19V8 — Triple-Diode Triode Vacuum Tube

The 19V8 is a multi-unit miniature vacuum tube containing three diodes and a high-mu triode within a single T-6 1/2 glass envelope. It is designed primarily for use in AM/FM receivers where circuit switching must be minimized, and in balanced low-impedance detector circuits. The tube features a 9-pin miniature button base (9AH) and can be mounted in any position.

The 19V8 is closely related to the 6V8, sharing similar internal structures but operating with a different heater rating.

Construction and Internal Structure

The 19V8 incorporates four distinct electrical sections sharing a common physical envelope:

• Two High-Perveance Diodes (designated Diode #1 and Diode #2).
• One Medium-Perveance Diode (designated Diode #3).
• One High-Mu Triode.

The tube uses a coated unipotential cathode structure. To facilitate balanced low-impedance detector circuits, Diode #1 is constructed with its own independent cathode. The remaining sections—the high-mu triode, Diode #2, and Diode #3—all share a single common cathode.

Heater Ratings

The heater is designed for series or parallel operation in receiving equipment.

Pinout and Base Connections

The 19V8 utilizes a 9-pin miniature button base (9AH). Published technical documentation contains two conflicting pinout configurations for this base. Both configurations are detailed below:

Configuration A

• Pin 1: Triode Plate (Pt)
• Pin 2: Triode Grid (G)
• Pin 3: Common Cathode (K for Triode, Diode #2, and Diode #3)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: Diode #2 Plate (P2)
• Pin 7: Diode #3 Plate (P3)
• Pin 8: Diode #1 Cathode (K - independent)
• Pin 9: Diode #1 Plate (P1)

Configuration B

• Pin 1: Diode #1 Plate (P1)
• Pin 2: Diode #2 Plate (P2)
• Pin 3: Diode #3 Plate (P3)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: Common Cathode (K for Triode, Diode #2, and Diode #3)
• Pin 7: Triode Grid (G)
• Pin 8: Diode #1 Cathode (K - independent)
• Pin 9: Triode Plate (T)

Electrical Characteristics and Operating Points

The triode section of the 19V8 is a high-mu voltage amplifier. Typical Class A1 operating conditions are documented at two distinct plate voltages:

Diode and Leakage Characteristics

The diode sections exhibit distinct perveance and current-carrying capabilities:

• Diode #2 and Diode #3 (High-Perveance / Medium-Perveance):
  • With 5 V DC applied, the average current for Diode #2 or Diode #3 is 40 mA.
  • The maximum ratio of current between these two diodes ($I_{D3}/I_{[D2](/tubes/D2)}$ or $I_{[D2](/tubes/D2)}/I_{D3}$) under a 5 V applied potential is 1.5.
  • Under zero applied voltage, the maximum current through a 40,000 Ω resistor is 24 µA.
• Diode #1 (High-Perveance, Independent Cathode):
  • With 10 V DC applied, the average current for Diode #1 is 2 mA.
• Heater-to-Cathode Leakage:
  • With ±100 V applied between the heater and all cathodes, the maximum leakage current is 5 µA.

Interelectrode Capacitances

Direct interelectrode capacitances, measured without an external shield, are as follows:

• Diode #1 Plate to Triode Grid (1P to G): 0.1 µµF (Max)
• Diode #2 Plate to Triode Grid (2P to G): 0.2 µµF (Max)
• Diode #3 Plate to Triode Grid (3P to G): 0.02 µµF (Max)
• Diode #1 Plate to All other electrodes: 1.3 µµF
• Diode #2 Plate to All other electrodes: 5.3 µµF
• Diode #3 Plate to All other electrodes: 5.3 µµF

Maximum Ratings

The absolute maximum ratings are established under the RMA Standard M8-210:

• Maximum Plate Voltage: 300 V
• Maximum Heater-Cathode Voltage: 200 V
• Maximum Positive DC Grid Voltage: 0 V
• Maximum Plate Dissipation: 1 W
• Maximum Continuous Diode #2 and Diode #3 Current: 10 mA
• Maximum Continuous Diode #1 Current: 1 mA

Technical Summary

The 19V8 is a versatile, multi-unit miniature tube containing a high-mu triode and three diodes (two high-perveance, one medium-perveance) in a T-6 1/2 envelope. Operating with an 18.9 V, 150 mA heater, it is optimized for AM/FM receiver applications. The inclusion of an independent cathode for one of the high-perveance diodes makes it highly suitable for balanced low-impedance detector circuits, while the high-mu triode (with an amplification factor of 70) serves as an effective voltage amplifier.