10LY8 — High-Mu Triode and Sharp-Cutoff Pentode

An indirectly heated, multi-section miniature vacuum tube containing a high-mu triode and a sharp-cutoff pentode in a single envelope, designed for television receiver applications.

Overview

The 10LY8 is a miniature multi-section vacuum tube incorporating two distinct and electrically independent active structures: a high-mu triode and a sharp-cutoff pentode. It is designed primarily for use in color and black-and-white television receivers. Within these applications, the sharp-cutoff pentode section is optimized for service as a video amplifier, while the high-mu triode section is designed for general-purpose voltage amplification and control functions.

The 10LY8 features a 10.5 V heater designed for series-string heater chains, drawing a nominal current of 0.45 A. An electrically identical counterpart, the 6LY8, is available for parallel heater circuits requiring a 6.3 V, 0.75 A supply. Both types share the same physical construction, internal electrode structures, and electrical characteristics, differing only in their heater ratings and warm-up specifications.

Construction, Physical Dimensions, and Basing

The 10LY8 is constructed in a miniature T-6 1/2 glass envelope. It utilizes a standard button 9-pin base (E9-1 designation) and requires a 9-contact socket for operation. The physical dimensions of the tube are as follows:

The tube uses the 9DX basing arrangement, which provides separate connections for the triode and pentode sections. The pentode section includes an internal shield (IS) connected internally to the pentode cathode and grid No. 3 (suppressor grid) to minimize internal feedback and capacitive coupling.

The pin connections, viewed from the underside of the base, are assigned as follows:

Heater and Heater-Cathode Ratings

The heater of the 10LY8 is designed for reliable operation in series-connected heater strings. It features a controlled warm-up time to ensure balanced voltage distribution across series elements during the initial power-up phase.

Heater-Cathode Voltage Limits

To prevent insulation breakdown and minimize hum or leakage between the heater and the individual cathodes, the following maximum voltage limits must be observed:

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

Direct Interelectrode Capacitances

The 10LY8 is designed to operate without an external shield. The direct interelectrode capacitances of the unshielded tube are specified separately for each section:

Pentode Section Capacitances

• Grid No. 1 to Plate: 0.075 pF
• Input (Grid No. 1 to Heater + Pentode Cathode + Grid No. 2 + Grid No. 3 + Internal Shield): 13 pF
• Output (Plate to Heater + Pentode Cathode + Grid No. 2 + Grid No. 3 + Internal Shield): 4.4 pF

Triode Section Capacitances

• Grid to Plate: 3.8 pF
• Input (Grid to Heater + Triode Cathode + Pentode Cathode + Grid No. 3 + Internal Shield): 2.6 pF
• Output (Plate to Heater + Triode Cathode + Pentode Cathode + Grid No. 3 + Internal Shield): 2.8 pF

These low capacitances, particularly the extremely low grid-to-plate capacitance of the pentode section (0.075 pF), are critical for maintaining stability and high gain in wideband video amplifier circuits.

Maximum Ratings (Design-Maximum System)

The maximum ratings of the 10LY8 are established under the Design-Maximum Rating System. Operating the tube beyond these limits may impair its performance and significantly shorten its service life.

Note: The maximum allowable screen voltage and screen dissipation for voltages between 165 V and 330 V are determined by established rating charts and curves.

Maximum Grid No. 1 Circuit Resistance

To prevent thermal runaway caused by grid emission or gas currents, the resistance in the control grid circuit must not exceed the following limits:

• Fixed-Bias Operation:
  • Pentode Section: 0.5 M̩
  • Triode Section: 0.5 M̩
• Cathode-Bias Operation:
  • Pentode Section: 1.0 M̩
  • Triode Section: 1.0 M̩

Electrical Characteristics and Typical Operation

The 10LY8 combines a high-transconductance, sharp-cutoff pentode with a high-mu triode. The typical operating points and characteristics for each section are detailed below.

Triode Section

The triode section is characterized by a high amplification factor (mu = 100), making it highly suitable for voltage preamplification, sync separation, or general-purpose control circuitry.

• Plate Voltage: 250 V
• Grid No. 1 Voltage: -2.0 V
• Amplification Factor: 100
• Plate Resistance (Approximate): 59,000 ̩
• Transconductance: 1,700 µmhos
• Plate Current: 1.0 mA
• Grid Voltage (Approximate) for Plate Current of 10 µA: -5 V

Pentode Section

The pentode section is designed to deliver high transconductance and high plate current at low plate voltages, which is essential for driving the capacitive loads encountered in video amplifier stages. Two typical operating points illustrate its performance:

At Operating Point B, the pentode section exhibits an exceptionally high transconductance of 20,000 µmhos with a plate current of 19.5 mA. This high transconductance-to-current ratio is achieved through advanced grid design and close electrode spacing, allowing the tube to provide high voltage gain over a wide frequency bandwidth.

Technical Summary

The 10LY8 is a versatile, high-performance miniature tube that combines a high-mu triode and a sharp-cutoff pentode in a single T-6 1/2 envelope. It is specifically optimized for television receiver applications, where the pentode serves as a high-transconductance video amplifier and the triode performs general-purpose voltage amplification.

With a 10.5 V, 0.45 A series-string heater (and its 6.3 V, 0.75 A parallel-heater counterpart, the 6LY8), the tube integrates seamlessly into standard television chassis designs. The pentode section's capability to deliver 20,000 µmhos of transconductance at a typical operating point of 200 V, combined with a robust 5.0 W plate dissipation rating, ensures excellent performance in demanding video output stages. Meanwhile, the triode section provides a high amplification factor of 100 with a modest 1.0 W plate dissipation rating, offering a compact and efficient solution for auxiliary signal processing.