12AQ5 — Beam Power Pentode

An indirectly heated, miniature beam power pentode designed for audio-frequency power amplification and television vertical deflection sweep circuits.

Overview

The 12AQ5 is a miniature 7-pin beam power pentode intended primarily for use in the audio-frequency output stage of automobile radio receivers operating from a 12-volt storage battery, as well as in the output stages of AC-operated receivers. Additionally, when triode-connected, the tube is designed for service as a vertical deflection amplifier in television sweep circuits. Within its maximum ratings, the electrical performance of the 12AQ5 is equivalent to that of the larger octal glass types 12V6-GT, 6V6, and 6V6GTA.

Construction, Heater, and Physical Dimensions

The 12AQ5 features a coated unipotential cathode and is housed in a T-5 1/2 glass bulb. It utilizes a small-button miniature 7-pin base (JETEC No. E7-1) with a 7BZ basing arrangement. The tube is designed to operate in any mounting position.

Physical Dimensions

• Maximum Overall Length: 2-5/8 inches
• Maximum Seated Length: 2-3/8 inches
• Length from Base Seat to Bulb Top (excluding tip): 2 inches  3/32 inch
• Maximum Diameter: 3/4 inch

Heater Characteristics

The heater is designed for operation from AC or DC power supplies:

• Nominal Heater Voltage: 12.6 V
• Nominal Heater Current: 0.225 A

Basing and Terminal Connections

Under the 7BZ basing configuration, viewed from the bottom of the tube, the pin connections are assigned as follows:

Grid No. 1 is internally connected to both Pin 1 and Pin 7.

Direct Interelectrode Capacitances

The approximate capacitances of the 12AQ5, measured without an external shield, are:

• Grid No. 1 to Plate: 0.35 pF to 0.4 pF
• Input (Grid No. 1 to Cathode, Heater, Grid No. 2, and Grid No. 3): 8.0 pF to 8.3 pF
• Output (Plate to Cathode, Heater, Grid No. 2, and Grid No. 3): 8.2 pF to 8.5 pF

Maximum Ratings and Rating Systems

The maximum ratings of the 12AQ5 depend on the rating system applied by the manufacturer (Design-Center System vs. Design-Maximum System) and the specific class of service.

Class A1 and Class AB1 Audio-Frequency Power Amplifier Ratings

• Plate Voltage: 250 V max (Design-Center) / 275 V max (Design-Maximum)
• Grid No. 2 (Screen-Grid) Voltage: 250 V max (Design-Center) / 275 V max (Design-Maximum)
• Plate Dissipation: 12 W max
• Grid No. 2 Input / Dissipation: 2 W max
• Bulb Temperature (at hottest point): 250 C max
• Heater-Cathode Voltage:
  • Heater negative with respect to cathode: 90 V max (RCA Design-Center), 100 V max (Tung-Sol/Westinghouse Design-Center), 200 V max (Design-Maximum peak value).
  • Heater positive with respect to cathode: 90 V max (RCA Design-Center), 100 V max (Tung-Sol/Westinghouse Design-Center DC or total DC and peak), 100 V max average / 200 V max peak (Design-Maximum).
• Maximum Grid No. 1 Circuit Resistance:
  • Fixed Bias: 0.1 megohm max
  • Cathode Bias: 0.5 megohm max
  • Note: If the Grid No. 1 circuit resistance is common to two tubes, these maximum values per tube must be halved.

Typical Operating Conditions

The 12AQ5 can be operated as a single-tube Class A1 amplifier or in a push-pull Class AB1 configuration.

Class A1 Pentode Amplifier (Single Tube)

Class AB1 Push-Pull Amplifier (Two Tubes)

Values are for two tubes unless otherwise specified.

• Plate Voltage: 250 V
• Grid No. 2 Voltage: 250 V
• Grid No. 1 Voltage: -15 V
• Peak AF Grid-to-Grid Voltage: 30 V
• Zero-Signal Plate Current: 70 mA
• Maximum-Signal Plate Current: 79 mA
• Zero-Signal Grid No. 2 Current (approx.): 5.0 mA
• Maximum-Signal Grid No. 2 Current (approx.): 13 mA
• Plate Resistance (per tube, approx.): 60,000 ̐
• Transconductance (per tube): 3,750 mhos
• Effective Plate-to-Plate Load Resistance: 10,000 ̐
• Total Harmonic Distortion: 5%
• Maximum-Signal Power Output: 10 W

Triode-Connected Characteristics and Vertical Deflection Service

When triode-connected (with Grid No. 2 connected directly to the plate), the 12AQ5 exhibits the following typical characteristics at a plate voltage of 250 V and a Grid No. 1 voltage of -12.5 V:

• Plate Current: 49.5 mA
• Amplification Factor: 9.5 to 9.8
• Plate Resistance (approx.): 1,960 ̐ to 1,970 ̐
• Transconductance: 4,800 mhos to 5,000 mhos
• Grid No. 1 Voltage (approx.) for a plate current of 0.5 mA: -36 V to -37 V

Vertical Deflection Amplifier Service (Triode Connection)

For operation in a 525-line, 30-frame television system, the maximum ratings are defined as:

• DC Plate Voltage: 275 V max (Design-Maximum) / 315 V max (Design-Center)
• Peak Positive-Pulse Plate Voltage: 1,100 V absolute max (Design-Maximum) / 1,200 V absolute max (Design-Center)
• Peak Negative-Pulse Grid No. 1 Voltage: 250 V max (Design-Center) / 275 V max (Design-Maximum)
• Average Cathode Current: 35 mA max (Design-Center) / 40 mA max (Design-Maximum)
• Peak Cathode Current: 105 mA max (Design-Center) / 115 mA max (Design-Maximum)
• Plate Dissipation: 9 W max (Design-Center) / 10 W max (Design-Maximum)
• Maximum Grid No. 1 Circuit Resistance (Cathode Bias): 2.2 megohms
• Note on Pulse Duration: The pulse duration must not exceed 15% of a vertical scanning cycle (equivalent to 2.5 milliseconds).
• Note on Bias: In stages operating with grid leak bias, an adequate cathode bias resistor or other suitable means is required to protect the tube in the absence of excitation.

Storage-Battery Operating Considerations

When the 12AQ5 is employed in storage-battery-operated equipment (such as automobile receivers), specific design guidelines must be followed to prevent exceeding the maximum ratings:

• Without Charger: The equipment should be designed so that the published maximum values of plate voltage, Grid No. 2 voltage, and dissipations are never exceeded for a terminal potential at the battery source of 2.0 volts per cell.
• With Charger: The equipment should be designed so that 90 percent of the published maximum values is never exceeded for a terminal potential at the battery source of 2.2 volts per cell.

Thermal and Shielding Recommendations

High ambient temperatures and the use of tube shields may necessitate a reduction in operating dissipation. When tube shields are utilized, it is recommended to paint both the inside and outside surfaces of the shield a dull black and to provide ventilation slots to reduce the operating temperature of the bulb.

Technical Summary

The 12AQ5 is a versatile, high-efficiency miniature beam power pentode with a 12.6 V, 0.225 A heater and a 7-pin base. It is electrically equivalent to the octal 12V6-GT and provides high power sensitivity and output in both Class A1 and Class AB1 audio amplifier applications. Additionally, its triode-connected characteristics make it highly effective as a vertical deflection amplifier in television sweep circuits. Designers must carefully observe the differences between Design-Center and Design-Maximum ratings, particularly when implementing the tube in automotive storage-battery environments or pulsed television deflection circuits.