19T8 — Triple-Diode High-Mu Triode Vacuum Tube

A miniature multi-unit tube containing three high-perveance diodes and a single high-mu triode, designed for AM/FM detection, ratio-detector circuits, and audio voltage amplification.

Overview

The 19T8 is a multi-unit miniature vacuum tube designed to consolidate several detection and amplification functions within a single glass envelope. It contains three independent high-perveance diode units and one high-mu triode unit. The tube is primarily intended for use in AM/FM radio receivers, where it performs AM detection, FM detection (or ratio-detector service), and audio preamplification with a minimum of circuit switching. Additionally, it finds application in color television receivers as a clamper or detector voltage amplifier.

The 19T8 is electrically identical to the 6T8 and 6T8A types, and closely related to the 5T8, differing only in its heater ratings. The inclusion of three diodes, with one diode plate having an independent cathode connection in certain configurations, facilitates balanced, low-impedance detector circuits (such as FM ratio detectors) while minimizing the overall tube count in compact receiver designs.

Physical Construction and Heater Ratings

The 19T8 is constructed in a miniature T-6 1/2 glass envelope with a 9-pin miniature button base (classified under the B9A or E9-1 designations). The tube features coated unipotential cathodes and can be mounted in any physical position.

Physical Dimensions

• Maximum Diameter: 0.875 inches (approx. 22.2 mm)
• Maximum Seated Height: 1.937 inches (approx. 49.2 mm)
• Maximum Overall Height: 2.187 inches (approx. 55.5 mm)

Heater Characteristics

The heater is designed for series or parallel AC/DC operation, featuring a controlled warm-up time to ensure reliable operation in series-string entertainment receivers.

Heater-Cathode Voltage Ratings

The maximum allowable potential difference between the heater and cathode varies depending on the rating system and polarity:

• Peak Value (General): 90 V max
• Heater Negative with respect to Cathode: 90 V to 200 V max (depending on rating system)
• Heater Positive with respect to Cathode (DC): 100 V max
• Heater Positive with respect to Cathode (Total DC and Peak): 90 V to 200 V max

Pinout Configurations

Due to variations in manufacturing designs and historical standards, several distinct pinout configurations (all utilizing the 9E or B9A basing diagram) are documented for the 19T8. Designers must verify the specific pinout of the target device.

Pinout Configuration A (Variant 1)

• Pin 1: Diode No. 2 Plate
• Pin 2: Diode No. 3 Plate
• Pin 3: Diode No. 2 Cathode, Internal Shield
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Diode No. 1 Plate
• Pin 7: Triode Cathode, Diode No. 1 & Diode No. 3 Cathodes, Internal Shield
• Pin 8: Triode Grid
• Pin 9: Triode Plate

Pinout Configuration B (Variant 2)

• Pin 1: Triode Plate
• Pin 2: Triode Grid
• Pin 3: Cathode (Common for Triode, Diode 1, and Diode 3)
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Diode 3 Plate
• Pin 7: Diode 2 Cathode
• Pin 8: Diode 1 Plate
• Pin 9: Diode 2 Plate

Pinout Configuration C (Variant 3)

• Pin 1: Cathode (Diode 1)
• Pin 2: Common Anode (Diode 1 & Diode 2)
• Pin 3: Cathode (Diode 2)
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Anode (Diode 3)
• Pin 7: Common Cathode (Triode & Diode 3)
• Pin 8: Grid (Triode)
• Pin 9: Anode (Triode)

Pinout Configuration D (Variant 4)

• Pin 1: Diode Plate No. 3
• Pin 2: Diode Cathode
• Pin 3: Diode Plate No. 2
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Diode Plate No. 1
• Pin 7: Triode Cathode
• Pin 8: Triode Grid
• Pin 9: Triode Plate

Pinout Configuration E (Variant 5)

• Pin 1: Triode Plate
• Pin 2: Diode 1 Plate
• Pin 3: Diode 2 Plate
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Diode 3 Plate
• Pin 7: Diode 3 Cathode
• Pin 8: Common Cathode for Triode, Diode 1, and Diode 2
• Pin 9: Triode Grid

Pinout Configuration F (Variant 6)

• Pin 1: Number 3 Diode Plate
• Pin 2: Number 2 Diode Plate
• Pin 3: Number 2 Diode Cathode and Shield
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Number 1 Diode Plate
• Pin 7: Triode Cathode and Shield
• Pin 8: Triode Grid
• Pin 9: Triode Plate

Pinout Configuration G (Variant 7)

• Pin 1: Triode Plate
• Pin 2: Diode No. 1 Plate
• Pin 3: Diode No. 2 Plate
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Diode No. 3 Plate
• Pin 7: Cathode (Diode 1, Diode 2), Internal Shield
• Pin 8: Cathode (Triode, Diode 3), Internal Shield
• Pin 9: Triode Grid

Electrical Characteristics and Typical Operation

The triode section of the 19T8 is a high-mu voltage amplifier, while the three diode sections are high-perveance units designed for low-current detection.

