18 — Power Pentode Vacuum Tube

The 18 is an indirectly heated power pentode designed primarily for audio power amplification. Classified as a replacement type, it is characterized by its 14.0-volt heater and is designed to operate in any mounting position. The tube is electrically similar to types 2A5 and 42, and shares characteristic curves and general operating parameters with type 6F6G, differing primarily in its heater ratings.

Physical Specifications and Mounting

The type 18 is constructed with an ST 14 glass envelope and is equipped with a 6-pin base. The physical dimensions of the tube are as follows:

• Maximum Diameter: 1-9/16 inches
• Maximum Overall Length: 4-7/8 inches
• Mounting Position: Any

Heater Ratings

The heater of the type 18 is designed for indirect heating and can be operated using either an AC or DC supply.

• Heater Voltage: 14.0 V
• Heater Current: 0.3 A
• Calculated Heater Power: 4.2 W (calculated as 14.0 V multiplied by 0.3 A)

Pin Configurations

There are two distinct pinout configurations recorded for the 6-pin base, depending on whether it is classified as a U.X. base or a medium 6-pin 6-B base.

U.X. Base (6-Pin) Configuration

In this layout, the pin connections are assigned as follows:

• Pin 1: Heater (H)
• Pin 2: Anode (A)
• Pin 3: Screen Grid (G2)
• Pin 4: Control Grid (G1)
• Pin 5: Suppressor Grid (G3) / Cathode
• Pin 6: Heater (H)

Medium 6-Pin (6-B) Configuration

In this alternative layout, the pin connections are assigned as follows:

• Pin 1: Screen Grid (G2)
• Pin 2: Plate (P)
• Pin 3: Heater (H)
• Pin 4: Heater (H)
• Pin 5: Cathode (K) / Suppressor Grid (G3) (internally connected to Cathode)
• Pin 6: Control Grid (G1)

Typical Operating Conditions and Characteristics

Under typical Class A amplifier operating conditions, the type 18 power pentode exhibits the following electrical characteristics:

For additional operating conditions, characteristics, and curves, the type 18 is comparable to type 42 and type 6F6G, which share similar electrical parameters under typical operation.

Related Designations and Comparisons

The type 18 is closely related to several other power pentodes:

• Type 42 and Type 2A5: The type 18 is described as similar to these types, with the primary distinction being the heater voltage and current ratings.
• Type 6F6G: The type 18 shares characteristic curves and general operating information with this type.

These relationships highlight the tube's design heritage as an indirectly heated power pentode, but direct substitution requires accounting for the different heater voltages and pin configurations.

Technical Summary

The 18 is an indirectly heated power pentode featuring a 14.0 V, 0.3 A heater, housed in an ST 14 envelope with a 6-pin base. It is capable of delivering 3.2 watts of audio power output under typical Class A conditions with a 250 V anode and screen supply. Its electrical characteristics align closely with types 42, 2A5, and 6F6G, making it a specialized replacement type for applications requiring a 14-volt heater supply.