12DW5 — Beam Power Pentode

The 12DW5 is a miniature beam power pentode designed primarily for vertical deflection amplifier service in television receivers employing 110-degree deflection systems. It is characterized by a high zero-bias plate current and is engineered to operate efficiently at relatively low B supply voltages.

The tube features controlled heater thermal characteristics to minimize heater voltage surges during the warm-up cycle when used in series-string arrangements with other similarly controlled tubes. Except for its heater ratings and heater warm-up time, the 12DW5 is identical to the 6DW5.

Construction, Physical Dimensions, and Pinout

The 12DW5 is constructed in a 9-pin miniature glass envelope (T-6 1/2 bulb) with a small button 9-pin base (9CK arrangement). It features a coated unipotential cathode and can be operated in any mounting position.

The physical dimensions of the tube are as follows:

• Bulb type: T-6 1/2
• Maximum overall length: 3-1/16 inches
• Maximum seated height: 2-13/16 inches
• Maximum diameter: 7/8 inches

The 9CK base connections, viewed from the bottom of the tube, are assigned as follows:

Grid No. 1 is internally connected to both Pin 1 and Pin 6. Grid No. 3 (the beam-forming plates) is internally connected to the cathode at Pin 7.

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 12DW5, measured without an external shield, are:

• Grid No. 1 to Plate (G1 to P): 0.5 pF
• Input (G1 to H+K): 14 pF
• Output (P to H+K): 9 pF

Heater Characteristics and Controlled Warm-up

The 12DW5 heater is designed for operation from either an AC or DC supply. The heater has controlled thermal characteristics specifically designed for series-string television receivers to minimize voltage surges during the initial warm-up phase.

Heater Warm-up Time Calculation and Definition

The heater warm-up time is defined as the time required for the voltage across the heater to reach 80% of its rated voltage (which is 12.6 V * 0.8 = 10.08 V) after applying four times the rated heater voltage (4 * 12.6 V = 50.4 V) to a test circuit. This test circuit consists of the tube heater connected in series with an external resistance.

The value of this series resistance is calculated as three times the nominal heater operating resistance:

• Nominal Heater Operating Resistance (Rh): 12.6 V / 0.6 A = 21 ohms
• Series Test Resistance (Rseries): 3 * 21 ohms = 63 ohms

Under these conditions, the heater voltage reaches the 80% threshold in approximately 11.0 seconds.

Electrical Characteristics and Typical Operating Points

The 12DW5 is optimized for high current delivery at low plate voltages, which is essential for driving the magnetic deflection coils in 110-degree television sweep circuits.

Typical Class A Operating Conditions (Pentode Connected)

When operated as a pentode-connected vertical deflection amplifier, the typical operating parameters are:

• Plate Voltage: 200 V
• Grid No. 2 (Screen Grid) Voltage: 150 V
• Grid No. 1 (Control Grid) Voltage: -22.5 V
• Plate Current: 55 mA
• Grid No. 2 Current: 2.0 mA
• Transconductance: 5,500 umhos
• Plate Resistance (Approximate): 15,000 ohms
• Grid No. 1 Voltage for Ib = 0.1 mA (Cutoff): -55 V

Zero-Bias Performance (Instantaneous Values)

To demonstrate the tube's high peak current capability, the instantaneous characteristics at zero bias (Ec1 = 0) with a low plate voltage are specified as follows:

• Plate Voltage (Eb): 60 VDC
• Grid No. 2 Voltage (Ec2): 150 VDC
• Instantaneous Plate Current: 260 mA
• Instantaneous Grid No. 2 Current: 20 mA

Triode-Connected Characteristics

When triode-connected (with the plate and Grid No. 2 tied together), the tube exhibits the following characteristic:

• Triode Amplification Factor (mu): 4.3 (measured with Eb = Ec2 = 150 V and Ec1 = -22.5 V)

Design-Center Ratings and Circuit Constraints

The maximum ratings for the 12DW5 are established under the Design-Center System for vertical deflection amplifier service (pentode connected). These ratings represent limiting values that must not be exceeded under the worst probable operating conditions, taking into account supply-voltage variations, component tolerances, and environmental factors.

Heater-Cathode Voltage Limits

To prevent insulation breakdown between the heater and the cathode, the following voltage limits must be observed:

• Heater Negative with Respect to Cathode:
  • Maximum Total DC and Peak Voltage: 200 V
• Heater Positive with Respect to Cathode:
  • Maximum DC Voltage: 100 V
  • Maximum Total DC and Peak Voltage: 200 V

Circuit Design and Operational Constraints

1. Television Scanning System Standards: The ratings for vertical deflection service are specified for operation in a standard 525-line, 30-frame television system. Under these conditions, the duty cycle of the peak positive voltage pulse must not exceed 15% of one scanning cycle.
2. Grid Leak Bias Protection: In stages designed to operate with grid leak bias, the tube is vulnerable to excessive dissipation if the excitation signal is lost. To protect the tube in the absence of excitation, an adequate cathode bias resistor or other suitable protective means must be incorporated into the circuit design.

Technical Summary

The 12DW5 is a 9-pin miniature beam power pentode designed specifically for vertical deflection amplification in 110-degree television receivers. Operating from a 12.6 V, 0.6 A heater supply, it features a controlled 11-second warm-up time to prevent voltage surges in series-wired heater strings.

The tube is optimized for low-voltage, high-current operation, capable of delivering an instantaneous plate current of 260 mA at a plate voltage of only 60 V under zero-bias conditions. With a plate dissipation rating of 11 W and a peak positive pulse plate voltage rating of 2,200 V, the 12DW5 provides the robust power handling and high transconductance (5,500 umhos) required for demanding television sweep applications. Except for its heater characteristics, it is identical to the 6DW5.