12JS6 — Beam Power Pentode Horizontal Deflection Amplifier

A high-power Compactron beam power pentode designed for horizontal deflection amplifier service in color television receivers.

Overview

The 12JS6 is a specialized beam power pentode designed primarily for horizontal deflection amplifier service in color television receivers. Housed in a 12-pin Compactron envelope, the tube is engineered to deliver high peak plate currents at low plate voltages, ensuring high efficiency in scanning circuits. The 12JS6 is part of a family of structurally similar horizontal deflection tubes, including the 6JS6C, 23JS6A, and 31JS6C, but is distinguished by its specific heater operating parameters.

Construction, Physical Dimensions, and Basing

The 12JS6 is constructed using the Compactron T-12 configuration, featuring a glass bulb with a 12-pin button base (E12-74). To safely isolate the high peak positive voltages encountered in horizontal deflection circuits, the plate connection is brought out to a top cap (C1-1).

The physical dimensions of the tube are:

• Maximum Diameter: 1.563 inches
• Maximum Seated Height: 3.750 inches
• Maximum Overall Height: 4.125 inches
• Outline Designation: 12-89

The tube utilizes the 12FY basing arrangement. Looking at the bottom of the base, the pin connections are assigned as follows:

• Pin 1: Grid No. 3 (Beam Plates)
• Pin 2: Grid No. 1 (Control Grid)
• Pin 3: Cathode
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Grid No. 2 (Screen Grid)
• Pin 7: No Connection
• Pin 8: Grid No. 1 (Control Grid)
• Pin 9: Internal Connection (do not use for external circuit connections)
• Pin 10: Grid No. 3 (Beam Plates)
• Pin 11: No Connection
• Pin 12: Grid No. 2 (Screen Grid)
• Top Cap: Plate (Anode)

The duplicate connections for Grid No. 1, Grid No. 2, and Grid No. 3 are designed to reduce lead inductance and distribute current loads across multiple pins.

Heater Characteristics and Ratings

The 12JS6 is designed to operate with a nominal heater voltage of 12.6 V.

Calculation of Nominal Heater Power: Using the nominal heater voltage ($V_f = 12.6\text{ V}$) and nominal heater current ($I_f = 1.125\text{ A}$), the nominal power dissipated by the heater is calculated as: $P_f = V_f \times I_f = 12.6\text{ V} \times 1.125\text{ A} = 14.175\text{ W}$

(Note: While some early registration records list the heater current as 12.125 A, this is a typographical anomaly; the standard operating current is 1.125 A, which corresponds to 1125 mA.)

The heater-to-cathode insulation is rated to withstand significant potential differences, which is critical in television receiver circuits where the cathode may operate at elevated potentials:

• Heater Negative with Respect to Cathode: 200 V (Total DC and Peak)
• Heater Positive with Respect to Cathode: 100 V DC, 200 V (Total DC and Peak)

Electrical Characteristics and Typical Operation

Under typical operating conditions as a class A amplifier, the 12JS6 exhibits high transconductance and moderate plate resistance. The typical operating parameters are defined below:

When triode-connected (with 125 V on the plate and Grid No. 2, and -25 V on Grid No. 1), the tube exhibits an amplification factor ($\mu$) of 3.

Design Maximum Ratings

The ratings of the 12JS6 are established under the Design Maximum Rating System. These ratings represent the limiting values of operating and environmental conditions that should not be exceeded under any normal service condition.

• DC Plate Supply Voltage (Boost + DC Power Supply): 990 V Max
• Peak Positive Plate Voltage: 7500 V Max
• Peak Negative Plate Voltage: 1200 V Max
• Positive DC Beam Plate (Grid No. 3) Voltage: 75 V Max
• Grid No. 2 (Screen Grid) Voltage: 220 V Max
• Peak Negative Grid No. 1 Voltage: 330 V Max
• Plate Dissipation: 30 W Max
• Grid No. 2 Input: 5.5 W Max
• Average Cathode Current: 350 mA Max
• Peak Cathode Current: 1200 mA Max
• Grid No. 1 Circuit Resistance (with Grid Bias Feedback HV Regulation): 0.47 MΩ Max
• Grid No. 1 Circuit Resistance (with DC or Pulse Shunt HV Regulation): 10.0 MΩ Max
• Bulb Temperature (at hottest point): 225 °C Max

For horizontal deflection service in a 525-line, 30-frame television system, the duty cycle of the voltage pulse must not exceed 15% of one horizontal scanning cycle.

Average Plate Characteristics and Knee Values

The 12JS6 is designed to deliver high current at low plate voltages, which is a critical requirement for horizontal deflection amplifiers to ensure high efficiency. This capability is characterized by the "knee" of the plate characteristic curve, where the plate current reaches a high value before saturating.

At the instantaneous plate knee, the typical values are:

• Plate Voltage ($E_b$): 70 V
• Grid No. 2 Voltage ($E_{c2}$): 125 V
• Grid No. 1 Voltage ($E_{[c1](/tubes/C1)}$): 0 V
• Instantaneous Plate Current ($I_b$): 570 mA
• Instantaneous Grid No. 2 Current ($I_{c2}$): 34 mA

The average plate characteristics show that with Grid No. 1 at 0 V, the plate current rises rapidly as plate voltage increases, reaching the knee near 70 V. Above this voltage, the plate current remains relatively flat, demonstrating a high dynamic output impedance. Grid No. 2 current peaks at very low plate voltages and drops significantly once the plate voltage exceeds the knee, minimizing screen grid dissipation during the conduction cycle.

Related Designations and Comparisons

The 12JS6 is closely related to several other horizontal deflection amplifiers:

• 6JS6 / 6JS6C: The 12JS6 is electrically similar to the 6JS6 and 6JS6C, except for its heater ratings. The 6JS6C operates with a 6.3 V heater drawing 2250 mA, whereas the 12JS6 operates at 12.6 V drawing 1125 mA.
• 23JS6A: This variant operates with a 23.6 V heater drawing 600 mA and has a controlled heater warm-up time of 11 seconds.
• 31JS6C: This variant operates with a 31.5 V heater drawing 450 mA and also features an 11-second heater warm-up time.

While these tubes share the same 12FY basing, physical dimensions, and internal plate structures, they are not direct drop-in substitutes for one another due to their incompatible heater voltage and current requirements.

Technical Summary

The 12JS6 is a high-performance, Compactron-style beam power pentode designed for horizontal deflection amplifier service in color television receivers. Operating with a 12.6 V, 1.125 A heater, it features a 12-pin button base with a top cap plate connection to handle peak positive plate voltages up to 7500 V. With a plate dissipation rating of 30 W and a peak cathode current capability of 1200 mA, the 12JS6 is optimized for high-efficiency, high-current operation at low plate voltages, as evidenced by its knee characteristic of 570 mA at a plate voltage of 70 V.