12A8GT — Pentagrid Converter

An indirectly heated pentagrid converter designed for superheterodyne receiver applications, featuring a low-current heater optimized for series-string AC-DC mains operation.

Overview

The 12A8GT is a multi-electrode vacuum tube of the pentagrid converter type, designed primarily for frequency mixer and oscillator service in superheterodyne radio receivers. It features an indirectly heated, unipotential cathode and is housed in a compact glass envelope with an octal base.

The electrical characteristics of the 12A8GT are designed to match those of the 6.3-volt types 6A8, 6A8G, 6ABGT, and the single-ended 7B8, with the primary differences being its heater rating and specific interelectrode capacitances. The 12A8GT is specifically tailored for series-string heater arrangements in AC-DC receivers, where its low current draw significantly reduces overall power consumption and simplifies power supply design.

Physical Construction, Base, and Pinout

The 12A8GT is constructed in a T-9 glass bulb with a small wafer 8-pin octal base featuring a metal shell. The control grid (Grid #4) is terminated at a top cap to minimize input-to-output coupling and optimize RF performance.

Physical Dimensions

• Maximum Overall Height: 3-5/16 inches (approx. 84.1 mm)
• Maximum Seated Height: 2-3/4 inches (approx. 69.9 mm)
• Maximum Bulb Diameter: 1-3/16 inches (approx. 30.2 mm)
• Maximum Base Diameter: 1-5/16 inches (approx. 33.3 mm)
• Top Cap Diameter: 1/4 inch nominal

Pin Configuration

The tube utilizes an 8A pinout arrangement, viewed from the bottom of the base:

Heater Characteristics and Series-String Application

The heater of the 12A8GT is designed for either AC or DC operation. By operating at a nominal current of 0.15 A, the tube is highly efficient compared to older 0.3 A designs.

In AC-DC receivers, the 12A8GT's heater is connected in series with other 150 mA tubes. This configuration allows the series string to be powered directly from the mains supply through a small series resistor, eliminating the need for bulky, heat-generating line cords or plug-in dropping resistors. This design reduces the total heater power required for the entire receiver by approximately 50% compared to systems utilizing 0.3 A tubes.

Maximum Ratings

The following absolute maximum ratings must be observed to ensure reliable operation and prevent premature tube failure:

Typical Operating Conditions and Characteristics

The 12A8GT operates as a pentagrid converter where Grid #1 (Go) acts as the oscillator grid, Grid #2 (Ga) acts as the oscillator anode, Grid #4 (G) acts as the RF control grid, and Grids #3 and #5 (Gs) are tied together internally to act as the screen grid.

Converter Service Operating Points

¹ Applied through a properly by-passed 20,000 Ω dropping resistor. ² The published typical total cathode current is 4.6 mA for the low-voltage point and 10.6 mA for the high-voltage point.

Conversion Transconductance and AGC Action

The conversion transconductance ($S_c$) of the 12A8GT is highly dependent on the control grid bias, enabling effective Automatic Gain Control (AGC/AVC) in receiver circuits:

• At 100 V Plate Voltage:

  • $S_c = 350\text{ to } 360\ \mu\text{mhos}$ at $E_g = -1.5\text{ V}$
  • $S_c = 180\ \mu\text{mhos}$ at $E_g = -3.0\text{ V}$
  • $S_c = 50\ \mu\text{mhos}$ at $E_g = -6.0\text{ V}$
  • $S_c = 3\ \mu\text{mhos}$ (Approx.) at $E_g = -20\text{ V}$
  • Cut-off bias for $S_c = 2\ \mu\text{mhos}$ is $-20\text{ V}$

• At 250 V Plate Voltage:

  • $S_c = 500\text{ to } 550\ \mu\text{mhos}$ at $E_g = -3.0\text{ V}$
  • $S_c = 325\ \mu\text{mhos}$ at $E_g = -6.0\text{ V}$
  • $S_c = 100\ \mu\text{mhos}$ at $E_g = -10\text{ V}$
  • $S_c = 6\ \mu\text{mhos}$ (Approx.) at $E_g = -35\text{ V}$
  • Cut-off bias for $S_c = 2\ \mu\text{mhos}$ is $-45\text{ V}$

Oscillator Section Characteristics (Non-Oscillating)

When the oscillator section of the 12A8GT is evaluated under static, non-oscillating conditions, it exhibits the following characteristics:

• Transconductance: Approximately $1150\ \mu\text{mhos}$
• Amplification Factor ($\mu$): 75
• Oscillator Anode Current: 4.0 mA

These values are measured under the following specific test conditions:

• Plate Voltage: 250 V
• Oscillator Anode Voltage: 100 V
• Screen Voltage: 55 V
• Control Grid Voltage: -2.0 V
• Oscillator Grid Voltage: -1.0 V

Interelectrode Capacitances

The interelectrode capacitances of the 12A8GT vary slightly depending on the shielding configuration.

Capacitances with External Shield Connected to Cathode

• Control Grid (G) to Mixer Plate (P): 0.26 pF (max)
• Control Grid (G) to Oscillator Anode (Ga): 0.19 pF (max)
• Control Grid (G) to Oscillator Grid (Go): 0.16 pF (max)
• Oscillator Grid (Go) to Oscillator Anode (Ga): 1.1 pF
• RF Input (Control Grid to all other electrodes): 9.5 pF
• Oscillator Input (Go to all other electrodes except Ga): 6.0 pF
• Oscillator Output (Ga to all other electrodes except Go): 4.6 pF
• Mixer Output (Plate to all other electrodes): 12 pF

Capacitances with Standard Shield Can

• Grid #4 to Plate: 0.20 pF
• Grid #4 to Grid #2: 0.19 pF
• Grid #4 to Grid #1: 0.15 pF
• Grid #1 to Grid #2: 1.0 pF
• Grid #4 to all other electrodes: 7.5 pF
• Grid #2 to all other electrodes: 5.0 pF
• Plate to all other electrodes: 7.5 pF
• Grid #1 to all other electrodes: 6.2 pF

Technical Summary

The 12A8GT is a versatile, octal-based pentagrid converter designed for superheterodyne frequency conversion in space- and power-constrained AC-DC receivers. By combining the mixer and oscillator functions into a single envelope and utilizing a highly efficient 12.6 V, 150 mA heater, it significantly reduces the component count and power dissipation of radio receivers. With a maximum plate rating of 300 V and a conversion transconductance of up to 550 $\mu$mhos, the 12A8GT provides robust performance and excellent AGC characteristics, making it a staple of mid-20th-century series-string receiver designs.