19KG8 — Triode-Pentode Vacuum Tube

The 19KG8 is a multi-section miniature vacuum tube containing a sharp-cutoff pentode and a triode within a single envelope. It is designed primarily for use in television and radio receivers as a combined triode oscillator and pentode mixer. The tube features a T-6 1/2 glass envelope, a small button 9-pin base, and coated unipotential cathodes. It is designed to operate in any mounting position.

Heater Characteristics and Ratings

The 19KG8 is equipped with a heater designed for series-string operation. The heater current must be maintained within tight tolerances to ensure reliable operation and tube longevity.

• Heater Voltage (AC or DC): 18.9 V (for a bogey tube at $I_f = 0.15$ A)
• Heater Current: $0.15 \pm 0.01$ A (150 mA)
• Series Operation Requirement: The heater current must be centered at the specified bogey value, and supply variations must be restricted to keep the current within the specified tolerance.

Heater-Cathode Voltage Limits

The insulation between the heater and the cathodes is rated to withstand specific voltage potentials:

• Heater Positive with Respect to Cathode:
  • Maximum DC Component: 100 V
  • Maximum Total DC and Peak: 200 V
• Heater Negative with Respect to Cathode:
  • Maximum Total DC and Peak: 200 V

Physical Dimensions and Basing Discrepancies

The 19KG8 is housed in a miniature T-6 1/2 glass envelope with an EIA 6-2 outline (also described as Outlines section 6B). It requires a standard miniature 9-contact socket.

• Maximum Diameter: 7/8 inches (0.875 in)
• Maximum Overall Length: 2 3/16 inches (2.187 in)
• Maximum Seated Height: 1 15/16 inches (1.937 in)

Basing and Pinout Configurations

While the tube is universally designated with a 9LY basing diagram, published technical specifications contain three completely different pinout configurations. Designers must verify which pinout corresponds to the specific physical device being utilized.

Maximum Ratings

The maximum ratings for the 19KG8 are defined under the Design-Maximum Rating System. Two distinct sets of maximum ratings exist in the technical literature, representing different design limits for the tube's sections.

Rating Set 1

This set of ratings applies equal plate dissipation limits to both the triode and pentode sections:

• Plate Voltage: 300 V (Pentode and Triode)
• Screen Supply Voltage: 300 V (Pentode)
• Positive DC Grid-Number 1 Voltage: 0 V (Pentode and Triode)
• Plate Dissipation: 2.5 W (Pentode and Triode)
• Screen Dissipation: 0.55 W (Pentode)
• Grid-Number 1 Circuit Resistance (Fixed Bias): 2.2 MΩ (Pentode and Triode)
• Grid-Number 1 Circuit Resistance (Cathode Bias): 2.2 MΩ (Pentode and Triode)

Rating Set 2

This set of ratings permits a higher plate voltage and a higher pentode plate dissipation, but restricts the triode plate dissipation and grid circuit resistances significantly:

• Plate Voltage: 330 V (Pentode and Triode)
• Screen Supply Voltage: 330 V (Pentode)
• Positive DC Grid-Number 1 Voltage: 0 V (Pentode and Triode)
• Plate Dissipation: 3.0 W (Pentode), 0.55 W (Triode)
• Screen Dissipation: 0.55 W (Pentode)
• Grid-Number 1 Circuit Resistance (Fixed Bias): 0.25 MΩ (Pentode), 0.5 MΩ (Triode)
• Grid-Number 1 Circuit Resistance (Cathode Bias): 1.0 MΩ (Pentode and Triode)

Screen Grid Dissipation Derating

For both rating sets, the permissible screen dissipation is flat at 0.55 W for screen voltages from 0 V up to 150 V. For screen voltages between 150 V and 300 V, the maximum screen dissipation derates linearly down to 0 W at 300 V.

Electrical Characteristics and Typical Operation

The 19KG8 exhibits two completely different sets of typical operating characteristics depending on the manufacturer's design of the triode section. One version features a medium-mu triode with high transconductance and plate current, while the other features a high-mu triode operating at very low plate current.

Operating Point 1: Medium-Mu Triode / Sharp-Cutoff Pentode

In this operating configuration, the triode section functions as a medium-mu, low-resistance, high-current amplifier:

Operating Point 2: High-Mu Triode / Sharp-Cutoff Pentode

In this alternative configuration, the triode section behaves as a high-mu, high-resistance, low-current amplifier, and the pentode transconductance is slightly lower:

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 19KG8 vary depending on whether an external shield (EIA 315) is connected to the cathode of the section under test.

Pentode Section Capacitances

• Grid No. 1 to Plate: 0.01 pF maximum (with shield), 0.016 pF (unshielded)
• Input ($g_1$ to $h + p_k + g_2 + g_3 + i.s.$): 5.5 pF (both shielded and unshielded)
• Output ($p$ to $h + p_k + g_2 + g_3 + i.s.$): 3.4 pF (shielded), 2.4 pF (unshielded)
• Heater to Cathode: 2.8 pF (both shielded with shield grounded, and unshielded)

Triode Section Capacitances

• Grid to Plate: 1.7 pF (shielded, per some specifications) or 0.9 pF (both shielded and unshielded, per alternative specifications)
• Input:
  • Specification A (with shield): 3.2 pF
  • Specification B (with shield): 1.9 pF
  • Specification B (unshielded): 1.7 pF
• Output:
  • Specification A (with shield): 2.2 pF
  • Specification B (with shield): 2.6 pF
  • Specification B (unshielded): 1.7 pF
• Heater to Cathode: 2.8 pF (both shielded with shield grounded, and unshielded)

Technical Summary

The 19KG8 is a versatile miniature triode-pentode designed for series-string heater applications requiring 18.9 V / 0.15 A. It combines a sharp-cutoff pentode and a triode in a single T-6 1/2 glass envelope with a 9-pin button base.

Depending on the specific manufacturer's design, the triode section can be either a medium-mu (46) high-transconductance (8,500 µmhos) unit or a high-mu (70) low-current (0.8 mA) unit. Similarly, the pentode section offers a transconductance of 6,500 to 7,500 µmhos with a plate resistance of 200,000 Ω.

Due to significant discrepancies in published literature regarding the 9LY pinout configuration, maximum plate and screen ratings, and grid circuit resistance limits, circuit designers must carefully verify the specifications of the exact 19KG8 variant being employed in their application.