18GB5 — Beam Power Tube

The 18GB5 (also designated as LL500) is a beam power tube designed primarily for use as a horizontal-deflection amplifier in television receivers. It belongs to the 6GB5/EL500 family of tubes, which includes the 6GB5/EL500, 13GB5/XL500, 27GB5/PL500, and 28GB5. These types are identical to the 18GB5 except for their heater ratings.

Mechanical Construction and Base Configuration

The 18GB5 is constructed in a Magnoval T-9 glass envelope with a maximum diameter of 1.188 inches, a maximum seated height of 3.750 inches, and a maximum overall height of 4.125 inches. It requires a neonoval 9-contact socket.

The tube uses the 9NH basing diagram, though different technical sources describe the specific pin assignments with some variations:

• Heater (H): Pins 4 and 5 are consistently assigned to the heater.
• Internal Connection (IC): Pin 9 is designated as an internal connection.
• Plate (P): Connected to the top cap (C1-2). (Note: One source lists Pin 6 as Plate, while others specify the top cap for the plate connection).
• Control Grid (G1): Variously assigned to Pin 1 and Pin 2, or Pin 1 and Pin 6.
• Screen Grid (G2): Variously assigned to Pin 7 (with Pin 6 or Pin 2), or Pins 2 and 7, or Pins 6 and 7.
• Cathode and Suppressor Grid (K, G3): Assigned to Pin 3 and Pin 8, which may be internally connected or separate.

This variation in documented pinouts highlights the importance of verifying the physical socket wiring against the specific device implementation.

Heater Ratings and Heater-Cathode Limits

The 18GB5 heater is designed for series or parallel AC/DC operation:

• Heater Voltage: 18 V
• Heater Current: 0.45 A (450 mA)

Heater-Cathode Voltage Limits

To prevent insulation breakdown, the voltage between the heater and cathode must be kept within specified limits:

• Peak Value: ±250 V max (Total DC and Peak Max)
• Average/DC Value: 125 V max (Heater positive or negative with respect to cathode)

Electrical Characteristics and Typical Operation

The 18GB5 exhibits high peak current capability at low plate voltages, which is a key requirement for horizontal deflection service.

Typical Instantaneous Characteristics

When measured with a recurrent waveform to prevent exceeding maximum ratings, the tube exhibits the following knee characteristics:

• Plate Voltage: 75 V
• Grid-No. 2 (Screen-Grid) Voltage: 200 V
• Grid-No. 1 (Control-Grid) Voltage: −10 V
• Plate Current: 440 mA
• Grid-No. 2 Current: 37 mA

Standard Operating Points

In typical horizontal deflection circuits, the operating conditions vary based on the supply voltage. A minimum screen-grid resistor ($R_{[g2](/tubes/G2)}$) is required to prevent excessive screen-grid dissipation during the receiver warm-up period.

Note: The plate voltage at the end of scan is specified at nominal line voltage. The tube operates above the knee at 10% below nominal line voltage.

Non-Stabilized Circuits (Drive Below the Knee)

For non-stabilized circuits where the tube is driven below the knee, the typical operating points are:

The peak plate current values specified should not be exceeded at nominal line voltage to allow for tube spread, deterioration, and a 10% low line voltage condition.

Maximum Ratings and Circuit Values

The maximum ratings are defined under the Design-Maximum system for operation in a 525-line, 30-frame television system as a horizontal-deflection amplifier.

Design-Maximum Ratings

• DC Plate Voltage: 275 V max
• Peak Positive-Pulse Plate Voltage: 7700 V max (The pulse duration must not exceed 15% of a horizontal scanning cycle, or 10 microseconds)
• DC Grid-No. 2 (Screen-Grid) Voltage: 275 V max
• Average Cathode Current: 275 mA max
• Plate Dissipation: 17 W max (A bias resistor or other protective means is required to protect the tube in the absence of excitation)
• Grid-No. 2 Input (Screen-Grid Dissipation): 5 W max (May reach 6 W if the plate dissipation is below 11 W)

Maximum Circuit Values

• Grid-No. 1 (Control-Grid) Circuit Resistance:
  • Without grid current: 0.5 MΩ max
  • With grid current (horizontal-output service only): 2.2 MΩ max

Circuit-Specific Operating Notes

During the flyback interval, a minimum control-grid voltage of −120 V is required to ensure complete cut-off of the tube.

Technical Summary

The 18GB5 (LL500) is a Magnoval-based beam power pentode designed specifically for horizontal-deflection amplifier service in television receivers. It features an 18 V, 0.45 A heater and is part of a larger family of identical tubes (6GB5/EL500, 13GB5/XL500, 27GB5/PL500, 28GB5) differing only in heater characteristics. The tube is characterized by its high peak plate current capability (up to 440 mA at a 75 V plate knee) and its ability to withstand peak positive-pulse plate voltages of up to 7700 V. Proper circuit design requires a minimum screen-grid resistor to protect the tube during warm-up and a bias resistor or other protective means to prevent damage in the event of a loss of excitation.