Class A1 Triode Amplifier Characteristics

At typical operating points, the triode unit exhibits the following characteristics:

Diode Unit Characteristics

• Average Diode Current (Each Section): 20 mA when measured with 5.0 Volts DC applied.

Direct Interelectrode Capacitances

Capacitances vary depending on whether an external shield (such as Shield #315) is employed:

• Triode Unit (Unshielded / Shielded):
  • Grid to Plate: 1.7 pF to 2.4 pF / 1.7 pF
  • Input (Grid to Cathode, Heater, Shield): 1.5 pF to 1.6 pF / 1.7 pF
  • Output (Plate to Cathode, Heater, Shield): 1.0 pF to 1.2 pF / 2.4 pF
• Diode Units (Unshielded / Shielded):
  • Diode 1 Input: 3.8 pF / 3.8 pF
  • Diode 2 Input: 2.2 pF to 3.8 pF / 3.8 pF (with shield connected to Pin 3 or Pin 7)
  • Diode 3 Input: 3.4 pF to 4.5 pF / 3.6 pF
  • Diode 2 Cathode to All: 7.5 pF / 8.5 pF
• Inter-unit Coupling:
  • Triode Grid to any Diode Plate (Max): 0.03 pF to 0.034 pF (with or without shield)
  • Triode Grid to each Diode (Max): 0.35 pF

Maximum Ratings

The maximum ratings for the 19T8 are defined under both the Design-Center and Design-Maximum rating systems. These limits must not be exceeded under the worst-case operating conditions.

Resistance-Coupled Amplifier Operation

The triode section of the 19T8 is highly suited for resistance-coupled audio preamplifier service. Operating parameters for various plate supply voltages ($E_{bb}$) and load configurations are detailed below.

Example 1: Fixed Bias Operation (at 400 Hz)

• Plate Supply Voltage ($E_{bb}$): 90 V
• Control Grid Voltage: 0 V
• Plate Load Resistor ($R_l$): 220,000 Ω
• Control Grid Resistor ($R_{g1}$): 10.0 MΩ
• Input / Output Coupling Condensers: 0.01 µF
• Grid Resistor of Following Stage ($R_c$): 470,000 Ω
• Signal Source Impedance (Max): 1,000 Ω
• Total Harmonic Distortion (THD): 5%
• Output Voltage ($E_o$): 8.5 V RMS
• Voltage Gain: 35

Example 2: Cathode Bias and Grid-Leak Bias Configurations

The following table outlines typical operating conditions at 5% total harmonic distortion (THD). Voltage gain is measured at an output level of 2.0 V RMS.

For alternative resistance-coupled operating details, reference can also be made to the operating characteristics of the 6AT6 type.

Qualitative Curve Observations

Triode Plate and Transfer Characteristics

The triode section exhibits typical triode behavior. The plate current versus plate voltage curves show a well-defined knee characteristic below 100 V, transitioning to highly linear slopes at higher plate voltages. At zero grid bias ($E_c = 0\text{ V}$), the plate current reaches approximately 3.0 mA at a plate voltage of 130 V.

Transfer characteristics (plate current versus grid voltage) demonstrate a sharp cutoff behavior. The cutoff point is highly dependent on plate voltage, occurring near $-2.5\text{ V}$ when the plate voltage is 100 V, and extending to approximately $-5.5\text{ V}$ when the plate voltage is increased to 300 V.

Triode Parameter Variations

At a constant plate voltage of 250 V, the triode's small-signal parameters vary dynamically with the operating plate current:

• Amplification Factor ($\mu$): Remains remarkably stable, staying relatively flat between 70 and 75 as plate current varies from 0.5 mA to 4.0 mA.
• Transconductance ($g_m$): Increases steadily from approximately 800 µmhos at 0.5 mA to nearly 1,800 µmhos at 4.0 mA.
• Plate Resistance ($r_p$): Decreases significantly from approximately 85,000 Ω (0.085 MΩ) at low plate currents down to 40,000 Ω (0.04 MΩ) as the plate current rises toward 4.0 mA.

Diode Characteristics

The diode units show highly linear detection behavior at higher signal inputs (up to 15 V to 30 V RMS). Standard load lines plotted for load resistances ($R_L$) between 0.25 MΩ and 2.0 MΩ demonstrate excellent rectification efficiency and linearity, making the diode units highly effective for low-distortion AM and FM detection.

Technical Summary

The 19T8 is a versatile, 9-pin miniature triple-diode high-mu triode designed for space-saving applications in AM/FM receivers and television circuitry. Operating with an 18.9 V / 0.15 A heater, it integrates three high-perveance diodes and a single high-mu triode ($\mu = 70$) in a T-6 1/2 glass envelope. The triode section provides high voltage gain (up to 54 in resistance-coupled circuits) with low distortion, while the three diodes provide flexible detection capabilities, including FM ratio-detector service (typically utilizing Diode No. 2 and Diode No. 3). By offering multiple independent diode and triode elements with various pinout configurations, the 19T8 serves as a highly efficient single-tube solution for detection and audio preamplification.