12AS5 — Beam Pentode Power Amplifier

The 12AS5 is a miniature, heater-cathode beam pentode power amplifier designed for use as an output tube in automobile and AC-operated receivers. It features a 7-pin miniature construction and is electrically identical to the 6AS5, except for its heater characteristics.

Physical Specifications and Basing

The 12AS5 is constructed in a T-5 1/2 glass bulb. It has the following physical dimensions:

• Maximum Diameter: 3/4 inch
• Maximum Seated Height: 2 1/8 inches
• Maximum Overall Length: 2 5/8 inches

The tube utilizes a miniature button 7-pin base with a 7CV basing arrangement. It can be mounted in any position. The pin connections, viewed from the bottom of the tube, are as follows:

• Pin 1: Grid No. 1 (G1)
• Pin 2: Cathode (K) and Grid No. 3 (G3)
• Pin 3: Heater (H)
• Pin 4: Heater (H)
• Pin 5: Grid No. 1 (G1)
• Pin 6: Grid No. 2 (G2)
• Pin 7: Plate (P)

Direct Interelectrode Capacitances

Measured without an external shield, the approximate interelectrode capacitances are:

• Grid No. 1 to Plate (G1 to P): 0.6 µµF
• Input [G1 to (H + K + G2 + G3)]: 12 µµF
• Output [P to (H + K + G2 + G3)]: 6.2 µµF

Technical Ratings

The ratings for the 12AS5 are interpreted according to the Design Center Maximum System:

Additionally, the maximum Grid No. 1 circuit resistance is restricted based on the biasing method:

• Fixed Bias: 0.1 Megohm maximum
• Self Bias: 0.5 Megohm maximum

Typical Operating Conditions and Characteristics

When operated as a Class A1 amplifier, the typical operating parameters of the 12AS5 are as follows:

• Heater Voltage: 12.6 Volts
• Heater Current: 0.4 Amperes
• Plate Voltage: 150 Volts
• Grid No. 2 Voltage: 110 Volts
• Grid No. 1 Voltage: -8.5 Volts
• Peak AF Grid No. 1 Voltage: 8.5 Volts
• Zero-Signal Plate Current: 35 mA
• Maximum-Signal Plate Current: 36 mA
• Zero-Signal Grid No. 2 Current (Approximate): 2 mA
• Maximum-Signal Grid No. 2 Current (Approximate): 6.5 mA
• Transconductance: 5600 µmhos
• Load Resistance: 4500 Ohms
• Total Harmonic Distortion: 10%
• Maximum-Signal Power Output: 2.2 Watts

Analysis of Performance Curves

Grid No. 2 Rating Curve

The Grid No. 2 rating curve defines the area of permissible operation for the screen grid. At Grid No. 2 voltages up to 75 Volts, the maximum allowable dissipation is 1.0 Watt. As the Grid No. 2 voltage increases beyond 75 Volts, the maximum permissible dissipation decreases along a curve, reaching 0 Watts at 150 Volts.

Plate Characteristics

The plate characteristic curves show the relationship between plate voltage and plate current (or Grid No. 2 current) across various Grid No. 1 bias voltages ranging from 0 Volts down to -17.5 Volts, with the heater voltage at 12.6 Volts and Grid No. 2 voltage held at 110 Volts. The plate current curves exhibit a typical pentode knee, where plate current rises rapidly at low plate voltages and flattens out as plate voltage increases. Grid No. 2 current is highest at low plate voltages and decreases as plate voltage rises.

Load Resistance Performance

With a constant plate voltage of 150 Volts, Grid No. 2 voltage of 110 Volts, Grid No. 1 bias of -8.5 Volts, and an RMS signal input of 6.0 Volts, the performance of the tube varies significantly with load resistance:

• Power Output: Power output rises with load resistance, peaking at approximately 2.2 Watts at a load resistance of 4500 Ohms. Beyond 4500 Ohms, the power output gradually decreases, falling to approximately 1.8 Watts at 10000 Ohms.
• Total Harmonic Distortion: Distortion reaches a minimum of 10% at the optimal load resistance of 4500 Ohms. Operating the tube with a lower load resistance (e.g., 1000 Ohms) increases distortion to approximately 18%, while operating with a higher load resistance (e.g., 10000 Ohms) increases distortion to approximately 24%.

Technical Summary

The 12AS5 is a compact, 7-pin miniature beam pentode power amplifier designed for low-voltage audio output applications in automotive and AC-operated receivers. Operating with a 12.6 V, 0.4 A heater, it provides a maximum power output of 2.2 Watts into a 4500 Ohm load under typical Class A1 conditions. Its design center ratings limit plate voltage to 150 Volts and plate dissipation to 5.5 Watts, requiring careful attention to Grid No. 2 dissipation limits and Grid No. 1 circuit resistance constraints to ensure reliable operation.