16A5 — Indirectly Heated Output Pentode

The 16A5 is an indirectly heated output pentode designed primarily for use in television receivers as a frame time base output valve or as an audio (sound) output valve. It is specifically engineered for operation in series heater chains.

Physical Characteristics, Heater, and Basing

The 16A5 is constructed with a T6 1/2 glass envelope and utilizes a small button Noval 9-pin (B9A) base. The physical dimensions of the tube are specified as follows:

The tube can be mounted and operated in any position.

Heater Specifications

The cathode is a coated, unipotential type. The heater is designed for series-string operation with a constant current.

• Heater Current: 0.3 A (300 mA)
• Heater Voltage: 16.5 V
• Calculated Heater Power: 4.95 W (calculated as 16.5 V multiplied by 0.3 A)

Pin Connections

The 16A5 uses the JETEC 9BL basing arrangement. The pin assignments, viewed from the underside of the base, are:

Maximum Ratings

The maximum ratings of the 16A5 must be strictly observed to ensure reliable operation and longevity. These ratings represent the absolute limits of the device:

* The maximum peak positive anode voltage rating of 2.5 kV is permitted only under pulsed conditions, with a maximum pulse duration of 10% of one cycle and a maximum absolute duration of 2 milliseconds.

Grid No. 1 Circuit Resistance Limits

The maximum allowable resistance in the control grid circuit depends on the biasing method and the specific application:

• With Automatic Bias: 1.0 MΩ
• With Fixed Bias: 0.4 MΩ
• For Frame Output Operation: 2.2 MΩ

Grid Current Starting Point

The control grid (Grid No. 1) voltage at which grid current begins (defined as a grid current of +0.3 µA) is -1.3 V.

Electrical Characteristics

The typical electrical characteristics of the 16A5 measured under static Class A conditions are outlined below:

Typical Operating Configurations

Frame Time Base Output

When utilized as a frame time base output valve, the circuit must be designed to accommodate manufacturing variations and tube aging over its operating life. The design should be centered around the following peak plate current limits:

• At Anode Voltage of 50 V and Screen Voltage of 170 V: The peak plate current should not exceed 90 mA (compared to 140 mA for an average new valve).
• At Anode Voltage of 60 V and Screen Voltage of 200 V: The peak plate current should not exceed 120 mA (compared to 175 mA for an average new valve).

In both cases, the control grid voltage is maintained at -1.0 V.

Class A Sound Output (Single Valve)

For single-ended Class A audio output service, the typical operating conditions are:

Push-Pull Class AB Audio Amplifier (Two Valves)

For push-pull audio output configurations operating in Class AB (without grid No. 1 current), the typical operating parameters for two valves are:

Interelectrode Capacitances

The direct interelectrode capacitances of the 16A5, measured without an external shield, are:

• Grid No. 1 to all other elements except plate (Cin): 11.0 pF
• Plate to all other elements except grid No. 1 (Cout): 5.9 pF to 6.2 pF
• Plate to Grid No. 1 (Ca-g1): < 1.0 pF
• Grid No. 1 to Heater (Cg1-h): < 0.15 pF

Technical Summary

The 16A5 is a versatile, high-performance indirectly heated output pentode housed in a Noval 9-pin (B9A) envelope. Designed with a 16.5 V, 0.3 A heater, it is optimized for series heater chains in television and audio equipment.

The tube is characterized by its dual-purpose design, serving efficiently as a frame time base output valve or as a Class A or Class AB push-pull audio output valve. With an anode dissipation rating of 9 W and a peak positive anode voltage rating of 2.5 kV under pulsed conditions, the 16A5 provides robust performance in demanding deflection circuits while delivering up to 12 W of low-distortion audio power in push-pull configurations.