16A8 — High-Mu Triode and Power Pentode

A miniature multi-unit vacuum tube containing a high-mu triode and a power pentode in a single envelope, designed for television receiver and audio amplifier applications.

Overview

The 16A8 (also designated as the PCL82) is a miniature, 9-pin vacuum tube combining a high-mu triode and a power pentode within a single glass envelope. The triode section is designed for service as a vertical deflection oscillator or as an audio-frequency (AF) voltage amplifier. The pentode section is optimized for use as a vertical deflection output stage (frame output) or as an audio power output amplifier.

The tube is designed with a miniature T-6 1/2 envelope and requires a standard miniature 9-contact socket. The 16A8 is electrically and structurally identical to the 8B8 and 11BM8 designations, differing only in its heater ratings.

Construction, Heater, and Physical Dimensions

The 16A8 features an indirectly heated, unipotential cathode system. The heater is designed for series-string or parallel operation depending on the specific designation. The heater characteristics for the related family of tubes are as follows:

The maximum allowable heater-cathode voltage is ±200 V (both heater-negative and heater-positive with respect to the cathode). To ensure reliable operation, the maximum circuit resistance between the heater and cathode must not exceed 20,000 Ohms.

Physically, the tube conforms to the 6-4 outline with the following maximum dimensions:

• Maximum Diameter: 0.875 inches
• Maximum Seated Height: 2.812 inches
• Maximum Overall Height: 3.062 inches

Basing and Pin Connections

The 16A8 utilizes the 9EX basing arrangement (E9-1 button base). The pin assignments, viewed from the bottom of the tube, are:

• Pin 1: Grid No. 1, Pentode ($G_{1p}$)
• Pin 2: Cathode, Grid No. 3, and Internal Shield, Pentode ($K_p, G_{3p}, IS$)
• Pin 3: Grid, Triode ($G_t$)
• Pin 4: Heater ($H$)
• Pin 5: Heater ($H$)
• Pin 6: Plate, Pentode ($P_p$)
• Pin 7: Grid No. 2 (Screen Grid), Pentode ($G_{2p}$)
• Pin 8: Cathode, Triode ($K_t$)
• Pin 9: Plate, Triode ($P_t$)

Electrical Characteristics and Operating Points

The triode and pentode sections operate independently, each exhibiting distinct electrical properties optimized for their respective roles.

Triode Section

At a typical Class A1 operating point, the triode section exhibits the following characteristics:

• Plate Voltage: 100 V
• Grid-No. 1 Voltage: 0 V
• Plate Current: 3.5 mA
• Transconductance: 2500 µmhos
• Amplification Factor ($μ$): 70

Pentode Section

The pentode section is characterized across several typical Class A1 operating points, reflecting its versatility in both low-voltage and high-voltage circuits:

† Note on Discrepancies: In some published specifications, a typographical transposition occurs for Point D, listing the plate current as 7 mA and the Grid-No. 2 current as 35 mA. Corrected technical records resolve this to 35 mA of plate current and 7.0 mA of Grid-No. 2 current.

Maximum Ratings and Circuit Values

The 16A8 is rated under both the Design-Maximum and Design-Center rating systems. Designers must adhere to these limits to prevent premature tube failure.

† Pulse Rating Conditions: The peak plate voltage ratings of 600 V (triode) and 2500 V (pentode) are valid only under pulsed conditions, such as in vertical deflection service, with a maximum duty factor of 0.04 (or 4% of one cycle) and a maximum pulse duration of 0.8 milliseconds.

Maximum Circuit Resistance Values

To prevent thermal runaway and maintain bias stability, the control grid (Grid-No. 1) circuit resistance must be kept within the following limits:

• Triode Section:
  • Fixed-bias operation: Max 1 Megohm
  • Automatic/Cathode-bias operation: Max 3 Megohms
  • Grid Circuit Impedance (at 50 Hz): Max 0.5 Megohm
• Pentode Section:
  • Fixed-bias operation: Max 1 Megohm
  • Automatic/Cathode-bias operation: Max 2 Megohms

Interelectrode and Coupling Capacitances

Because the triode and pentode sections reside in close proximity within the same miniature envelope, interelectrode and cross-unit coupling capacitances are critical design considerations, particularly in high-gain or high-frequency applications. The following unshielded values are specified:

Triode Section Capacitances

• Grid to Plate: 4.2 pF
• Grid to All Other Elements Except Plate: 2.7 pF
• Plate to All Other Elements Except Grid: 4.3 pF
• Grid to Heater (Max): 0.02 pF

Pentode Section Capacitances

• Grid No. 1 to All Other Elements Except Plate: 9.3 pF
• Plate to All Other Elements Except Grid No. 1: 8.0 pF
• Plate to Grid No. 1 (Max): 0.3 pF
• Grid No. 1 to Heater (Max): 0.3 pF

Cross-Unit Coupling Capacitances

• Triode Plate to Pentode Grid No. 1 (Max): 0.02 pF
• Triode Grid to Pentode Plate (Max): 0.02 pF
• Triode Grid to Pentode Grid No. 1 (Max): 0.025 pF
• Triode Plate to Pentode Plate (Max): 0.25 pF

Technical Summary

The 16A8 (PCL82) is a highly integrated, space-saving miniature high-mu triode and power pentode designed for dual-role efficiency in television and audio systems. Operating with a 16 V, 300 mA heater, it provides a high-gain triode ($μ = 70$) suitable for voltage amplification or oscillation, alongside a robust power pentode capable of up to 5 W of continuous plate dissipation (7 W peak in audio service). Its 9EX pinout configuration and carefully specified interelectrode capacitances ensure stable operation when both sections are active, making it a classic choice for compact vertical deflection and audio output stages.