11D5 — Double Diode Triode Vacuum Tube

An indirectly heated double diode triode vacuum tube designed for signal detection, automatic volume control (AVC) rectification, and voltage amplification in radio receiving equipment.

Overview

The 11D5 is a multi-unit thermionic valve containing two diode systems and a single triode system within a single glass envelope. The diode and triode sections share a common indirectly heated cathode. Designed as a replacement type, the 11D5 features a classic 7-pin English base and an external top cap connection for the triode control grid. It is characterized by a medium-mu triode section, making it suitable for general-purpose audio-frequency amplification and detection circuits.

Physical Characteristics and Base Configuration

The 11D5 is constructed with a dome-top glass bulb and a standard 7-pin English Base. The physical dimensions of the device are as follows:

• Maximum Bulb Diameter: 1 9/16 inches (approximately 39.7 mm)
• Maximum Overall Height: 3 1/8 inches (approximately 79.4 mm)

The electrical connections are distributed between the 7-pin base and a top cap terminal. The pinout configuration, viewed from the bottom of the tube base, is defined as follows:

This configuration isolates the sensitive control grid at the top of the envelope, reducing parasitic interelectrode capacitances and enhancing stability in high-gain stages. The two diode anodes (a'd and a''d) are brought out to separate pins, allowing for independent diode operation, such as full-wave detection or combined detection and automatic volume control (AVC) rectification.

Heater and Electrical Characteristics

The 11D5 features an indirectly heated, coated unipotential cathode. The heater is designed for operation under the following nominal ratings:

• Heater Voltage: 13.0 V
• Heater Current: 0.15 A

Triode Section Class A Operating Point

The triode section of the 11D5 is designed for Class A voltage amplification. Under typical operating conditions, the triode section exhibits the following characteristics:

• Anode Voltage: 250 V
• Grid Voltage: -3 V
• Anode Current: 3.8 mA
• Cathode Bias Resistor: 750 ̤
• Anode Impedance (Plate Resistance): 26,700 ̤
• Mutual Conductance (Transconductance): 1.5 mA/V (1,500 µmhos)
• Amplification Factor (μ): 40

The fundamental relationship between the tube's mutual conductance ($g_m = 1.5\text{ mA/V}$) and anode impedance ($r_p = 26,700\ \Omega$) mathematically determines the amplification factor:

$\mu = g_m \times r_p = 0.0015\text{ S} \times 26,700\ \Omega \approx 40$

This calculated value matches the specified amplification factor of 40, demonstrating the consistency of the published operating parameters.

Comparison with Related Types

The 11D5 is closely related to the 11D3, another double diode triode sharing the same physical dimensions, English 7-pin base, and 13.0 V heater voltage. However, the two types are optimized for different circuit requirements and are not direct electrical substitutes:

• Heater Current: The 11D5 has a lower heater current requirement of 0.15 A, compared to the 0.2 A drawn by the 11D3.
• Amplification Factor: The 11D5 is a medium-mu triode with an amplification factor of 40, whereas the 11D3 is a high-mu triode with an amplification factor of 100 (with triode characteristics equivalent to the type 75 tube).
• Anode Impedance and Current: At an anode voltage of 250 V, the 11D5 operates at a significantly higher anode current (3.8 mA vs. 0.4 mA for the 11D3) and a much lower anode impedance (26,700 ̤ vs. 90,000 ̤ for the 11D3).
• Cathode Bias: The typical operating point of the 11D5 utilizes a 750 ̤ cathode bias resistor at a grid voltage of -3 V, while the 11D3 requires a 5,000 ̤ resistor at a grid voltage of -2 V.

These differences indicate that the 11D5 is better suited for applications requiring lower output impedance and higher signal-handling capability, whereas the 11D3 is optimized for maximum voltage gain.

Technical Summary

The 11D5 is a double diode triode vacuum tube featuring a 13.0 V, 0.15 A heater, an English 7-pin base, and a top-cap control grid. Its triode section provides a medium amplification factor of 40, a transconductance of 1.5 mA/V, and an anode impedance of 26,700 ̤ at a typical operating anode voltage of 250 V. The combination of two independent diode anodes and a medium-impedance triode stage sharing a common cathode makes the 11D5 a versatile component for detection, rectification, and audio preamplification in vintage receiving systems.