12BK5 — Beam Power Amplifier

The 12BK5 is a 9-pin miniature beam power amplifier designed primarily for service in the power output stages of television and radio receivers. It is also highly suitable for use as a video amplifier. The tube is characterized by high power sensitivity, high transconductance, and high plate efficiency. Except for its heater ratings, the 12BK5 is electrically and physically identical to the 6BK5.

Overview and Series-Heater Design

The 12BK5 is specifically engineered for use in receivers that employ series-connected heater strings. This is particularly common in television receivers where the picture-tube heater is wired in series with other receiving tubes. In such series circuits, individual tubes can experience severe voltage surges during the warm-up phase if their thermal characteristics are mismatched. To minimize these surges, the 12BK5 features a strictly controlled heater warm-up characteristic, ensuring safe and reliable operation when integrated into a 600 mA series-connected heater string.

Heater Specifications and Warm-up Measurements

The 12BK5 utilizes a coated unipotential cathode. The heater is designed to operate from either an AC or DC supply.

Heater Warm-up Test Conditions

The approximate warm-up time of 10.5 seconds is defined as the time required for the voltage across the heater terminals to rise from zero to a specified test voltage ($V_1$) in a standardized test circuit. The measurement is conducted using the following parameters:

• Applied Voltage ($E$, RMS or DC): 50 V
• Total Series Resistance ($R$): 63 Ω
• Heater Test Voltage ($V_1$, RMS or DC): 10.0 V
• Rated Heater Voltage ($E_f$): 12.6 V
• Rated Heater Current ($I_f$): 0.6 A

Physical Characteristics and Pinout Configuration

The 12BK5 is housed in a T-6 1/2 glass bulb with an E9-1 small button 9-pin base. The tube can be mounted and operated in any position.

The device utilizes the 9BQ basing arrangement. The pin connections, viewed from the bottom of the base, are as follows:

Grid 1 is connected to three separate pins (Pins 1, 3, and 7) to provide flexibility in circuit layout and minimize lead inductance. Grid 3 (the beam-forming plates) is internally tied to the cathode at Pin 6.

Direct Interelectrode Capacitances

When operated without an external shield, the direct interelectrode capacitances of the 12BK5 are:

• Grid 1 to Plate: 0.6 µµF
• Input (Grid 1 to Cathode, Heater, Grid 2, and Grid 3): 13 µµF
• Output (Plate to Cathode, Heater, Grid 2, and Grid 3): 5.0 µµF

Maximum Ratings (Design Center Values)

The maximum ratings represent the limiting values for safe operation of the 12BK5 under the Design Center system:

Maximum Circuit Values

To prevent thermal runaway and ensure bias stability, the resistance in the Grid 1 circuit must not exceed the following limits:

• Fixed Bias Operation: 0.1 MΩ maximum
• Cathode Bias Operation: 0.5 MΩ maximum

Class A1 Amplifier Characteristics and Typical Operation

Under typical Class A1 operating conditions, the 12BK5 delivers high power output with relatively low input signal requirements, demonstrating its high power sensitivity.

Performance Curves and Operation Characteristics

The electrical behavior of the 12BK5 is illustrated through several characteristic relationships:

• Transfer Characteristics: The relationship between plate/screen current and Grid 1 voltage (at a constant plate voltage of 250 V) shows that the tube achieves its nominal transconductance of 8,500 µmhos near the -5.0 V bias point. Curves plotted for screen voltages ($E_{c2}$) of 100 V, 150 V, 200 V, and 250 V demonstrate how the cutoff point shifts more negative as screen voltage increases, reaching approximately -12 V when the screen is at 250 V.
• Plate Characteristics: The plate current curves plotted against plate voltage (at a screen voltage of 250 V) exhibit the characteristic sharp knee of a beam power tube. This knee allows the plate voltage to swing very low before plate current drops off, contributing to the tube's high plate efficiency.
• Screen Voltage Effects: At a constant Grid 1 voltage of 0 V, varying the screen voltage from 100 V to 250 V significantly scales the plate current capacity, with the plate current plateau rising from approximately 25 mA at $E_{c2} = 100\text{ V}$ to nearly 80 mA at $E_{c2} = 250\text{ V}$.
• Load Resistance and Distortion: The operation characteristics under typical Class A1 conditions (with a peak AC signal of 3.54 V RMS) show that the power output peaks at approximately 3.5 W with a 6,500 Ω load. Total harmonic distortion reaches a minimum of approximately 5.5% at a load resistance of 4,500 Ω, rises to 7% at the recommended 6,500 Ω load, and increases rapidly at higher load resistances, exceeding 12% as the load approaches 12,000 Ω.

Technical Summary

The 12BK5 is a high-performance, 9-pin miniature beam power amplifier featuring a 12.6 V, 0.6 A heater with a controlled 10.5-second warm-up time designed for series-connected heater strings. It is electrically identical to the 6BK5 except for its heater ratings. Operating under Class A1 conditions with a 250 V plate and screen supply, it provides a transconductance of 8,500 µmhos, a plate resistance of approximately 100,000 Ω, and delivers 3.5 W of power output into a 6,500 Ω load with approximately 7% total harmonic distortion. Its high power sensitivity and efficiency make it an excellent choice for audio power output and video amplifier stages in television and radio receivers.