12B8GT — Triode-Pentode Midget Vacuum Tube

The 12B8GT is a multi-unit midget vacuum tube combining a remote cut-off pentode and an independent high-mu triode within a single glass envelope. Designed primarily for use in compact AC-DC radio receivers where space is highly limited, this tube combines two distinct amplification systems sharing a common heater but utilizing separate, unipotential cathodes.

Physical Construction and Pinout

The 12B8GT is constructed in a T9-C glass bulb with a small wafer 8-pin octal base featuring a metal shell. The physical dimensions of the tube are compact, with a maximum overall height of 3 9/16 inches, a maximum seated height of 3 inches, and a maximum bulb diameter of 1 5/16 inches.

The tube features an 8-pin octal base configuration combined with a top cap. The control grid of the pentode section is routed to the top cap to minimize input-to-output coupling and optimize high-frequency performance. The remaining electrodes are connected to the base pins.

The pin connections, viewed from the bottom of the tube, are assigned as follows:

The heater is designed for series or parallel operation in AC-DC circuits, requiring a nominal voltage of 12.6 V and a current of 0.3 A. The heater can be operated on either AC or DC. Both the triode and pentode sections utilize independent, coated unipotential cathodes, allowing them to be biased separately.

Electrical Characteristics and Operating Points

The 12B8GT operates both of its internal sections at a typical plate potential of 90 V. The electrical characteristics of each section under standard Class A1 operating conditions are detailed below.

Pentode Section

The pentode section is designed as a remote cut-off high-frequency amplifier. Under typical operating conditions, its parameters are:

The remote cut-off characteristic is demonstrated by the control grid voltage of -42.5 V required to reduce the transconductance to a nominal 2 µmhos, enabling effective automatic volume control (AVC) action.

Triode Section

The triode section is a high-mu voltage amplifier. Under typical operating conditions, its parameters are:

The triode section features a sharp cut-off characteristic, with plate current reaching cut-off at approximately -2.5 V of grid bias.

Direct Interelectrode Capacitances

The close proximity of the two independent tube structures within a single small envelope requires careful shielding and layout. The direct interelectrode capacitances of the individual sections and the cross-coupling capacitances between the pentode and triode sections are as follows:

Pentode Section Capacitances

• Control Grid to Plate: 0.015 µµF
• Input (Control Grid to Cathode): 5.0 to 5.2 µµF
• Output (Plate to Cathode): 9.5 to 9.6 µµF

Triode Section Capacitances

• Control Grid to Plate: 2.3 to 2.4 µµF
• Control Grid to Cathode: 5.0 µµF
• Plate to Cathode: 6.3 to 6.5 µµF

Inter-Section Capacitances

• Pentode Control Grid to Triode Grid: 0.002 µµF
• Pentode Control Grid to Triode Plate: 0.003 µµF
• Pentode Plate to Triode Plate: 0.08 µµF
• Pentode Plate to Triode Grid: 0.078 µµF

These low inter-section capacitances indicate that despite sharing a single bulb, the two sections are electrostatically well-isolated, minimizing unwanted feedback or crosstalk when operated in separate stages.

Applications and Circuit Service

The 12B8GT is specifically tailored for space-saving applications in small AC-DC receivers. The dual-section design allows a single tube envelope to perform multiple critical receiver functions:

• Pentode Section: The remote cut-off pentode section is optimized for use as a conventional radio-frequency (RF) or intermediate-frequency (IF) amplifier. Its remote cut-off characteristic allows it to handle a wide range of signal amplitudes without severe distortion when controlled by an automatic volume control bias.
• Triode Section: The high-mu triode section is designed to operate as a detector. It can be configured either as a biased detector (plate detection) or as a grid-leak detector, providing efficient demodulation of the RF/IF signal.

By combining these two stages, the 12B8GT reduces the total tube count and physical footprint required for the receiver's front-end and detector stages.

Technical Summary

The 12B8GT is a compact, indirectly heated triode-pentode vacuum tube designed for space-constrained AC-DC receiver applications. Operating from a 12.6 V, 0.3 A heater, it houses a remote cut-off pentode (amplification factor of 360, transconductance of 1800 µmhos) and an independent high-mu triode (amplification factor of 90, transconductance of 2400 µmhos). With its small T9-C glass envelope, 8-pin octal base, and top-cap control grid connection, the 12B8GT provides a highly integrated solution for RF/IF amplification and detection in compact radio designs.