10JT8 — High-Mu Triode and Sharp-Cutoff Pentode

Overview

The 10JT8 is a neonoval-type multi-section vacuum tube containing a high-mu triode and a sharp-cutoff pentode within a single envelope. Designed primarily for use in color and black-and-white television receivers, the tube's triode section is tailored for service as a voltage amplifier or sync separator, while its frame-grid pentode section is optimized for video amplifier applications.

The 10JT8 features a 10.2 V heater designed for series-string heater circuits, drawing 0.45 A. It is electrically identical to the 6JT8 and 8JT8 except for its heater voltage and current ratings.

Physical Characteristics and Basing

The 10JT8 is constructed using the 9T-9 envelope style with a 9-69 outline. It utilizes a small-button miniature 9-pin base (E9-68 designation) and requires a standard miniature 9-contact socket.

Physical Dimensions

• Maximum Diameter: 1.188 inches
• Maximum Seated Height: 2.320 inches
• Maximum Overall Height: 2.630 inches

Pin Connections (9DX Basing)

The 9DX basing arrangement provides the following pin connections, viewed from the underside of the base:

Special Construction Note

The pentode section of the 10JT8 features an extremely close control-grid-to-cathode spacing. Because of this tight physical tolerance, the voltage between the control grid and cathode must not exceed 50 volts DC or peak AC when testing the tube in commercial tube checkers or shorts-indicating devices. This restriction is especially critical when mechanical excitation (tapping or vibration) is applied to the tube during testing.

Heater Specifications

The 10JT8 heater is designed for operation from either an AC or DC supply. It features a controlled warm-up time for series-string integration.

Heater-Cathode Voltage Ratings

• Heater Negative with Respect to Cathode:
  • Total DC and Peak: 200 V max
  • Peak Value: 200 V max
  • Average Value: 100 V max
• Heater Positive with Respect to Cathode:
  • DC: 100 V max
  • Total DC and Peak: 200 V max
  • Peak Value: 100 V max
  • Average Value: 100 V max

Direct Interelectrode Capacitances

The following capacitances are measured without an external shield:

Triode Section

• Grid to Plate: 3.2 pF
• Input (g to h + Tk + Pk, g3, IS): 1.7 pF
• Output (p to h + Tk + Pk, g3, IS): 1.6 pF

Pentode Section

• Grid No. 1 to Plate (Max): 0.10 pF
• Input (g1 to h + Pk, g3, IS + g2): 12 pF
• Output (p to h + Pk, g3, IS + g2): 3.0 pF

Inter-Section Coupling

• Triode Grid to Pentode Plate (Max): 0.02 pF
• Pentode Grid No. 1 to Triode Plate (Max): 0.004 pF
• Pentode Plate to Triode Plate (Max): 0.13 pF

Maximum Ratings (Design-Maximum System)

The maximum ratings define the absolute operating boundaries for each section under Class A1 amplifier service:

Maximum Circuit Values (Grid No. 1 Circuit Resistance)

• Triode Section:
  • Fixed Bias Operation: 0.5 MΩ max
  • Cathode Bias Operation: 1.0 MΩ max
• Pentode Section:
  • Fixed Bias Operation: 0.25 MΩ max
  • Cathode Bias Operation: 1.0 MΩ max

Electrical Characteristics and Typical Operation

The 10JT8 operates as a Class A1 amplifier with the following typical characteristics:

Triode Section

• Plate Voltage: 250 V
• Grid No. 1 Voltage: -2 V
• Plate Current: 1.5 mA
• Transconductance: 2700 µmhos
• Amplification Factor (μ): 100
• Plate Resistance (Approx.): 37,000 Ω
• Grid No. 1 Voltage for Ib = 20 µA (Approx.): -5.3 V

Pentode Section

• Plate Supply Voltage: 200 V
• Grid No. 2 Supply Voltage: 100 V
• Cathode Bias Resistor: 82 Ω
• Plate Current: 17 mA
• Grid No. 2 Current: 3.5 mA
• Transconductance: 20,000 µmhos
• Plate Resistance (Approx.): 50,000 Ω
• Grid No. 1 Voltage for Ib = 100 µA (Approx.): -5 V

Instantaneous Plate Knee Characteristics (Pentode Section)

Under pulsed or instantaneous operating conditions near the plate knee, the pentode section exhibits high current capability. These values are measured using a recurrent waveform method to prevent exceeding the tube's maximum dissipation ratings:

• Plate Voltage (Eb): 50 V
• Grid No. 2 Voltage (Ec2): 100 V
• Grid No. 1 Voltage (Ec1): 0 V
• Plate Current (Ib): 55 mA
• Grid No. 2 Current (Ic2): 18 mA

Related Designations

The 10JT8 belongs to a family of identical multi-section tubes that differ only in their heater configurations:

• 6JT8: Designed for standard parallel heater circuits, requiring a 6.3 V supply at 0.725 A.
• 8JT8: Designed for series-string heater circuits, requiring a 7.7 V supply at 0.600 A (600 mA).

These tubes share the same physical dimensions, 9DX basing, and internal electrical characteristics for both the triode and pentode sections, allowing direct circuit design translation across different heater supply topologies.

Technical Summary

The 10JT8 is a versatile, high-performance neonoval double-unit tube combining a high-mu triode (μ = 100) and a high-transconductance, sharp-cutoff frame-grid pentode (gₘ = 20,000 µmhos) in a single 9-pin envelope. Operating with a 10.2 V, 0.45 A heater, it is optimized for series-string television receiver designs. The triode section serves effectively in voltage amplification and sync-separator roles, while the pentode section's high transconductance and robust plate dissipation rating (4.0 W) make it highly suited for video output and wideband amplification.