12B6GM — Diode High-Mu Triode Vacuum Tube

An indirectly heated, unipotential cathode receiving tube designed for voltage amplification service, combining a high-mu triode system with a single diode plate in a single envelope.

Physical Construction and Basing

The 12B6GM features a T9 metal spray bulb construction with a small octal shell base and a miniature metal top cap. The physical dimensions of the tube are specified as follows:

• Maximum Diameter: 1-3/16 inches
• Maximum Overall Length: 4-1/4 inches
• Maximum Seated Height (including top cap): 3-11/16 inches

The tube is designed to operate in any mounting position. It utilizes the 6Y basing designation, which provides the following pin and cap connections when viewed from the bottom of the base:

Heater and Electrical Ratings

The 12B6GM is equipped with an indirectly heated, unipotential cathode. The heater is designed to operate from either an alternating current (AC) or direct current (DC) supply.

Note: The heater power of 1.89 W is a calculated value derived from the nominal heater voltage of 12.6 V and nominal heater current of 0.150 A (12.6 V multiplied by 0.150 A).

For Class A voltage amplifier service, the tube is constrained by the following maximum and minimum ratings:

• Maximum Plate Voltage: 250 V
• Minimum Grid Bias Voltage: -1 V

Typical Class A Operating Characteristics

Under typical Class A voltage amplifier operating conditions, the triode section of the 12B6GM exhibits a high amplification factor combined with a high plate resistance. The typical operating parameters are defined as:

• Plate Voltage: 250 V
• Grid Voltage: -2 V
• Plate Current: 0.9 mA
• Plate Resistance: 91,000 ohms
• Transconductance: 1,100 micromhos (µmhos)
• Amplification Factor (μ): 100

The relationship between these typical characteristics is consistent with standard triode design principles, where the amplification factor is approximately equal to the product of transconductance and plate resistance. A calculation using these typical values yields:

$\mu \approx g_m \times r_p = 0.0011\text{ S} \times 91,000\ \Omega = 100.1$

This calculated value closely matches the nominal amplification factor of 100.

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the triode unit are measured approximately as follows:

• Grid-to-Plate (Grid-Plate): 1.7 µµf
• Input (Grid-to-Cathode and Shield): 2.5 µµf
• Output (Plate-to-Cathode and Shield): 7.0 µµf

These capacitance values are critical for determining the high-frequency response and input loading effects of the triode stage when configured as a high-gain voltage amplifier.

Technical Summary

The 12B6GM is a specialized diode high-mu triode receiving tube featuring a unipotential cathode, a 12.6 V heater, and a T9 metal spray bulb with a small octal shell base. Designed for Class A voltage amplification, its triode section provides a high amplification factor of 100 with a transconductance of 1,100 µmhos and a plate current of 0.9 mA at a plate voltage of 250 V. The inclusion of a separate diode plate and an integrated metal spray shield connected to Pin 1 makes it a compact solution for combined detection and high-gain audio voltage amplification in radio receiver designs.