12HL7 — Sharp-Cutoff Frame-Grid Pentode

The 12HL7 is a miniature, sharp-cutoff pentode featuring frame-grid construction. It is designed primarily for use as a video output amplifier in color television receivers. The tube is housed in a T-6 1/2 glass envelope with a 9-pin miniature button base (E9-1) and utilizes the 9BF basing diagram.

Heater Characteristics and Configurations

The 12HL7 features a versatile heater design consisting of two sections that can be configured in series or parallel depending on the circuit requirements:

• Series Heater Configuration: When the two heater sections are connected in series, the nominal heater voltage is 12.6 V (with a permissible range of 12.6 ± 1.2 V) and the heater current is 0.3 A (300 mA). This configuration is typically used in parallel heater circuits.
• Parallel Heater Configuration: When the two heater sections are connected in parallel, the nominal heater voltage is 6.3 V (with a permissible range of 6.3 ± 0.6 V) and the heater current is 0.6 A (600 mA). This configuration can be operated in series with other heaters sharing the same bogey current (0.6 A) or in parallel with other heaters sharing the same bogey voltage.
• Heater Warm-up Time: For the 6.3 V connection, the average heater warm-up time is 11 seconds. This warm-up time represents the time required for the voltage across the heater to reach 80 percent of its bogey value when 4 times the bogey heater voltage is applied to a circuit consisting of the tube heater in series with a resistance equal to 3 times the bogey heater voltage divided by the bogey heater current.
• Heater-Cathode Voltage Ratings:
  • Heater negative with respect to cathode: Maximum total DC and peak voltage of 200 V.
  • Heater positive with respect to cathode: Maximum DC component of 100 V, and maximum total DC and peak voltage of 200 V.

Physical and Mechanical Specifications

The 12HL7 is designed for flexible physical installation and has the following mechanical characteristics:

• Operating Position: Any
• Envelope: T-6 1/2 glass
• Base: E9-1 small button 9-pin
• Basing Diagram: EIA 9BF
• Outline Drawing: EIA 6-3
• Physical Dimensions:
  • Maximum Diameter: 0.875 inches
  • Minimum Diameter: 0.750 inches
  • Maximum Seated Height: 2.375 inches
  • Maximum Overall Length: 2.625 inches

Terminal Connections (9BF Pinout)

The 9-pin miniature base provides the following connections:

Electrical Characteristics and Typical Operation

The 12HL7 is characterized by very high transconductance, made possible by its frame-grid construction.

Typical Class A Operating Point

Under typical operating conditions, the pentode exhibits the following characteristics:

• Plate Voltage: 250 V
• Grid No. 2 (Screen) Supply Voltage: 150 V
• Grid No. 1 (Control Grid) Voltage: 0 V
• Grid No. 3 (Suppressor Grid) Voltage: 0 V (referred to the negative end of the cathode resistor)
• Cathode Bias Resistor (Bypassed): 122 Ω
• Plate Current: 25 mA
• Grid No. 2 Current: 6.0 mA
• Transconductance (Grid No. 1 to Plate): 21,000 µmhos
• Plate Resistance (Approximate): 55,000 Ω
• Grid No. 1 Voltage for Plate Current of 100 µA (approximate, with Rk = 0): -7.2 V

Instantaneous Plate Knee Characteristics

Under short-interval conditions (to prevent damage to the tube), the 12HL7 exhibits the following knee characteristics:

• Plate Voltage (Eb): 50 V
• Grid No. 2 Voltage (Ec2): 125 V
• Grid No. 1 Voltage (Ec1): 0 V
• Grid No. 3 Voltage (Ec3): 0 V
• Plate Current (Ib): 76 mA
• Grid No. 2 Current (Ic2): 32 mA

Maximum Ratings and Circuit Values

The 12HL7 is rated under the Design-Maximum Rating System. Operating conditions must not exceed these limits under the worst probable conditions:

• Maximum Plate Voltage: 400 V
• Maximum Grid No. 2 (Screen) Supply Voltage: 330 V
• Maximum Grid No. 2 Voltage: See Screen Rating Chart (permissible screen dissipation is 1.0 W up to 165 V, and decreases linearly to 0 W at 330 V)
• Maximum Positive DC Grid No. 1 Voltage: 0 V
• Maximum Plate Dissipation: 10.0 W
• Maximum Grid No. 2 Dissipation: 1.0 W
• Maximum Grid No. 1 Circuit Resistance:
  • With Fixed Bias: 0.1 MΩ
  • With Cathode Bias: 0.25 MΩ

Testing Precaution

Due to the extremely close spacing between the control grid and the cathode (typical of frame-grid construction), the voltage between these elements must be restricted to a maximum of 50 V DC or peak AC in commercial tube checkers or shorts-indicating devices. This is especially critical when mechanical excitation (tapping) of the tube is employed.

Direct Interelectrode Capacitances

The capacitances of the 12HL7 are measured without an external shield:

• Grid No. 1 to Plate (g1 to p): 0.15 pF (maximum)
• Input (g1 to k + g3 + IS + g2 + h): 15.0 pF
• Output (p to k + g3 + IS + g2 + h): 6.0 pF

Characteristic Curves and Interpretation

The published curves for the 12HL7 include average plate characteristics and transfer characteristics.

Average Plate Characteristics

• At a fixed screen voltage (Ec2) of 150 V and suppressor voltage (Ec3) of 0 V: The plate current (Ib) rises sharply at low plate voltages (below 50 V) and flattens out, demonstrating typical pentode behavior. For a control grid voltage (Ec1) of 0 V, the plate current exceeds 100 mA at plate voltages above 100 V, while at Ec1 of -5.5 V, the plate current is reduced to near zero.
• At a fixed control grid voltage (Ec1) of 0 V and suppressor voltage (Ec3) of 0 V: Curves for various screen voltages (Ec2 from 50 V to 175 V) show that higher screen voltages significantly shift the plate current curves upward. For example, at Ec2 = 175 V, the plate current plateaus near 150 mA.

Average Transfer Characteristics

• At a plate voltage (Eb) of 250 V and suppressor voltage (Ec3) of 0 V: Transfer curves show plate current and screen current as a function of grid-number 1 voltage (from 0 V down to -10 V) for screen voltages (Ec2) ranging from 50 V to 175 V.
• Plate Current Transfer: The plate current transfer curves show a sharp cutoff characteristic, with current approaching zero near -7.2 V for typical operating screen voltages. At Ec2 = 150 V, the plate current rises from near zero at -6 V to 25 mA at 0 V.
• Screen Current Transfer: The screen current transfer curves follow a similar profile but at a lower magnitude, rising to about 6 mA at 0 V grid bias under typical operating conditions.

Technical Summary

The 12HL7 is a high-performance, frame-grid, sharp-cutoff pentode designed for video output amplifier service in color television receivers. Housed in a 9-pin miniature T-6 1/2 envelope, it features a flexible heater that can be operated at 12.6 V / 0.3 A (series) or 6.3 V / 0.6 A (parallel). With an exceptionally high transconductance of 21,000 µmhos and a plate dissipation rating of 10.0 W, it provides robust amplification capabilities. Designers must observe the strict 50 V limit between the control grid and cathode during testing due to the extremely close frame-grid spacing, and adhere to the screen dissipation limits outlined in the screen rating chart.