18GD6A — Sharp-Cutoff Pentode

The 18GD6A is a miniature, sharp-cutoff pentode featuring a coated unipotential cathode and a heater with a controlled warm-up time. It is primarily designed for radio frequency (RF) and intermediate frequency (IF) amplifier applications, as well as autodyne converter service in AC/DC radio receivers and other electronic equipment. The 18GD6A directly replaces the obsolete 18GD6, with which it is identical except for its controlled heater warm-up characteristics.

Mechanical Specifications and Pinout

The 18GD6A is constructed in a T-5 1/2 glass envelope with a small-button miniature 7-pin base (JEDEC designation E7-1). The tube can be operated in any mounting position.

Physical Dimensions

• Maximum Overall Length: 2-1/8 inches (2.125 in)
• Maximum Seated Height: 1-7/8 inches (1.875 in)
• Length from Base Seat to Bulb Top (excluding tip): 1-1/2 ± 3/32 inches
• Diameter: 0.650 to 0.750 inches (maximum 0.750 in)

Pin Connections (JEDEC 7BK Basing, Bottom View)

The 7-pin base configuration provides the following connections:

• Pin 1: Grid No. 1 (G1)
• Pin 2: Grid No. 3 (G3) / Internal Shield (IS)
• Pin 3: Heater (H)
• Pin 4: Heater (H)
• Pin 5: Plate (P)
• Pin 6: Grid No. 2 (G2)
• Pin 7: Cathode (K)

Heater and Maximum Ratings

The heater is designed for series string operation in AC/DC equipment and features a controlled warm-up time to ensure reliable operation.

Heater Specifications

• Heater Voltage (AC or DC): 18 V
• Heater Current: 0.100 A (nominal), with an acceptable range of 94–106 mA (±6% tolerance)
• Heater Warm-up Time: 20 seconds. This is defined as the time required to reach 80% of the rated heater voltage when 4 times the rated voltage is applied in series with a resistor equal to 3 times the nominal heater operating resistance.
• Maximum Heater-Cathode Voltage: 100 V (total DC and peak) with the heater either positive or negative with respect to the cathode.

Design-Maximum Ratings (EIA Standard RS-239)

• Maximum Plate Voltage: 150 V
• Maximum Grid No. 2 (Screen-Grid) Supply Voltage: 150 V
• Maximum Plate Dissipation: 2.5 W
• Maximum Grid No. 2 Dissipation: 0.6 W (for Grid No. 2 voltages up to 75 V; for voltages between 75 V and 150 V, dissipation limits are subject to specific rating charts)

Electrical Characteristics and Typical Operation

Under typical Class A1 amplifier operating conditions, the 18GD6A exhibits high transconductance and high plate resistance.

Typical Operating Conditions

• Plate Voltage: 100 V
• Grid No. 3 (Suppressor Grid): Connected to cathode at socket
• Grid No. 2 Voltage: 100 V
• Cathode Bias Resistor: 150 Ω
• Plate Current: 5 mA
• Grid No. 2 Current: 2.0 mA
• Transconductance: 4300 µmhos
• Plate Resistance (Approximate): 0.5 MΩ
• Grid No. 1 Voltage for Cutoff (approximate, at Ib = 10 µA): -4.7 V

Direct Interelectrode Capacitances

Capacitances are measured either with or without an external shield (JEDEC No. 316) connected to the cathode:

• Grid No. 1 to Plate (Maximum): 0.0035 pF
• Input (G1 to H + K + G2 + G3 + IS): 6.0 pF
• Output (P to H + K + G2 + G3 + IS): 5.0 pF

Qualitative Curve Analysis

The average plate characteristics of the 18GD6A show a sharp knee below 25 V, which is characteristic of sharp-cutoff pentodes. The screen-grid (Grid No. 2) current decreases as the plate voltage increases. Under typical supply conditions (Eb = 100 V, Ec2 = 100 V), the transconductance falls from approximately 5300 µmhos at zero grid bias (Ec1 = 0 V) to near zero at approximately -4.5 V. Similarly, the plate current drops from approximately 10.5 mA at zero bias to zero near -4.5 V.

Technical Summary

The 18GD6A is a miniature sharp-cutoff pentode designed for RF/IF amplification and autodyne converter service in series-string AC/DC receivers. Operating with an 18 V, 100 mA heater with a controlled 20-second warm-up time, it provides a transconductance of 4300 µmhos and a plate resistance of 0.5 MΩ under typical 100 V operating conditions. Its design-maximum ratings restrict plate and screen supply voltages to 150 V, with a maximum plate dissipation of 2.5 W.