12B4A — Low-Mu High-Perveance Triode

Overview

The 12B4A (also written as 12B4-A) is a miniature, low-mu, high-perveance single triode designed primarily for service as a vertical-deflection amplifier in television receivers. The tube features high plate current capabilities at relatively low plate voltages and is engineered to withstand the high-amplitude positive pulse voltages typically encountered in vertical-deflection circuits.

In addition, the 12B4A exhibits a controlled heater warm-up characteristic, making it particularly suited for television receivers employing series-connected heater strings. When operated in series with other 600-milliampere tubes possessing similar controlled warm-up times, heater voltage surges across individual tubes are minimized during the initial warm-up cycle. Except for this controlled heater thermal characteristic, the electrical and physical characteristics of the 12B4A are identical to those of the standard 12B4.

Mechanical Specifications and Pinout

The 12B4A is constructed in a T-6 1/2 glass envelope and utilizes a Small-Button Noval 9-pin base (JETEC No. E9-1). The tube requires a standard miniature 9-contact socket and can be mounted in any position.

Its physical dimensions are as follows:

• Maximum Overall Length: 2-5/8 inches
• Maximum Seated Length: 2-3/8 inches
• Length from Base Seat to Bulb Top (excluding tip): 2 inches ± 3/32 inch
• Maximum Diameter: 7/8 inch

The tube uses the 9AG basing arrangement, viewed from the bottom of the base. The pin connections are assigned as follows:

The control grid is internally connected to both Pin 2 and Pin 7, providing dual terminal access. The heater is center-tapped at Pin 3, allowing for either series or parallel heater supply configurations.

Heater and Warm-Up Characteristics

The 12B4A features a coated unipotential cathode. The heater is designed for versatile operation, supporting both series and parallel electrical connections:

• Series Connection: Under series operation, the heater voltage is 12.6 V (AC or DC) at a nominal current of 0.3 A (or 0.300 A).
• Parallel Connection: Under parallel operation, the heater voltage is 6.3 V (AC or DC) at a nominal current of 0.6 A (or 0.600 A).

The average heater warm-up time is 11 seconds. The control of heater thermal characteristics is specifically applied to the 600-milliampere (parallel) heater connection.

Heater warm-up time is defined as the time required for the voltage across the heater terminals to reach 80% of its rated value when a voltage of four times the rated heater voltage (25 V, RMS or DC) is applied to a test circuit. This test circuit consists of the tube heater connected in series with a resistance equal to three times the nominal heater operating resistance (31.5 ohms), with the target test voltage across the heater being 5.0 V (RMS or DC).

Electrical Characteristics and Class A1 Operation

At typical operating points, the 12B4A exhibits low plate resistance and high transconductance, reflecting its high-perveance design. The direct interelectrode capacitances, measured without an external shield, are approximately:

• Grid to Plate (G to P): 4.8 pF
• Input (Grid to Cathode and Heater): 5.0 pF
• Output (Plate to Cathode and Heater): 1.5 pF

The tube's electrical characteristics under Class A1 amplifier conditions are documented at two distinct operating points:

The fundamental triode relationship can be mathematically illustrated using the values from Operating Point A: $\mu \approx g_m \times r_p$ Substituting the typical values: $0.0063 \text{ S} \times 1030 \Omega = 6.489$ This calculated value closely aligns with the published nominal amplification factor of 6.5.

Vertical-Deflection Amplifier Service

When utilized as a vertical-deflection amplifier in a 525-line, 30-frame television system, the 12B4A is subjected to demanding pulse conditions. The maximum ratings for this service are defined under the Design-Center system (except where noted):

• DC Plate Voltage: 550 max. volts
• Peak Positive-Pulse Plate Voltage (Absolute Maximum): 1000 max. volts. This is an absolute limit that must not be exceeded under any circumstances. The pulse duration is restricted to a maximum of 15% of one vertical scanning cycle, which corresponds to 2.5 milliseconds (or 2.6 milliseconds).
• Peak Negative-Pulse Grid Voltage: 250 max. volts
• Peak Cathode Current: 105 max. milliamperes
• Average Cathode Current: 30 max. milliamperes
• Plate Dissipation: 5.5 max. watts
• Maximum Grid-Circuit Resistance (Cathode Bias): 2.2 max. megohms

In stages operating with grid-leak bias, the tube must be protected against the absence of excitation. An adequate cathode-bias resistor or other suitable protective means is required to prevent excessive plate dissipation and cathode current if the drive signal is lost.

Maximum Ratings and Circuit Values (Class A1)

For general Class A1 amplifier service, the maximum ratings (Design-Center Values) and circuit limits are:

• Plate Voltage: 550 max. volts
• Grid Voltage, Negative-bias value: 50 max. volts
• Plate Dissipation: 5.5 max. watts
• Maximum Grid-Circuit Resistance:
  • For fixed-bias operation: 0.47 max. megohm
  • For cathode-bias operation: 2.2 max. megohms
• Heater-Cathode Voltage:
  • Heater negative with respect to cathode (Total DC and Peak): 200 max. volts
  • Heater positive with respect to cathode (Total DC and Peak): 200 max. volts, provided the DC component does not exceed 100 volts.

Analysis of Characteristic Curves

The average plate and transfer characteristics of the 12B4A illustrate its high-perveance design and low-mu behavior:

• Average Plate Characteristics: The plate-current-versus-plate-voltage curves demonstrate that the 12B4A can conduct high currents at low plate potentials. For instance, at zero grid bias (E_c = 0 V), the plate current rises rapidly, exceeding 80 mA before the plate voltage reaches 100 V. The curves are bounded by a dashed boundary representing the 5.5-watt plate dissipation limit. At more negative grid biases, the curves shift to the right, showing a gradual and linear control characteristic typical of a low-mu triode.
• Average Transfer Characteristics: The transfer curves plot plate current against grid voltage for various plate voltages ranging from 50 V to 550 V. These curves show that at a typical plate voltage of 150 V, the plate current is cut off (approaching zero) near a grid bias of -32 V, which matches the typical characteristic value of -32 V for a 200 \mu$A plate current. At the maximum rated plate voltage of 550 V, the grid voltage required to achieve cutoff shifts to approximately -100 V. The relatively wide grid voltage swing required to control the plate current is a direct consequence of the low amplification factor (\mu = 6.5).

Technical Summary

The 12B4A is a miniature, low-mu, high-perveance triode housed in a 9-pin T-6 1/2 glass envelope with a 9AG basing configuration. It is designed primarily for vertical-deflection amplifier service in television receivers, where it handles high peak currents at low plate voltages and withstands peak positive-pulse plate voltages up to 1000 V. The tube features a controlled heater warm-up time of 11 seconds for 600 mA series-string applications, and can be operated with either a 12.6 V / 0.3 A series heater or a 6.3 V / 0.6 A parallel heater. With an amplification factor of 6.5, a low plate resistance of approximately 1030 ohms, and a plate dissipation rating of 5.5 watts, the 12B4A provides robust performance in both Class A1 amplifier and vertical-deflection circuits.