10LZ8 — High-Mu Triode and Sharp-Cutoff Pentode

Overview

The 10LZ8 (also designated as 10JA8/10LZ8) is a miniature multi-unit vacuum tube containing a high-mu triode and a sharp-cutoff pentode in a single envelope. It is designed primarily for use in color and black-and-white television receiver applications. In these circuits, the triode section is typically employed as a sync separator, sync clipper, or phase inverter, while the pentode section is optimized for service as a video amplifier. The triode section is also suitable for general-purpose voltage amplification.

The tube features a 10.5-volt heater designed for series-string heater circuits, requiring a nominal current of 0.45 amperes. It is housed in a standard T-6 1/2 glass envelope with a miniature 9-pin button base, utilizing the 9DX basing arrangement.

Mechanical Specifications and Pinout

The 10LZ8 is constructed in a T-6 1/2 glass envelope and utilizes a small button 9-pin base (E9-1). It can be mounted and operated in any position.

The physical dimensions of the tube are as follows:

• Maximum Diameter: 0.875 inches (22.2 mm)
• Minimum Diameter: 0.750 inches (19.1 mm)
• Maximum Seated Height: 2.375 inches (60.3 mm)
• Maximum Overall Length: 2.625 inches (66.7 mm)

The tube requires a standard miniature 9-contact socket. The 9DX basing diagram provides the following pin connections, viewed from the underside of the base:

Heater Characteristics and Ratings

The 10LZ8 is designed with an indirectly heated, coated unipotential cathode structure. The heater is intended for operation in series-string heater chains, with controlled warm-up characteristics to minimize voltage surges across other tubes in the string during start-up.

Heater-Cathode Voltage Limits

To prevent insulation breakdown between the heater and the cathodes, the following design-maximum voltage limits must be observed:

• Heater Negative with Respect to Cathode:
  • Total DC and Peak Voltage: 200 V max
• Heater Positive with Respect to Cathode:
  • DC Component: 100 V max
  • Total DC and Peak Voltage: 200 V max
• Alternative Rating System Limits: Some specifications define these limits as a peak value of ±200 V max and an average value of 100 V max.

Electrical Characteristics and Operating Points

Because the 10LZ8 contains two highly distinct tube structures, its electrical characteristics are documented separately for each section. Multiple operating points are established depending on the specific application and supply voltages.

Triode Section

The triode section is a high-mu unit. Depending on the operating point, its amplification factor ranges from 60 to 110, and its plate resistance varies significantly.

Pentode Section

The pentode section is a sharp-cutoff unit designed for high transconductance and high plate current capability at low plate voltages, which is highly beneficial for video amplifier service.

—

​​​​​​​Note: Measured using a recurrent waveform method to avoid exceeding the maximum ratings of the tube.

An additional operating point with cathode bias is specified for the pentode section:

• Plate Voltage: 200 V
• Screen Voltage: 100 V
• Cathode-Bias Resistor: 82 Ω
• Plate Resistance: 60,000 Ω
• Transconductance: 19,000 μmhos
• Plate Current: 16.5 mA
• Screen Current: 2.8 mA
• Grid-No. 1 Voltage for $I_b = 500\,\mu\text{A}$: -5.5 V

Maximum Ratings and Circuit Constraints

The maximum ratings of the 10LZ8 are defined under the Design-Maximum Rating System. These ratings are limiting values of operating and environmental conditions and should not be exceeded under the worst probable conditions.

