10LW8 — High-Mu Triode and Sharp-Cutoff Pentode
The 10LW8 is a miniature triode-pentode vacuum tube containing a high-mu triode and a sharp-cutoff pentode in a single envelope. The pentode section is designed for service as a video amplifier in television receivers, while the triode section is intended for general-purpose applications.
Overview
The 10LW8 is a multi-unit miniature tube featuring two independent electrical systems: a high-mu triode and a sharp-cutoff pentode. The tube is constructed with a T-6 1/2 glass envelope and utilizes a small button 9-pin (E9-1) base. It is designed to operate in any position.
The heater is designed for series-string television receivers, requiring a nominal voltage of 10.5 V and a current of 0.45 A (with a tolerance of 0.03 A). The heater features a controlled warm-up time of 11 seconds to ensure reliable operation in series-connected heater strings.
Physical Construction, Basing, and Pinout
The 10LW8 utilizes the 9DX basing diagram, which provides separate connections for the triode and pentode electrodes, with the exception of a shared heater. The pentode section includes an internal shield connected to its cathode and grid number 3.
Terminal Connections (EIA 9DX)
- Pin 1: Triode Cathode
- Pin 2: Triode Grid
- Pin 3: Triode Plate
- Pin 4: Heater
- Pin 5: Heater
- Pin 6: Pentode Cathode, Grid Number 3, and Internal Shield
- Pin 7: Pentode Grid Number 1
- Pin 8: Pentode Grid Number 2 (Screen)
- Pin 9: Pentode Plate
Physical Dimensions
- Envelope: T-6 1/2 Glass
- Base: E9-1, Small Button 9-Pin
- Maximum Diameter: 0.875 inches
- Maximum Seated Height: 2.375 inches
- Maximum Overall Length: 2.625 inches
Direct Interelectrode Capacitances
The capacitances of each section, measured without an external shield, are as follows:
Electrical Characteristics and Typical Operation
The 10LW8 provides distinct operating characteristics for its two sections. The pentode section is characterized by high transconductance and sharp-cutoff behavior, making it highly suitable for wideband video amplification. The triode section offers a high amplification factor (mu = 75) for general-purpose voltage amplification or control circuitry.
Typical Operating Points (Class A1 Amplifier)
Maximum Ratings
The maximum ratings are established under the Design-Maximum Rating System. These limiting values must not be exceeded under the worst probable operating conditions.
Analysis of Characteristic Curves
The graphical data for the 10LW8 illustrates the performance of both sections under varying bias and voltage conditions:
Pentode Section Curves
- Average Plate Characteristics: At a fixed screen voltage (Ec2) of 150 V, the plate current (Ib) rises rapidly at low plate voltages (below 50 V) and remains highly constant across a wide range of plate voltages up to 500 V. This flat characteristic is typical of a sharp-cutoff pentode. The curves are plotted for grid-No. 1 voltages (Ec1) from 0 V down to -3.0 V. Screen current (Ic2) curves show a complementary behavior, decreasing as plate voltage increases.
- Average Transfer Characteristics: Plotted at a plate voltage (Eb) of 200 V, these curves show plate current, screen current, and transconductance as functions of grid-No. 1 voltage for screen voltages ranging from 25 V to 150 V. The transconductance curves demonstrate a steep slope at low bias voltages, peaking near 30,000 micromhos at Ec2 = 150 V and Ec1 = 0 V, and dropping sharply to cutoff near -5.5 V.
Triode Section Curves
- Average Plate Characteristics: The triode plate curves show plate current plotted against plate voltage for grid bias values from 0 V to -9.0 V. The curves exhibit the typical exponential rise of a high-mu triode.
- Average Transfer Characteristics: Plotted for plate voltages from 100 V to 300 V, these curves show plate current vs. grid voltage, illustrating a cutoff region near -4.0 V for a 200 V plate supply.
- Average Characteristics vs. Plate Current: This set of curves plots the amplification factor (mu), plate resistance (rp), and transconductance (gm) against plate current up to 7 mA. The amplification factor remains remarkably stable around 75 across the entire operating range. The plate resistance decreases from over 40,000 Ohms at low currents to approximately 15,000 Ohms at 7 mA, while the transconductance rises from under 1,000 micromhos to over 5,000 micromhos.
Technical Summary
The 10LW8 is a versatile 9-pin miniature triode-pentode designed for television receiver applications. It combines a high-mu triode (mu = 75) and a sharp-cutoff pentode with exceptionally high transconductance (19,000 micromhos at typical operating conditions) in a single T-6 1/2 glass envelope. Featuring a controlled 11-second heater warm-up time and a 10.5 V / 0.45 A heater rating, it is optimized for series-string heater circuits. The pentode section is highly effective for video amplifier service, while the triode section serves general-purpose voltage amplification and control functions.
