12DZ8 — High-Mu Triode Power Pentode

An indirectly heated, multi-section miniature vacuum tube combining a high-mu triode and a power pentode in a single envelope, designed for two-stage audio frequency amplification.

Overview

The 12DZ8 is a multi-section miniature vacuum tube containing a high-mu triode and a power pentode within a single glass envelope. It is designed specifically for use as a two-stage audio frequency amplifier in applications where the plate supply voltage is obtained directly from a single half-wave rectifier connected to a 120-volt AC power line. The tube features a 9-pin miniature base and is designed to operate with a nominal 12.0 V heater supply.

Construction, Mechanical Specifications, and Pinout

The 12DZ8 is constructed with a T-6 1/2 glass bulb and a miniature 9-pin base. It utilizes a coated unipotential cathode and is designed to operate in any mounting position.

Physical Dimensions

• Maximum overall length: 2 5/8 inches
• Maximum seated height: 2 3/8 inches
• Maximum diameter: 7/8 inch

Pin Connections

The bottom view of the miniature 9-pin base provides the following electrode connections:

The heater is designed for operation on either AC or DC with a nominal voltage of 12.0 Volts and a nominal current of 0.45 Ampere.

Electrical Characteristics and Typical Operation

The 12DZ8 contains two independent electrical sections with distinct characteristics optimized for their respective roles in a two-stage audio amplifier.

Triode Section

The triode section is designed as a high-gain voltage amplifier, featuring a high amplification factor. Under typical operating conditions, it exhibits the following characteristics:

• Plate Voltage: 120 Volts
• Grid No. 1 Voltage: 0 Volts
• Cathode Bias Resistor: 1500 Ohms
• Plate Current: 0.8 mA
• Transconductance: 1400 µmhos
• Amplification Factor (μ): 100
• Grid Cutoff Voltage (for a plate current of 20 µA): -2.5 Volts

Pentode Section

The pentode section is designed as a power output stage capable of delivering audio power to a load. Under typical operating conditions, it exhibits the following characteristics:

• Plate Voltage: 145 Volts
• Grid No. 2 Voltage: 120 Volts
• Grid No. 1 Voltage: 0 Volts
• Cathode Bias Resistor: 180 Ohms
• Plate Current: 45 mA
• Grid No. 2 Current: 6 mA
• Transconductance: 7500 µmhos
• Load Resistance: 2500 Ohms
• Power Output: 2.0 Watts

Maximum Ratings

The maximum ratings for the 12DZ8 are established under the Design Center System. These limits represent the maximum values that should not be exceeded under any normal operating conditions.

Similar Designations and Family Relations

The 12DZ8 belongs to a family of identical triode-pentode tubes that share the same internal structure, electrode geometry, and electrical characteristics, differing only in their heater ratings. Except for these heater specifications, the 12DZ8 is identical to the following tube types:

• 6DZ8
• 9DZ8
• 18DZ8
• 35DZ8

These related designations represent versions of the same basic design optimized for different heater supply configurations, such as series-string heater circuits in television or radio receivers.

Technical Summary

The 12DZ8 is a miniature 9-pin triode-pentode vacuum tube featuring a 12.0 V, 0.45 A heater and a coated unipotential cathode. It is designed specifically as a two-stage audio frequency amplifier operating directly from a half-wave rectified 120-volt AC line. The high-mu triode section provides an amplification factor of 100 with a transconductance of 1400 µmhos, while the power pentode section delivers 2.0 Watts of audio power into a 2500 Ohm load with a transconductance of 7500 µmhos. The tube is physically housed in a T-6 1/2 glass bulb and can be operated in any mounting position. Except for its heater ratings, the 12DZ8 is identical to the 6DZ8, 9DZ8, 18DZ8, and 35DZ8 vacuum tubes.