12AL5 — High-Perveance Twin Diode

A miniature, high-perveance double diode designed for detector service in frequency modulation (FM) and television circuits, automatic-volume-control (AVC) rectifiers, and low-power half-wave rectification.

Overview

The 12AL5 is a miniature seven-pin twin diode containing two independent, high-perveance diode units within a single T-5 1/2 glass envelope. The independent construction of each diode section allows them to be operated separately, connected in parallel for increased current capacity, or configured in a full-wave arrangement.

Due to its high-perveance design, the 12AL5 exhibits low internal voltage drop and high operating efficiency, making it highly effective as an AM or FM detector, particularly in ratio detector circuits for AC-operated receivers. Additionally, the tube is designed to operate at high frequencies, with each diode unit possessing a resonant frequency of approximately 700 MHz (megacycles per second).

Construction, Base, and Pinout

The 12AL5 is constructed with coated unipotential cathodes and an internal shield to minimize coupling between the two diode sections. The tube is housed in a T-5 1/2 glass bulb and utilizes a small-button miniature 7-pin base. It can be mounted and operated in any physical position.

Physical Dimensions

• Maximum Diameter: 0.750 inches
• Maximum Seated Height: 1.500 inches
• Maximum Overall Height: 1.750 inches

Pin Connections (Basing Diagram 6BT)

Viewed from the bottom of the tube, the 7-pin miniature base connections are assigned as follows:

Heater Characteristics and Family Variations

The 12AL5 belongs to a family of identical twin diodes that differ only in their heater ratings. The heater is designed for operation on either AC or DC power.

The 3AL5 variant features a controlled heater warm-up time of approximately 11 seconds, specifically designed to minimize voltage surges across individual tubes when employed in series-connected heater strings (such as in television receivers with 600 mA series circuits).

Direct Interelectrode Capacitances

The internal shield of the 12AL5, when properly grounded, provides excellent isolation between the two diode sections. The direct interelectrode capacitances vary depending on whether an external shield is utilized. The values below represent the capacitances for each section:

Note: Shielded values are measured using an external shield (such as No. 316) connected directly to Pin 6 (Internal Shield).

Maximum Ratings

The maximum ratings for the 12AL5 are established under the Design-Center or Design-Maximum Rating Systems. These limits must not be exceeded under any operating conditions.

Heater-Cathode Voltage Limits

• Maximum Peak Heater-Cathode Voltage: 330 Volts
• Heater Negative with Respect to Cathode (Total DC + Peak): 330 Volts max.
• Heater Positive with Respect to Cathode (DC Component): 100 Volts max.
• Heater Positive with Respect to Cathode (Total DC + Peak): 200 Volts max.

Typical Operation and Characteristics

When operated as a low-power half-wave rectifier (with the units used either separately or in parallel), the typical operating parameters are:

• AC Plate Voltage per Plate (RMS): 117 Volts
• Minimum Total Effective Plate Supply Impedance per Plate: 300 Ohms
• Average Output Current per Plate: 9.0 mA
• Tube Voltage Drop: 10 Volts DC at a plate current ($I_b$) of 60 mA

This low voltage drop (10 V at 60 mA) is a direct consequence of the tube's high perveance, enabling efficient rectification and signal detection with minimal signal loss.

Analysis of Characteristic Curves

The published performance curves for the 12AL5 illustrate two primary operational aspects:

Average Plate Characteristics

The plate current versus DC plate voltage curve demonstrates the high-perveance nature of the diode. The curve shows a rapid, non-linear rise in plate current as plate voltage increases from zero. At a plate potential of approximately 5.0 Volts DC, the plate current reaches 25 mA. When the plate potential is increased to 10.0 Volts DC, the current rises to approximately 60 mA. This steep curve indicates a low internal plate resistance during conduction.

Half-Wave Rectification Characteristics

The rectification curves plot the DC developed voltage against the rectified microamperes for a single diode unit operating under rated filament conditions. With an RMS signal input of up to 30 Volts, the developed DC voltage is shown as a function of various load resistance values ranging from 0.1 Megohm to 2.0 Megohms:

• Low Load Resistance (e.g., 0.1 Megohm): Results in higher rectified current (microamperes) but a lower developed DC voltage for a given AC input.
• High Load Resistance (e.g., 2.0 Megohms): Restricts the rectified current to very low levels but maximizes the developed DC voltage, approaching the peak value of the AC input signal.

These curves allow designers to select the optimum load resistance for detector and AVC circuits to balance signal voltage recovery against circuit loading.

Technical Summary

The 12AL5 is a miniature, 7-pin twin diode featuring two independent high-perveance sections with a common 12.6 V, 0.15 A heater. It is electrically identical to the 6AL5 and 3AL5 except for its heater configuration. With a resonant frequency of 700 MHz, low internal capacitance, and a low voltage drop of 10 V at 60 mA, the 12AL5 is optimized for high-frequency detection, ratio detectors, and low-current power supply rectification in television and FM receiver designs.