10C8 — Miniature Triode-Pentode Vacuum Tube

The 10C8 is a miniature, multi-section vacuum tube containing a high-mu triode and a power pentode within a single glass envelope. It is designed specifically for use in compact television receivers employing 300-milliampere series-string heater circuits. The pentode section is engineered for general-purpose amplification or, when triode-connected, as a vertical-deflection amplifier. The high-mu triode section is designed for service as a vertical oscillator, audio-frequency (AF) voltage amplifier, sync separator, clipper, or amplifier.

The tube features a controlled heater warm-up characteristic to ensure reliable operation in series-connected heater strings, alongside a compact physical footprint, low power requirements, and high performance capabilities when operating at the low DC supply voltages typical of these receiver designs.

Construction, Heater, and Pinout

The 10C8 is constructed with coated unipotential cathodes for both sections. The heater is designed for series-string applications and has a controlled warm-up time.

Heater Characteristics

The nominal heater parameters and the specific test conditions for the controlled warm-up characteristic are:

The heater warm-up time is defined as the time required for the voltage across the heater to reach 80% of its rated value (8.4 V) when four times the rated heater voltage (42.0 V) is applied to a circuit consisting of the tube heater in series with a specific resistance. This series resistance is calculated as:

$\text{Resistance} = 3 \times \frac{\text{Rated Heater Voltage}}{\text{Rated Heater Current}} = 3 \times \frac{10.5\text{ V}}{0.3\text{ A}} = 105\ \Omega$

Mechanical Specifications and Basing

The 10C8 is housed in a T-6 1/2 glass envelope with a small button 9-pin E9-1 base. It can be mounted in any position.

• Maximum Overall Length: 2-3/16 inches
• Maximum Seated Height: 1-15/16 inches
• Maximum Diameter: 7/8 inches

The tube utilizes the RETMA 9DA basing diagram, which provides the following pin connections (viewed from the bottom of the base):

Interelectrode Capacitances

The direct interelectrode capacitances of the 10C8 are measured without an external shield. The internal shield is connected to Pin 9 (along with the pentode cathode and grid number 3).

Pentode Section Capacitances

• Grid-Number 1 to Plate (maximum): 0.04 μμF
• Input (Grid 1 to Cathode, Heater, Screen, Suppressor, and Shield): 7.0 μμF
• Output (Plate to Cathode, Heater, Screen, Suppressor, and Shield): 2.2 μμF

Triode Section Capacitances

• Grid to Plate: 1.6 μμF
• Input (Grid to Cathode and Heater): 2.4 μμF
• Output (Plate to Cathode and Heater): 0.20 μμF

Inter-Section Capacitances

• Pentode Grid-Number 1 to Triode Plate (maximum): 0.008 μμF
• Triode Grid to Pentode Plate (maximum): 0.006 μμF
• Pentode Plate to Triode Plate (maximum): 0.06 μμF

Electrical Characteristics and Typical Operation

The 10C8 sections exhibit distinct electrical characteristics optimized for their respective roles. Typical operating points are established for Class A1 amplifier service.

Typical Operating Points

* Screen connected to plate. ** Applied for a short interval (two seconds maximum) to prevent damage to the tube.

The triode-connected pentode section provides a lower amplification factor (\mu = 40) but is capable of handling higher plate current (33 mA at 135 V) compared to the high-mu triode section (\mu = 53, $I_b = 7.3\text{ mA}$ at 250 V).

Maximum Ratings

The maximum ratings for the 10C8 are defined under the Design-Maximum system. These ratings represent the limiting values at which satisfactory tube life can be expected under the worst probable operating conditions.

Class A1 Amplifier Service

Vertical Oscillator and Amplifier Service

For vertical deflection and oscillator applications, the ratings are tailored to the pulsed nature of the television sweep circuits. The pentode section is rated for vertical deflection amplifier service in a triode connection (screen connected to plate), while the triode section is rated for vertical oscillator service.

* Screen connected to plate. ** For operation in a 525-line, 30-frame television system. The duty cycle of the voltage pulse must not exceed 15 percent of one scanning cycle. *** In stages operating with grid-leak bias, an adequate cathode-bias resistor or other suitable means is required to protect the tube in the absence of excitation.

Screen Rating Chart

The pentode screen grid (Grid Number 2) dissipation is bounded by both voltage and wattage limits. The maximum permissible screen dissipation is 0.55 Watts for screen voltages up to approximately 165 Volts. Above this voltage, the maximum allowable dissipation decreases linearly, reaching 0 Watts at a screen voltage of 300 Volts.

Analysis of Characteristic Curves

The average plate and transfer characteristics of the 10C8 sections illustrate their performance under varying bias and voltage conditions.

Triode Section Curves

The plate characteristics for the triode section show plate current ($I_b$) plotted against plate voltage ($E_b$) for grid voltages ($E_c$) ranging from +4.0 Volts down to -9.0 Volts. At a typical plate voltage of 250 Volts, the curves demonstrate a highly linear relationship between grid voltage and plate current in the region of -1.0 to -4.0 Volts. The transfer characteristics confirm this linearity, showing a steady transconductance ($g_m$) of approximately 4,400 μmhos and an amplification factor (\mu) that remains relatively constant around 53 across a wide range of plate currents.

Pentode Section Curves

The pentode section curves are provided for both pentode and triode connections:

• Triode-Connected Pentode: With the screen grid tied to the plate, the plate characteristics resemble those of a low-mu power triode. The curves show rapid increases in plate current with plate voltage, reflecting a low plate resistance and a triode amplification factor of approximately 40.
• Pentode Connection: With the screen grid maintained at a constant potential (such as 135 Volts or 150 Volts), the plate characteristics display the classic pentode knee. Above approximately 50 Volts on the plate, the plate current curves flatten out, indicating a very high plate resistance (approximately 190,000 Ohms) and making the section highly suitable for high-gain voltage amplification or constant-current drive applications. The screen current ($I_{c2}$) curves mirror the plate current behavior at low plate voltages, rising as plate voltage drops below the knee.

Technical Summary

The 10C8 is a versatile, space-saving triode-pentode designed for 300 mA series-string television receivers. It features a 10.5 V, 0.3 A heater with a controlled 11-second warm-up time to prevent voltage imbalances in series-connected heater strings.

The pentode section provides high transconductance (8,000 μmhos) and can be operated as a pentode amplifier or triode-connected as a vertical-deflection amplifier capable of handling peak positive pulse plate voltages up to 1000 V. The high-mu triode section (\mu = 53) is optimized for vertical oscillator, AF voltage amplifier, or sync circuit functions. With its compact T-6 1/2 envelope, 9-pin miniature base, and robust design-maximum ratings, the 10C8 provides a highly integrated solution for television sweep and signal processing stages.