12K8 — Triode-Hexode Converter

The 12K8 (including its glass-octal variant, the 12K8-GT) is a metal-envelope triode-hexode converter designed to perform simultaneously the functions of mixer and oscillator in superheterodyne receivers, particularly those of the all-wave type. By combining a triode oscillator and a hexode mixer within a single envelope, the 12K8 provides efficient frequency conversion with minimized space and wiring requirements.

Physical Specifications and Basing

The 12K8 features a metal shell envelope (MT-8) with a phenolic small wafer octal 8-pin base (basing designation B8-21). The tube is designed to be mounted in any position. A miniature top cap (C1-4) provides the connection for the hexode control grid (Grid Number 3).

Physical Dimensions

• Maximum Overall Length: 3-1/8 inches (3.125 in)
• Maximum Diameter: 1-5/16 inches (1.3125 in)
• Seated Height: 2-7/16 inches ± 1/8 inch

Terminal Connections

The 12K8 utilizes the 8K basing arrangement, viewed from the bottom of the base:

The heater is designed for operation from either an AC or DC supply, with a nominal voltage of 12.6 V and a current draw of 0.15 A. The cathode is a coated unipotential type.

Interelectrode Capacitances

The direct interelectrode capacitances of the 12K8 are measured with the metal shell connected to the cathode:

Maximum Ratings

The 12K8 has maximum ratings established under both the Design Center and Absolute Maximum systems. Operating the tube beyond these limits can impair performance and shorten service life.

Characteristics and Typical Operation

In typical converter service, the 12K8 operates with a triode grid resistor of 50,000 ohms. The table below details the operating characteristics under two different hexode plate voltage conditions:

At these operating points, the triode oscillator section operates with an oscillator plate voltage of 100 V and an oscillator plate current of 3.8 mA. The conversion transconductance is highly responsive to the control-grid bias, dropping from over 300 micromhos down to approximately 2 micromhos when the hexode control-grid (Grid Number 3) bias is increased to -30 V. This wide range of conversion transconductance allows for effective automatic volume control (AVC) in receiver designs.

Operation Characteristics and Curves

The operational characteristics of the 12K8 illustrate the relationship between the triode grid/hexode grid number 1 D-C current and key performance parameters, including conversion transconductance (Sc), total cathode current (Ik), and hexode plate resistance (rp).

When the D-C current of the combined triode grid and hexode grid number 1 varies from 0 to 0.4 milliamperes:

• Conversion Transconductance (Sc): Reaches a peak of approximately 350 micromhos at around 0.1 to 0.15 mA of grid current, after which it gradually decreases as grid current increases.
• Total Cathode Current (Ik): Decreases steadily as the grid current increases, reflecting the self-biasing action of the triode grid resistor.
• Hexode Plate Resistance (rp): Shows a corresponding variation, maintaining a high value (typically in the range of 0.4 to 0.6 megohms) under normal operating grid currents.

These curves demonstrate that optimal frequency converter performance is achieved when the oscillator section is adjusted to produce a grid current of approximately 0.1 to 0.15 mA through the 50,000-ohm grid resistor.