19CL8A — Medium-Mu Triode and Sharp-Cutoff Tetrode/Pentode

The 19CL8A is a miniature multi-section vacuum tube designed for high-frequency applications in television and radio receivers. It combines a medium-mu triode and a sharp-cutoff (or semi-remote cutoff) tetrode or pentode within a single envelope. The tube is primarily intended for combined oscillator-mixer service in VHF television tuners (including color and black-and-white receivers) and FM receivers, performing well at high frequencies.

The 19CL8A is identical to the 6CL8A (and in some respects 5CL8A) except for its heater ratings. It features controlled thermal characteristics to minimize heater voltage surges during warm-up, making it suitable for series-string entertainment equipment.

Construction and Mechanical Characteristics

The 19CL8A is constructed in a miniature glass envelope (T-6 1/2 bulb) with a miniature button 9-pin base. The cathodes are coated unipotential types, with separate and electrically independent cathode connections for each section.

The physical dimensions of the tube are:

• Maximum overall height: 2.187 inches (approx. 2-3/16")
• Maximum seated height: 1.937 inches
• Maximum diameter: 0.875 inches (7/8")
• Mounting position: Any

Heater Ratings

The heater is designed for series-string operation with controlled warm-up characteristics.

• Heater Voltage (AC or DC): 18.9 V
• Heater Current: 0.15 A (150 mA)  0.010 A
• Heater Warm-up Time: 11.0 seconds (defined as the time to reach 80% of rated voltage when four times the rated voltage is applied in series with a resistance three times the nominal heater operating resistance).
• Heater-Cathode Voltage Limits:
  • Heater negative with respect to cathode: 200 V max (Total DC and Peak)
  • Heater positive with respect to cathode: 100 V max (DC component), 200 V max (Total DC and Peak)
  • Alternative rating: Peak heater-cathode voltage of 200 V max, average heater-cathode voltage of 100 V max.

Pinout and Electrode Connections

The tube uses a 9-pin miniature base. Depending on the specific design variant and standard (such as 9FX or 9FA), the pin connections are documented in several configurations:

Configuration A (Standard 9FX Tetrode Pinout)

This is the most common configuration for the triode-tetrode variant:

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

Configuration B (Alternative 9FX Pinout)

An alternative 9FX layout is documented as:

• Pin 1: Triode Plate
• Pin 2: Tetrode Grid #1
• Pin 3: Tetrode Grid #2
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Tetrode Plate
• Pin 7: Tetrode Cathode
• Pin 8: Triode Grid
• Pin 9: Triode Cathode

Configuration C (Alternative 9FX Pinout with swapped triode/tetrode pins)

Another variation of the 9FX layout lists:

• Pin 1: Plate (Triode)
• Pin 2: Grid (Triode)
• Pin 3: Cathode (Triode)
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Plate (Tetrode)
• Pin 7: Grid 2 (Tetrode)
• Pin 8: Cathode (Tetrode)
• Pin 9: Grid 1 (Tetrode)

Configuration D (Alternative 9FX Pinout with swapped triode/tetrode pins - Variant 2)

Another variation of the 9FX layout lists:

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

Configuration E (9FA Pentode Pinout)

For the triode-pentode variant:

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

Interelectrode Capacitances

The direct interelectrode capacitances vary depending on whether an external shield (such as Shield No. 315) is connected.

Triode Section Capacitances

• Grid to Plate: 1.8 pF (with or without shield); also listed as 1.7 pF in some configurations.
• Input (Grid to Heater + Cathode): 2.7 pF to 2.8 pF (with or without shield); also listed as 3.2 pF (including pentode cathode and shield).
• Output (Plate to Heater + Cathode): 1.2 pF (shielded), 0.4 pF (unshielded); also listed as 2.0 pF (shielded), 1.5 pF (unshielded), or 2.2 pF in alternative configurations.

Tetrode/Pentode Section Capacitances

• Grid No. 1 to Plate: 0.010 pF max (shielded), 0.020 pF to 0.028 pF max (unshielded); also listed as 0.01 pF in pentode configuration.
• Input (Grid No. 1 to Heater + Cathode + Grid No. 2): 5.0 pF (with or without shield); also listed as 5.5 pF in pentode configuration.
• Output (Plate to Heater + Cathode + Grid No. 2): 3.4 pF (shielded), 2.0 pF to 2.4 pF (unshielded).

Inter-Section Coupling and Heater-Cathode Capacitances

• Tetrode Grid No. 1 to Triode Plate: 0.01 pF max (shielded), 0.015 pF max (unshielded)
• Tetrode Plate to Triode Plate: 0.03 pF max (shielded), 0.15 pF max (unshielded)
• Cathode to Heater (either section): ~2.5 pF to 3.0 pF

Maximum Ratings (Design-Maximum or Design-Center Values)

The maximum ratings for the 19CL8A are defined under the Design-Maximum or Design-Center systems. (Note: Some documentation references the 6U8A for Class A1 maximum ratings, but specific limits are detailed below).

Grid No. 2 Input Limits

For Grid No. 2 (screen grid) voltages up to 150 V (or 170 V in some charts), the maximum dissipation is limited to 0.5 W or 0.55 W. For voltages between 150 V (or 165 V) and 300 V, the permissible dissipation decreases non-linearly to 0 W at 300 V, as defined by the screen rating chart.

Maximum Circuit Resistance (Grid No. 1)

• Fixed Bias: 0.5 M̩ (Triode), 0.25 M̩ (Tetrode/Pentode)
• Cathode Bias (Self Bias): 1.0 M̩ (Triode), 1.0 M̩ (Tetrode/Pentode); also listed as 2.2 M̩ for both sections in some configurations.

Typical Operating Characteristics

The 19CL8A operates as a Class A1 amplifier under the following typical conditions:

Triode Section

• Plate Voltage: 125 V
• Grid No. 1 Voltage: 0 V (with cathode bias) or -1.0 V
• Cathode Bias Resistor: 56 ̩ (when Grid No. 1 is at 0 V)
• Plate Current: 13.5 mA to 15 mA
• Amplification Factor (\mu): 40 to 46
• Plate Resistance (r_p): 5,000 ̩ to 5,400 ̩
• Transconductance (g_m): 8,000 mhos (8,500 mhos in some configurations)
• Grid No. 1 Cutoff Voltage (for I_b = 10 A or 20 A): -8 V to -9 V (approx.)

Tetrode / Pentode Section

The tetrode section can be operated under different typical conditions:

Technical Summary

The 19CL8A is a versatile 9-pin miniature triode-tetrode (or triode-pentode) designed for high-frequency applications, particularly as a combined VHF oscillator and mixer in television and FM receivers. It features a 18.9 V, 0.15 A heater with controlled warm-up characteristics for series-string operation. The triode section provides a medium amplification factor of 40 to 46 with a transconductance of 8,000 to 8,500 mhos, while the sharp-cutoff tetrode/pentode section offers high plate resistance and transconductance up to 8,200 mhos. With separate cathodes and low interelectrode capacitances, the 19CL8A delivers reliable, low-noise performance in demanding RF and converter stages.