12AW6 — Sharp Cut-Off RF Pentode

Overview

The 12AW6 is a miniature, sharp cut-off radio frequency (RF) pentode designed for use in compact, lightweight electronic equipment. It features a high transconductance and low input and output capacitances, making it highly effective as an RF amplifier at frequencies up to approximately 400 megacycles (MHz), as well as a high-frequency intermediate-frequency (IF) amplifier.

The tube is constructed with an indirectly heated unipotential cathode and is housed in a T-5 1/2 glass envelope with a 7-pin miniature button base. Electrically, the 12AW6 is equivalent to the 6AG5, sharing identical ratings and characteristics but utilizing a 12.6-volt heater.

Physical Construction and Pinout

The 12AW6 is built in a small T-5 1/2 glass bulb. Its physical dimensions are:

• Maximum overall length: 2 1/8 inches
• Maximum seated height: 1 7/8 inches
• Maximum diameter: 3/4 inch

The tube can be mounted in any position. It utilizes a standard 7-pin miniature button base with the following pin assignments, viewed from the bottom of the tube:

The separate suppressor grid connection (Pin 7) allows for direct grounding or flexible circuit configurations, which contributes to the tube's stable high-frequency performance.

Heater and Interelectrode Capacitances

The 12AW6 features a unipotential cathode heated by a 12.6-volt filament. The heater can be operated on either alternating current (AC) or direct current (DC).

Direct Interelectrode Capacitances

Measured without an external shield, the direct interelectrode capacitances of the 12AW6 are:

• Grid to Plate (G1 to P) (Maximum): 0.025 pF (uuf)
• Input (G1 to H + K + G2 + G3 & IS): 6.5 pF (uuf)
• Output (P to H + K + G2 + G3 & IS): 1.5 pF (uuf)

These low capacitances, particularly the very small grid-to-plate feedback capacitance, are critical for maintaining stability and preventing oscillation in high-frequency amplifier stages.

Maximum Ratings

The maximum ratings for the 12AW6 are interpreted according to the RMA Standard M8-210. The tube can be operated either in its standard pentode configuration or as a triode (with Grid No. 2 tied to the Plate and Grid No. 3 tied to the Cathode).

Typical Operating Conditions and Characteristics

The 12AW6 can be operated as a Class A1 amplifier in either pentode or triode connection.

Class A1 Pentode Connection

In the pentode configuration, Grid No. 3 is connected directly to the cathode at the socket. Typical operating points at three different plate voltages are:

Class A1 Triode Connection

When connected as a triode (with Grid No. 2 tied to the Plate and Grid No. 3 tied to the Cathode), the typical operating points are:

Characteristic Curves and Behavior

The published performance curves for the 12AW6 illustrate its behavior under both pentode and triode operating conditions.

Pentode Connection Curves

Under pentode operation with a fixed screen grid voltage (Ec2 = 150 V) and the suppressor grid at ground (Ec3 = 0 V), the plate current (Ib) and screen current (Ic2) are plotted against plate voltage:

• At a control grid bias (Ec1) of 0 V, the plate current rises sharply below 50 volts, leveling off to a nearly flat plateau of approximately 14 mA across the 100 V to 500 V range.
• As the negative grid bias increases (from -0.5 V down to -4.0 V), the plate current plateaus decrease progressively. At Ec1 = -4.0 V, the plate current is reduced to less than 1 mA, demonstrating the sharp cut-off characteristic of the tube.
• The screen grid current (Ic2) curves follow a similar trend but at lower magnitudes, with Ic2 reaching approximately 5.5 mA at Ec1 = 0 V and dropping to near zero as the control grid is biased more negatively.

Triode Connection Curves

In the triode-connected configuration, where Grid No. 2 is tied to the plate, the plate current curves exhibit typical triode behavior:

• The curves rise more gradually with plate voltage compared to the sharp knee of the pentode curves.
• At Ec = 0 V, plate current begins to flow immediately as plate voltage rises, reaching 15 mA at approximately 100 V.
• Successive negative grid bias curves (from -1 V down to -14 V) shift the plate current onset to higher plate voltages. For example, at a grid bias of -14 V, plate current only begins to flow as the plate voltage approaches 450 V.

Technical Summary

The 12AW6 is a high-performance, 7-pin miniature RF pentode featuring a sharp cut-off characteristic and high transconductance. Operating with a 12.6 V, 150 mA heater, it is electrically equivalent to the 6AG5 and is optimized for high-frequency amplification up to 400 MHz. Its design incorporates low interelectrode capacitances and a separate suppressor grid connection to ensure stable operation in compact RF and IF amplifier stages. With maximum ratings of 300 V on the plate and a plate dissipation limit of 2.0 W (pentode) or 2.5 W (triode), the 12AW6 provides a versatile and reliable solution for high-frequency receiving applications.