12BX6 — Miniature RF and Video Pentode

The 12BX6 is a 9-pin miniature pentode designed primarily for use as a radio frequency (R.F.) amplifier, intermediate frequency (I.F.) amplifier, video amplifier, or mixer in television receivers. It features a coated unipotential cathode and is housed in a standard T-6½ glass envelope. Except for its heater ratings, the 12BX6 is electrically and physically identical to the 3BX6.

Physical Construction, Heater, and Pinout

The 12BX6 is constructed in a compact 9-pin miniature configuration. It can be operated in any mounting position.

Physical Dimensions

• Maximum Overall Length: 2⅝ inches (2.625 in)
• Maximum Seated Height: 2⅜ inches (2.375 in)
• Maximum Bulb Diameter: ⅞ inch (0.875 in)
• Bulb Type: T-6½ glass

Heater Characteristics

The tube features an indirectly heated, coated unipotential cathode. The heater is designed for operation from either an AC or DC supply.

• Heater Voltage: 12.6 V
• Heater Current: 0.15 A

Basing and Pin Connections

The 12BX6 uses the 9A0 bottom-view basing arrangement. The pin connections are assigned as follows:

The dual cathode pin connections (Pin 1 and Pin 3) allow for low-inductance cathode connections, which are critical for high-frequency performance.

Maximum Ratings

The maximum ratings for the 12BX6 are established under the Design Center System. These limits must not be exceeded under the worst-case operating conditions.

Typical Operating Conditions and Characteristics

The 12BX6 can be operated under various plate and screen voltages. The typical characteristics as an R.F. amplifier are documented at three distinct operating points:

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 12BX6, measured without an external shield, are as follows:

• Grid No. 1 to all other elements except Anode: 7.5 µµF
• Plate to all other elements except Grid No. 1: 3.3 µµF
• Plate to Grid No. 1 (Max.): 0.007 µµF
• Plate to Cathode (Max.): 0.012 µµF
• Grid No. 2 to all other elements: 5.4 µµF
• Grid No. 2 to Grid No. 1: 2.6 µµF
• Grid No. 1 to Heater (Max.): 0.15 µµF
• Cathode to Heater: 5 µµF

Application and Circuit Considerations

High-Frequency Input Resistance

At high frequencies, input loading becomes a significant factor. When operating the 12BX6 at 50 Mc, connecting Pin 1 directly to Pin 3 (the two cathode pins) helps minimize internal lead inductance. Under these conditions, the input resistance ranges from 10,000 Ω (at 170 V plate/screen) up to 15,000 Ω (at 250 V plate/screen).

Microphonic Prevention in Video Amplifiers

When the 12BX6 is utilized as a video amplifier in television receivers, care must be taken to avoid microphonic feedback. To prevent microphonic effects, the voltage amplification between the input grid of the 12BX6 and the input of the cathode-ray tube (CRT) must be restricted. Specifically, this stage gain should not exceed a maximum value of 25.

Technical Summary

The 12BX6 is a versatile 9-pin miniature pentode designed for high-frequency television receiver applications, including R.F., I.F., and video amplification, as well as mixer service. Operating with a 12.6 V, 0.15 A heater, it is identical to the 3BX6 except for its heater ratings. Under the Design Center System, it supports a maximum plate voltage of 250 V and a plate dissipation of 2.5 W. At typical operating points, it provides high transconductance (up to 7,400 µmhos) and a high grid-to-screen amplification factor of 50. When implemented in video amplifier circuits, the stage gain to the CRT input must be kept below 25 to mitigate microphonics, while its dual cathode pinout allows designers to minimize input loading at high frequencies by tying Pin 1 and Pin 3 together.