18GV8 — High-Mu Triode and Power Pentode

The 18GV8 is a miniature multi-unit vacuum tube containing a high-mu triode and a power pentode within a single envelope. It is designed primarily for use in television receivers, specifically for vertical deflection amplifier, sync-amplifier, and video-output applications. The 18GV8 is electrically identical to the 6GV8 (also designated as ECL85) except for its heater ratings, and is closely related to the 9GV8. Its alternative European designation is PCL85.

Construction, Pinout, and Heater Ratings

The 18GV8 is constructed in a miniature T-6½ glass envelope with a button 9-pin base (E9-1 designation), requiring a standard miniature 9-contact socket. The physical dimensions of the tube include a maximum diameter of 0.875 inches, a maximum seated height of 2.812 inches, and a maximum overall height of 3.062 inches.

The tube features a 9LY basing arrangement, which provides separate connections for the triode and pentode sections, with the pentode's cathode internally tied to its third grid and an internal shield.

Pin Connection Table (9LY Basing)

Heater Specifications

The heater is designed for series or parallel operation with the following ratings:

• Heater Voltage (AC/DC): 18 V
• Heater Current: 0.3 A (300 mA)
• Maximum Heater-Cathode Voltage:
  • Heater negative with respect to cathode (Total DC and Peak): 200 V
  • Heater positive with respect to cathode (Total DC and Peak): 220 V (some specifications list a general peak limit of ±200 V)

Electrical Characteristics and Typical Operation

The 18GV8 combines a high-mu triode section suitable for voltage amplification or signal processing with a robust power pentode section capable of handling higher currents.

Class A Amplifier Characteristics

The typical operating parameters for each section under Class A conditions are outlined below:

Pentode Knee Current Characteristics

To evaluate the peak performance of the pentode section without exceeding maximum dissipation limits, measurements are conducted using a recurrent waveform method. The knee characteristics of an average tube under these conditions are:

• Condition 1: At a plate voltage of 50 V, screen grid voltage of 170 V, and control grid voltage of -1 V, the plate current reaches 200 mA and the screen grid current is 40 mA.
• Condition 2: At a plate voltage of 65 V, screen grid voltage of 210 V, and control grid voltage of -1 V, the plate current reaches 240 mA and the screen grid current is 50 mA.

Absolute Maximum Ratings

The absolute maximum ratings define the limiting operational boundaries for the 18GV8 to ensure reliable operation and prevent damage.

Pentode Section Limits

• Plate Supply Voltage (Cut-off Condition): 550 V
• DC Plate Voltage: 250 V
• Peak Plate Voltage: 2,000 V (maximum pulse duration of 5% of a cycle, not to exceed 1 ms)
• Screen Grid Supply Voltage (Cut-off Condition): 550 V
• Screen Grid Voltage: 250 V
• Screen Grid Dissipation: 2 W
• Plate Dissipation: 7 W
• Average Cathode Current: 75 mA
• Maximum Series Grid-No. 1 Resistor (Fixed Bias): 1 MΩ
• Maximum Series Grid-No. 1 Resistor (Automatic/Cathode Bias): 2.2 MΩ
• Maximum Heater-Cathode Resistance: 20,000 Ω

Triode Section Limits

• Plate Supply Voltage: 550 V
• DC Plate Voltage: 250 V
• Plate Dissipation: 0.5 W
• Average Cathode Current: 15 mA
• Peak Cathode Current: 200 mA (maximum pulse duration of 200 µs; if a larger flyback is required, this limit may be reduced to 100 mA with a maximum pulse duration of 400 µs)
• Maximum Series Grid-No. 1 Resistor (Fixed Bias): 1 MΩ
• Maximum Series Grid-No. 1 Resistor (Automatic/Cathode Bias): 3.3 MΩ
• Maximum Heater-Cathode Resistance: 20,000 Ω

Vertical Sweep Circuit Applications

The pentode section of the 18GV8 is highly optimized for vertical deflection sweep circuits. Designers can implement the tube in both stabilized and non-stabilized circuit configurations.

Operating Examples in Vertical Sweep Service

• Non-stabilized Circuits (Condition A): Operating with a screen grid voltage of 170 V and a minimum plate voltage of 55 V at the end of the scan. The control grid voltage at the end of the scan is -6 V, yielding a peak plate current of 135 mA and a nominal screen grid dissipation of 1.1 W.
• Non-stabilized Circuits (Condition B): Operating with a screen grid voltage of 200 V and a minimum plate voltage of 55 V at the end of the scan. The control grid voltage at the end of the scan is -9.5 V, yielding a peak plate current of 135 mA and a nominal screen grid dissipation of 1.4 W.
• Stabilized Circuits: Operating with a screen grid voltage of 210 V and a minimum plate voltage of 75 V at the end of the scan. The control grid voltage at the end of the scan is -11 V, yielding a peak plate current of 125 mA and a nominal screen grid dissipation of 1.35 W.

Circuit Design Considerations

When designing vertical sweep circuits to achieve full deflection at nominal line voltage, the circuit must account for tube aging. If the minimum plate voltage ($E_b$), screen grid voltage ($E_{c2}$), and plate current ($I_b$) are designed to their specified values, full deflection can still be maintained with an end-of-life tube at the lower limit of its characteristic spread. This is achievable provided the control grid voltage ($E_{[c1](/tubes/C1)}$) at the end of the scan is -1 V, even when the line voltage drops up to 10% below its nominal value.