18DZ8 — High-Mu Triode and Power Pentode Vacuum Tube

An indirectly heated, multi-unit miniature valve containing a high-mu triode and a power pentode in a single envelope, designed for compact audio-frequency amplification.

Overview

The 18DZ8 is a multi-unit vacuum tube of the miniature type, combining a high-mu triode and a power pentode within a single glass envelope. It is designed primarily for service as a two-stage audio-frequency amplifier in applications where the plate supply voltage is obtained from a single half-wave rectifier operating directly from a 120-volt AC power line. The tube features a coated unipotential cathode and is housed in a T-6 1/2 glass bulb with a 9-pin miniature base. It is designed to be mounted in any position.

The 18DZ8 is identical to the 6DZ8, 9DZ8, 12DZ8, and 35DZ8 vacuum tubes, with the sole exception of its heater ratings.

Heater and Physical Specifications

The 18DZ8 is equipped with an indirectly heated, coated unipotential cathode. The heater is designed to operate from either an AC or DC supply.

Pinout Configurations

The 18DZ8 is documented with two distinct pinout configurations (viewed from the bottom of the miniature 9-pin base). Circuit designers must verify the specific pin arrangement of the physical device being utilized.

Configuration A

In this arrangement, the triode and pentode connections are distributed as follows:

• Pin 1: Triode Grid (G)
• Pin 2: Triode Cathode (K)
• Pin 3: Pentode Grid #2 (Screen Grid, G2)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: Pentode Plate (P1)
• Pin 7: Pentode Cathode, Grid #3 (Suppressor Grid), and Shield (Ks)
• Pin 8: Pentode Grid #1 (Control Grid, G1)
• Pin 9: Triode Plate (P2)

Configuration B

In this alternative arrangement, the pin assignments are:

• Pin 1: Triode Plate (Pt)
• Pin 2: Pentode Grid #1 (Control Grid, G1)
• Pin 3: Pentode Grid #2 (Screen Grid, G2)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: Pentode Plate (Pp)
• Pin 7: Pentode Cathode, Grid #3 (Suppressor Grid, Kp, G3)
• Pin 8: Triode Cathode (Kt)
• Pin 9: Triode Grid (Gt)

Maximum Ratings (Design-Center System)

The maximum ratings for the triode and pentode sections of the 18DZ8 are defined under the Design-Center System. These limits must not be exceeded under the worst-case operating conditions.

Typical Operating Conditions and Characteristics

The typical operating parameters for each section of the 18DZ8 demonstrate its capability as a combined voltage amplifier and power output stage. The triode section provides high voltage gain, while the pentode section is optimized for power output.

Circuit Application and Design Considerations

The 18DZ8 is specifically tailored for compact, cost-effective audio amplification systems. A typical application is a two-stage audio-frequency amplifier where the high-mu triode serves as the initial voltage preamplifier stage, and the power pentode serves as the single-ended power output stage.

Because the tube is designed to operate with plate and screen voltages that can be derived directly from a 120-volt AC power line via a simple half-wave rectifier, it eliminates the need for an expensive power transformer. In this type of line-connected power supply, the typical operating plate voltage of 120 V for the triode and 145 V for the pentode, along with a screen grid voltage of 120 V, are easily achieved. Under these conditions, the pentode section can deliver a useful power output of 2.0 Watts into a 2500-Ohm load, which is sufficient for driving a loudspeaker in small receivers or utility amplifiers.

When designing circuits with the 18DZ8, the maximum grid circuit resistance limits must be strictly observed: 5.0 Megohms for the triode section and 0.5 Megohms for the pentode section. Additionally, the heater-to-cathode voltage limits must be respected, especially in series-string heater arrangements where the potential difference between the heater and cathode can be substantial. The maximum DC heater-cathode potential is limited to 110 Volts, while the peak heater-cathode voltage (with the heater either positive or negative with respect to the cathode) must not exceed 200 Volts.

Technical Summary

The 18DZ8 is a versatile, space-saving 9-pin miniature double-unit tube containing a high-mu triode (amplification factor of 100) and a power pentode capable of delivering 2.0 Watts of audio output. Operating with an 18.0 V, 0.30 A heater, it is designed for series-heater or transformerless line-operated audio equipment. By combining a high-gain voltage amplifier and a power output stage in a single T-6 1/2 envelope, the 18DZ8 enables the design of compact, low-cost two-stage audio amplifiers operating directly from rectified AC line voltages.