15A6 — Sharp-Cutoff Video Output Pentode

The 15A6 is an indirectly heated sharp-cutoff pentode designed primarily for use as a video output valve in television receivers employing series heater chains. It features high mutual conductance, high anode dissipation capabilities, and is housed in a standard miniature glass envelope. The tube is engineered to deliver significant peak output voltage swings when operated with typical video-frequency load resistances.

Physical Construction, Heater, and Base Connections

The 15A6 is constructed with a miniature glass envelope utilizing a B9A (Noval) nine-pin base and a top cap designated as CT1. The physical dimensions of the tube are compact, with a maximum overall length of 78.5 mm (3 3/32 inches), a maximum seated height of 71.5 mm (2 13/16 inches), and a maximum envelope diameter of 22.2 mm (7/8 inch). The tube is designed to be versatile in physical installation and can be mounted in any position.

The heater is designed for series-string operation, drawing a nominal current of 0.3 A at a nominal voltage of 15.0 V.

The 9AR base connection configuration, viewed from the underside of the base, provides the following pin assignments:

Electrical Characteristics and Maximum Ratings

The 15A6 is characterized by high transconductance and a sharp-cutoff control grid characteristic. The maximum ratings are defined under specific limiting conditions to ensure reliable operation in television receiver circuits.

Maximum Ratings

• Maximum DC Supply Voltage: 550 V
• Maximum Anode Voltage: 250 V
• Maximum Anode Dissipation: 9 W
• Maximum Screen Grid (g2) Voltage: 250 V
• Maximum Screen Grid Dissipation: 2 W
• Maximum Cathode Current: 70 mA
• Maximum Negative Grid Voltage (at Ig1 = +0.3 µA): -1.3 V
• Maximum Heater-to-Cathode Voltage (Vh-k): 150 V
• Maximum Heater-to-Cathode Resistance (Rh-k): 20 kΩ
• Maximum Control Grid-to-Cathode Resistance (Rg1-k):
  • Under Fixed Bias: 500 kΩ
  • Under Auto (Cathode) Bias: 1.0 MΩ

Interelectrode Capacitances

The internal capacitances of the 15A6, measured without an external shield, are:

• Input Capacitance (Cin): 10.4 pF
• Output Capacitance (Cout): 6.6 pF
• Anode to Control Grid Capacitance (Ca-g1): <0.1 pF
• Control Grid to Heater Capacitance (Cg1-h): <0.15 pF

Typical Operating Points and Characteristics

The 15A6 can be operated under different Class A amplifier conditions. The published specifications document characteristics at several distinct operating points:

Class A Amplifier Characteristics

Typical Video Output Operation

When configured specifically as a video output stage, the tube is operated with a higher negative bias to restrict quiescent current, allowing for large positive-going signal excursions:

• Anode Supply Voltage: 170 V
• Screen Voltage: 170 V
• Suppressor Grid Voltage: 0 V
• Control Grid Voltage: -6.7 V
• Anode Current: 4 mA
• Screen Current: 0.25 mA
• Anode Load Resistance: 2.2 kΩ
• Peak Output Voltage: >70 V

Under this video output operating point, the quiescent anode current is biased down to 4 mA, which provides a large dynamic range for amplifying video signals and achieving a peak-to-peak output voltage swing exceeding 70 V across the 2.2 kΩ load resistor.

Analysis of Characteristic Curves

The transfer and plate characteristics of the 15A6 illustrate its behavior across its operating envelope:

• Transfer Characteristics (Ia, Ig2 vs. Vg1): At a screen voltage of 200 V, the anode current reaches approximately 60 mA at zero grid bias (Vg1 = 0 V) and drops smoothly to near-zero as the grid bias approaches -10 V. At a screen voltage of 170 V, the zero-bias anode current is approximately 50 mA. The screen current tracks the anode current at a much lower level, peaking at approximately 8 to 10 mA at zero bias.
• Plate Characteristics (Ia vs. Va): The family of curves plotted for a screen voltage of 200 V shows a classic pentode profile with a sharp knee. At Vg1 = 0 V, the anode current rises rapidly below Va = 50 V and flattens out above 100 V, maintaining a level of over 70 mA. As the control grid voltage is stepped down from 0 V to -6 V in 1-volt increments, the plate current curves remain highly horizontal and evenly spaced, indicating high anode impedance and linear transconductance over a wide voltage range.

Technical Summary

The 15A6 is a high-performance, sharp-cutoff pentode housed in a B9A Noval envelope with a CT1 top cap. Featuring a 15.0 V, 0.3 A heater designed for series-string television receivers, it is optimized for video output service. It combines a high transconductance of 10.0 to 10.5 mA/V with an anode dissipation rating of 9 W and a maximum anode voltage of 250 V. Its electrical design allows it to deliver peak output swings greater than 70 V in typical video amplifier circuits, making it a highly effective solution for television receiver video stages.