12BQ6GTB — Beam Power Pentode

The 12BQ6GTB (also designated as 12BQ6GTB/12CU6) is a glass octal beam power tube designed primarily for use as a horizontal-deflection amplifier in both color and black-and-white television receivers. It features high power sensitivity, high plate current capability at low plate voltages, and is designed to withstand high-voltage pulses.

Construction and Physical Characteristics

The 12BQ6GTB is constructed with a T-9 glass envelope and utilizes a standard octal base. The tube's physical dimensions and mechanical specifications are as follows:

• Envelope: T-9 glass bulb
• Base: Octal type, with specific base designations including 85-187, 86-81, B6-84, B7-7, or B7-59. The tube may be supplied with pin number 1 omitted.
• Top Cap: C1-2, C1-3, or C1-33
• Basing Arrangement: 6AM
• Outline Designation: 9-49 or 9-50
• Maximum Diameter: 1.188 inches
• Maximum Seated Height: 3.313 inches
• Maximum Overall Height: 3.875 inches
• Mounting Position: Any

The 6AM basing diagram provides the following pin connections (viewed from the bottom of the base):

• Pin 1: No Connection (NC) / Omitted
• Pin 2: Heater (H)
• Pin 3: No Connection (NC)
• Pin 4: Grid No. 2 (Screen Grid)
• Pin 5: Grid No. 1 (Control Grid)
• Pin 7: Heater (H)
• Pin 8: Cathode (K) and Grid No. 3 (Suppressor Grid)
• Top Cap: Plate (P)

Heater Characteristics and Family Variations

The 12BQ6GTB belongs to a family of identical beam power tubes that differ only in their heater ratings and warm-up characteristics. The heater is designed for operation from either an AC or DC supply.

Heater Specifications by Type

Heater-Cathode Voltage Ratings

The insulation between the heater and cathode is rated to withstand significant potential differences:

• Heater Negative with Respect to Cathode:
  • Peak value: 200 V maximum
  • Total DC and peak: 200 V maximum
• Heater Positive with Respect to Cathode:
  • DC value: 100 V maximum (average value)
  • Total DC and peak: 200 V maximum

Direct Interelectrode Capacitances

The approximate direct interelectrode capacitances of the tube, measured without an external shield, are:

• Grid No. 1 to Plate: 0.6 pF
• Input (Grid No. 1 to Cathode, Heater, Grid No. 2, and Grid No. 3): 15 pF
• Output (Plate to Cathode, Heater, Grid No. 2, and Grid No. 3): 7.0 to 7.5 pF

Class A1 Amplifier Characteristics

Under typical Class A1 operating conditions, the 12BQ6GTB exhibits the following electrical characteristics at various operating points:

Typical Operating Points

Cutoff and Knee Characteristics

• Grid No. 1 Voltage for Plate Current of 1.0 mA (Approx.): -43 V (at a plate voltage of 250 V and Grid No. 2 voltage of 150 V).
• Instantaneous Plate Knee Values: At a plate voltage of 60 V, Grid No. 2 voltage of 150 V, and Grid No. 1 voltage of 0 V, the instantaneous plate current is 260 mA and the Grid No. 2 current is 26 mA.

Horizontal-Deflection Amplifier Service

The primary application of the 12BQ6GTB is as a horizontal-deflection amplifier in television receivers operating under a 525-line, 30-frame scanning system.

Maximum Ratings (Design-Center Values)

• DC Plate Voltage: 600 V
• Peak Positive-Pulse Plate Voltage (Absolute Maximum): 6000 V
• Peak Negative-Pulse Plate Voltage: 1250 V
• DC Grid No. 2 (Screen Grid) Voltage: 200 V
• Peak Negative-Pulse Grid No. 1 (Control Grid) Voltage: 300 V
• Peak Cathode Current: 400 mA
• Average Cathode Current: 110 mA
• Plate Dissipation: 11 W
• Grid No. 2 Dissipation: 2.5 W
• Bulb Temperature (at hottest point): 220 °C

Operating Constraints and Circuit Values

• Maximum Grid No. 1 Circuit Resistance: 0.47 MΩ
• Pulse Duration Limits: For the peak positive-pulse plate voltage rating of 6000 V, the pulse duration must not exceed 15% of a horizontal scanning cycle (equivalent to 10 microseconds). Under no circumstances should this absolute maximum value be exceeded.
• Tube Protection: A bias resistor or other suitable means is required to protect the tube from excessive dissipation in the absence of excitation.

Predecessor Comparison (6BQ6GT)

The earlier 6BQ6GT has different typical operating characteristics and ratings. For comparison, the typical single-tube Class A1 operation for the 6BQ6GT is specified as:

• Plate Voltage: 250 V
• Grid No. 2 Voltage: 250 V
• Grid No. 1 Voltage: -7.3 V
• Peak AF Grid No. 1 Voltage: 6.2 V
• Zero-Signal Plate Current: 48 mA
• Maximum-Signal Plate Current: 50.6 mA
• Zero-Signal Grid No. 2 Current: 5.5 mA
• Maximum-Signal Grid No. 2 Current: 10 mA
• Plate Resistance (Approx.): 38,000 Ω
• Transconductance: 11,300 µmhos
• Load Resistance: 4500 Ω
• Total Harmonic Distortion: 10%
• Maximum-Signal Power Output: 5.7 W

The 6BQ6GT also supports push-pull Class AB1 amplifier operation with plate and Grid No. 2 supply voltages of 250 V or 300 V, delivering maximum-signal power outputs of 11 W and 17 W respectively. However, the 12BQ6GTB/12CU6 features updated ratings, including an 11 W plate dissipation limit and a 2.5 W Grid No. 2 dissipation limit, which must be strictly observed in horizontal-deflection service.

Technical Summary

The 12BQ6GTB is an octal-based, glass-envelope beam power pentode designed for horizontal-deflection amplifier service in television receivers. It features a 12.6 V, 0.6 A heater with a controlled 11-second warm-up time, making it suitable for series-string heater designs. The tube is capable of handling peak positive plate pulses up to 6000 V and peak cathode currents of 400 mA. At a typical Class A1 operating point of 250 V on the plate and 150 V on Grid No. 2, it exhibits a transconductance of 5900 µmhos and a plate resistance of 14,500 Ω. The 12BQ6GTB provides robust performance and high current capability at low plate voltages, making it highly effective in television deflection circuits.