Because different manufacturers and specifications list slightly different maximum limits, these are presented as ranges or distinct sets of ratings:

Grid-No. 1 Circuit Resistance Limits

To prevent thermal runaway and maintain bias stability, the maximum resistance in the control grid circuit must be restricted based on the bias method employed:

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

Interelectrode Capacitances

The direct interelectrode capacitances of the 10LZ8 are measured without an external shield. The values vary slightly across different manufacturing specifications:

Triode Section Capacitances

• Grid to Plate ($T_g$ to $T_p$): 3.6 pF to 4.0 pF
• Input ($T_g$ to $h + T_k + P_k + P_{g3} + \text{IS}$): 2.6 pF to 2.8 pF
• Output ($T_p$ to $h + T_k + P_k + P_{g3} + \text{IS}$): 2.6 pF to 3.0 pF

Pentode Section Capacitances

• Grid-No. 1 to Plate ($P_{g1}$ to $P_p$): 0.075 pF to 0.10 pF (maximum value of 0.10 pF)
• Input ($P_{g1}$ to $h + P_k + P_{[g2](/tubes/G2)} + P_{g3} + \text{IS}$): 9.5 pF to 12.0 pF
• Output ($P_p$ to $h + P_k + P_{[g2](/tubes/G2)} + P_{g3} + \text{IS}$): 4.4 pF

Inter-Section Capacitances

Because both sections reside in close proximity within the same envelope, small parasitic capacitances exist between the triode and pentode elements:

• Grid-No. 1 to Triode Plate: 0.005 pF max
• Plate to Triode Grid: 0.018 pF max
• Plate to Triode Plate: 0.17 pF max

(Note: "IS" refers to the internal shield, "h" to the heater, "k" to the cathode, "g" to the grid, and "p" to the plate).

Analysis of Characteristic Curves

The published characteristic curves for the 10LZ8 illustrate the behavior of both sections under various operating conditions:

Pentode Section Curves

• Plate Characteristics: The plate current ($I_b$) versus plate voltage curves show a sharp knee at low plate voltages (typically below 50 V), which is characteristic of a sharp-cutoff pentode. Above the knee, the plate current remains relatively constant with increasing plate voltage, indicating high plate resistance. At a screen voltage ($E_{c2}$) of 140 V and grid-No. 1 voltage ($E_{[c1](/tubes/C1)}$) of 0 V, the plate current reaches approximately 35 mA.
• Transfer Characteristics: The curves of plate current and screen current versus grid-No. 1 voltage (at a plate voltage of 200 V) demonstrate the sharp-cutoff nature of the control grid. The transconductance ($g_m$) curve peaks at around 15,000 μmhos near $E_{[c1](/tubes/C1)} = 0$ V for a screen voltage of 160 V, and drops rapidly as the grid bias becomes more negative, reaching near-zero values around -5 V.

Triode Section Curves

• Plate Characteristics: The triode plate current versus plate voltage curves exhibit a typical exponential rise. At $E_c = 0$ V, the plate current rises steeply, reaching 5 mA at approximately 180 V.
• Transfer Characteristics: The curves of plate current versus grid voltage for various plate voltages (from 50 V to 300 V) show that the triode behaves as a high-mu device, requiring only a few volts of negative bias to completely cut off plate current. For example, at a plate voltage of 250 V, the plate current is cut off at approximately -3.6 V.
• Average Characteristics: The curves of amplification factor ($μ$), plate resistance ($r_p$), and transconductance ($g_m$) plotted against plate current show that the triode's amplification factor remains relatively stable between 100 and 120 for plate currents above 1.5 mA (at $E_b = 250$ V). In contrast, the plate resistance decreases and transconductance increases as plate current rises, which is typical for triode voltage amplifiers.

Technical Summary

The 10LZ8 is a versatile, space-saving miniature triode-pentode designed for television receiver applications. It combines a high-mu triode (with an amplification factor up to 110) and a high-transconductance, sharp-cutoff pentode (with transconductance up to 19,000 μmhos) in a single T-6 1/2 glass envelope.

Operating from a 10.5 V, 0.45 A heater supply with controlled warm-up characteristics, the tube is optimized for series-string heater configurations. Its 9DX basing arrangement provides independent connections for almost all electrodes, with the pentode's cathode, suppressor grid, and internal shield sharing a common pin (Pin 6) to minimize lead inductance and simplify layout.

By offering two distinct, high-performance stages in a single 9-pin envelope, the 10LZ8 serves as an efficient solution for video amplification, sync separation, sync clipping, and phase inversion in television circuitry.