12AU8 — Triode-Pentode Miniature Vacuum Tube

An indirectly heated, multi-section miniature valve combining a sharp-cutoff pentode and a medium-mu triode within a single envelope, designed for television receiver applications.

Overview

The 12AU8 is a 9-pin miniature vacuum tube containing two electrically independent sections: a sharp-cutoff pentode and a medium-mu triode. Each section is constructed with its own separate cathode, allowing them to operate independently in distinct circuit stages.

The tube is designed primarily for use in monochrome and color television receivers. Within these systems, the pentode section is specifically suited for service as a video amplifier, video intermediate-frequency (IF) amplifier, and sound IF amplifier. The triode section is intended for use as a sweep oscillator, sync amplifier, separator, or clipper. Except for its heater ratings, the 12AU8 is electrically and physically identical to the 6AU8.

Physical Construction, Heater, and Pinout

The 12AU8 is housed in a T-6½ glass bulb with a miniature button 9-pin base (designated JEDEC E9-1) and conforms to the JEDEC 6-3 outline drawing. The tube can be mounted and operated in any physical position.

Heater Characteristics

The tube features a coated unipotential cathode structure. The heater is designed for series or parallel operation on either AC or DC supplies.

The heater warm-up time is defined under specific test conditions: it is the time required for the voltage across the heater to reach 80% of its rated voltage after applying four times the rated heater voltage to a circuit consisting of the tube heater in series with a resistance valued at three times the nominal heater operating resistance.

Pin Connections

The JEDEC 9DY basing diagram provides the following pinout configuration, viewed from the bottom of the tube:

• Pin 1: Triode Cathode (IK)
• Pin 2: Triode Grid (G)
• Pin 3: Triode Plate (P)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: Pentode Grid #3, Pentode Cathode, Internal Shield (G3, K_p, IS)
• Pin 7: Pentode Grid #1 (G1)
• Pin 8: Pentode Grid #2 / Screen Grid (G2)
• Pin 9: Pentode Plate (P)

Direct Interelectrode Capacitances

These capacitance values are measured directly without the use of an external shield:

Additionally, the capacitive coupling between the independent internal sections is rated as follows:

• Pentode Grid #1 to Triode Plate: 0.005 pF
• Triode Grid to Pentode Plate: 0.002 pF
• Pentode Plate to Triode Plate: 0.12 pF

Maximum Ratings

The maximum ratings for the 12AU8 are established under the Design-Maximum System. These represent limiting values that must not be exceeded under the worst probable operating conditions, taking into account supply-voltage variations, component tolerances, equipment control adjustments, load variations, and environmental factors.

Typical Operating Conditions and Characteristics

When operated as a Class A1 amplifier, the typical operating parameters for each section are defined as follows:

Analysis of Characteristic Curves

Pentode Section Curves

• Plate Characteristics: At a screen grid voltage ($E_{c2}$) of 125 V, the plate current ($I_b$) rises rapidly at low plate voltages before flattening into a horizontal pentode knee. The curves demonstrate sharp-cutoff behavior as the negative grid #1 voltage ($E_{[c1](/tubes/C1)}$) is increased down to -5.0 V. A boundary line for the maximum plate dissipation of 3.0 W restricts the safe operating region at higher plate voltages.
• Positive Grid #1 Operation: Under positive grid #1 bias conditions (up to +6.0 V), grid #1 current ($I_{[c1](/tubes/C1)}$) and grid #2 current ($I_{c2}$) increase significantly, particularly at low plate voltages where the plate is less effective at collecting electrons.
• Transfer Characteristics: At a constant plate voltage ($E_b$) of 200 V, both plate current and screen grid current are plotted against grid #1 voltage for various screen potentials ($E_{c2} = 50\text{ V}$ to $150\text{ V}$). These curves show that higher screen voltages shift the transfer curves to the left, requiring a more negative grid bias to achieve cutoff.
• Transconductance ($g_m$): The transconductance curves show a steep rise as grid #1 bias approaches 0 V, peaking near 10,000 µmhos when the screen voltage is set to 150 V.
• Screen Rating Chart: The screen dissipation rating is bounded by a permissible operating area. Screen dissipation is allowed at its maximum of 0.6 W up to a screen potential of 150 V, above which it must be derated linearly down to 0 W at 300 V.

Triode Section Curves

• Plate Characteristics: The triode plate characteristics show plate current plotted against grid voltage (from 0 V to -14 V) and plate voltage (up to 400 V). The curves exhibit typical triode curvature, with a maximum plate dissipation limit line of 2.5 W defining the safe operating envelope.
• Dynamic Characteristics: Plots of transconductance ($g_m$), plate resistance ($r_p$), and amplification factor ($\mu$) against plate current ($I_b$) reveal that the amplification factor remains remarkably stable near 40 across most of the operating range. Meanwhile, transconductance increases and plate resistance decreases as plate current rises.

Technical Summary

The 12AU8 is a versatile 9-pin miniature triode-pentode designed for television receiver circuitry. It features a 12.6 V, 0.3 A heater with a controlled warm-up time of approximately 11 seconds. The sharp-cutoff pentode section provides high transconductance (7,000 µmhos) and high plate resistance (150,000 Ω) for intermediate-frequency and video amplification. The medium-mu triode section features an amplification factor of 40 and a plate resistance of 8,200 Ω, making it highly suitable for sweep oscillator, sync separator, and clipper applications. Both sections are electrically independent with separate cathodes, allowing flexible circuit configurations within a single compact envelope.