18GE6A — High-Mu Triode Double Diode

The 18GE6A is a miniature, multi-unit vacuum tube containing a high-mu triode and two independent diodes within a single envelope. It is designed primarily for detector-amplifier applications in AC/DC radio receivers. The tube features a coated unipotential cathode and a heater designed for series-string operation with controlled warm-up characteristics. The 18GE6A is identical to the older 18GE6 except for its controlled heater warm-up time, and it directly replaces the now-obsolete 18GE6.

Physical Construction, Base, and Pinout

The 18GE6A is constructed in a compact T-5 1/2 glass bulb outline (JEDEC 5-2) with a miniature button 7-pin base (E7-1). The tube can be mounted and operated in any position.

The physical dimensions of the envelope are:

• Maximum overall length: 2-1/8 inches
• Maximum bulb length: 1-7/8 inches
• Maximum diameter: 3/4 inch

The bottom-view basing diagram (JEDEC 7BT) provides the following pin connections:

Heater Ratings and Warm-up Control

The heater of the 18GE6A is designed for series-string operation in AC/DC receivers. To ensure reliable operation in series circuits, the heater current is tightly controlled, and the tube features a specified warm-up time to minimize voltage surges across other heaters in the string during start-up.

• Heater Voltage (Average): 18 Volts
• Heater Current (Average): 0.100 Amps (100 mA)
• Heater Current Limits (Design-Maximum): 0.094 to 0.106 Amps (94 to 106 mA)
• Heater Warm-up Time: 20 Seconds

The heater warm-up time is defined as the time required for the voltage across the heater to reach 80% of its rated voltage (14.4 V) when four times the rated heater voltage (72 V) is applied to a circuit consisting of the tube heater in series with a resistance equal to three times the nominal heater operating resistance. To maintain the heater current within the specified limits, heater voltage supply variations must be restricted.

Electrical Characteristics and Typical Operation

The triode section of the 18GE6A is designed for Class A1 voltage amplification, while the two diode units are intended for AM detection and automatic volume control (AVC) functions.

Class A1 Triode Amplifier Characteristics

At a typical operating point, the triode section exhibits the following characteristics:

As a calculation, multiplying the transconductance of 0.0017 mhos (1,700 µmhos) by the plate resistance of 40,000 Ohms yields a calculated amplification factor of 68, which closely matches the published nominal amplification factor of 70.

Diode Characteristics

The two diode plates are used for signal detection. For testing purposes, each diode unit is characterized by its average current under a specific test condition:

• Average Diode Current (Each Diode): 2.0 mA (with 10 Volts DC applied; test condition only)

Design-Maximum Ratings

The ratings of the 18GE6A are established under the Design-Maximum System. These limits must not be exceeded under the worst-case operating conditions of supply voltage, component tolerances, and environmental variations.

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 18GE6A are measured without an external shield. These values determine the high-frequency performance and isolation between the triode and diode sections:

• Grid to Plate: 1.8 pF
• Input (Grid to Heater + Cathode): 2.4 pF
• Output (Plate to Heater + Cathode): 0.2 pF
• Grid to Diode No. 2 Plate: 0.2 pF (Maximum)

Technical Summary

The 18GE6A is a 7-pin miniature high-mu triode double diode designed for detector-amplifier service in AC/DC radio receivers. Operating with an 18 V, 100 mA heater, it features a controlled 20-second warm-up time to facilitate safe series-string operation. The triode section provides an amplification factor of 70 with a transconductance of 1,700 µmhos at a typical plate voltage of 100 V. The dual independent diodes allow for simultaneous detection and AVC generation. Housed in a T-5 1/2 envelope, the 18GE6A serves as a direct, modern replacement for the obsolete 18GE6.