter (functioning as a combined oscillator and mixer), and a general-purpose audio-frequency (AF) voltage amplifier.

Mechanical Specifications and Physical Dimensions

The 12AZ7 is housed in a standard T-6 1/2 glass bulb with a small-button Noval 9-pin base (JETEC No. E9-1). The tube is designed to operate reliably in any mounting position.

Its physical dimensions are as follows:

• Maximum Overall Length: 2-3/16 inches
• Maximum Seated Length: 1-15/16 inches
• Length from Base Seat to Bulb Top (excluding tip): 1-9/16 inches  3/32 inch
• Maximum Diameter: 7/8 inch

Heater Configuration and Connections

The heater of the 12AZ7 is mid-tapped, allowing for operation from either a 12.6-volt or a 6.3-volt AC or DC power supply. This makes the tube highly adaptable to both series and parallel heater strings.

Pinout and Socket Connections

The 12AZ7 utilizes the 9A basing diagram. Looking at the bottom of the tube base, the pin connections are assigned as follows:

The separate cathode terminals (Pin 3 and Pin 8) ensure that the two triode units are electrically independent, preventing unwanted galvanic coupling and allowing different bias levels or circuit topologies to be implemented simultaneously.

Direct Interelectrode Capacitances

The interelectrode capacitances of the 12AZ7 vary depending on whether the circuit is configured for grid-drive or cathode-drive operation, and whether an external shield is employed. The values below represent typical measurements with and without an external shield (specifically JETEC No. 315, connected to the cathode of the unit under test).

Grid-Drive Operation

In standard grid-drive configurations, the capacitances for each triode unit are:

Cathode-Drive Operation

For cathode-drive applications, the effective capacitances are:

Maximum Ratings

The ratings for the 12AZ7 are established under the Design-Center System. These represent the maximum limits within which the tube can be operated safely and reliably. The values are specified per individual triode unit:

Note: The DC component of the heater-positive-to-cathode voltage must not exceed 100 volts.

Typical Operating Conditions and Characteristics

The 12AZ7 can be operated under various Class A1 amplifier conditions. Typical operating points for each triode unit are detailed below for two different plate supply voltages:

At a plate voltage of 250 V and a cathode bias resistor of 200 ohms, the tube draws 10 mA of plate current, exhibiting a transconductance of 5,500 mhos and a plate resistance of 10,900 ohms. Under these conditions, the amplification factor remains constant at 60.

Operating Considerations and Characteristic Curves

The maximum ratings of the 12AZ7 are working design-center maximums. To ensure satisfactory performance and longevity, equipment must be designed so that these maximum ratings are not exceeded under normal operating conditions. This includes accounting for power-line supply variations; the design should accommodate fluctuations of up to 10% around a nominal line-center voltage of 117 volts (for both AC and DC line supplies).

Analysis of Average Characteristics

The average plate characteristics (plate current vs. plate voltage) illustrate the behavior of the triode units across a range of grid bias voltages from 0 V down to -12 V.

• At lower plate voltages (e.g., 100 V), the plate current is highly sensitive to grid bias, dropping to near cutoff ($10\ \mu\text{A}$) at approximately -5 V.
• At higher plate voltages (e.g., 250 V), the grid bias must be increased to approximately -12 V to achieve the same cutoff current.

The relationship between the tube's primary parameterstransconductance ($g_m$), plate resistance ($r_p$), and amplification factor ()is also dependent on the operating point:

• Amplification Factor (): Remains remarkably stable near 60 across a wide range of plate currents, particularly when operating at a plate voltage of 250 V.
• Transconductance ($g_m$): Increases dynamically with plate current. At low plate currents (around 2 mA), the transconductance is approximately 3,000 mhos, rising to over 5,500 mhos as the plate current approaches 10 mA and exceeding 6,000 mhos at higher currents.
• Plate Resistance ($r_p$): Decreases as plate current increases. It starts at approximately 20,000 ohms at very low currents, drops to 15,000 ohms at 3.7 mA, and further decreases to 10,900 ohms at 10 mA.

This inverse relationship between $g_m$ and $r_p$ maintains the constant amplification factor ( \approx g_m \times r_p). For example, at the 250 V operating point, the calculation is: $0.0055\text{ S} \times 10,900\ \Omega = 59.95 \approx 60$

Technical Summary

The 12AZ7 is a versatile, 9-pin miniature high-mu twin triode featuring independent unipotential cathodes and a mid-tapped heater configurable for 12.6 V / 0.225 A or 6.3 V / 0.450 A operation. With an amplification factor of 60, a transconductance of up to 5,500 mhos, and a plate resistance of 10,900 ohms at typical operating points, it provides high gain and moderate plate impedance.

Its design-center ratings permit a maximum plate voltage of 300 V and a plate dissipation of 2.5 W per unit. The separate cathode connections and low interelectrode capacitances make the 12AZ7 highly effective for a range of demanding applications, including VHF television cathode-drive amplifiers, frequency converters, oscillators, mixers, and general-purpose AF voltage amplifiers.