15AF11 — Dissimilar Double Triode and Sharp-Cutoff Pentode

The 15AF11 is a multi-unit, indirectly heated vacuum tube of the duodecar (also referred to as compactron) type designed primarily for television receiver applications. It integrates three distinct, electrically independent active structures within a single T-9 glass envelope: a high-mu triode, a medium-mu triode, and a sharp-cutoff pentode. This combination is specifically tailored to perform multiple functions in television circuitry, where the high-mu triode serves as an automatic gain control (AGC) keyer, the medium-mu triode operates as a sync separator, and the pentode functions as a video amplifier.

Construction, Physical Characteristics, and Basing

The 15AF11 features a compactron T-9 glass bulb construction with a button 12-pin base (E12-70 designation), requiring a standard duodecar 12-contact socket. The internal elements utilize coated unipotential cathodes. The physical dimensions of the tube are as follows:

The 12DP basing configuration provides independent connections for the three units, allowing them to be biased and operated separately, subject to heater-cathode voltage limitations.

Heater Characteristics and Ratings

The 15AF11 is designed for series-string heater circuits. Its heater has controlled warm-up characteristics to ensure reliable operation in series configurations.

The average heater warm-up time of 11 seconds is defined under specific test conditions: it is the time required for the voltage across the heater to reach 80 percent of its nominal value when four times the nominal heater voltage is applied to a circuit consisting of the heater in series with an external resistance equal to three times the nominal heater voltage divided by the nominal heater current.

Heater-Cathode Voltage Limits

To prevent insulation breakdown between the heater and the separate cathodes, the following maximum voltage ratings must be observed:

• Heater Negative with Respect to Cathode: 200 V maximum (Total DC and Peak)
• Heater Positive with Respect to Cathode: 100 V maximum (DC), 200 V maximum (Total DC and Peak)

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 15AF11 are measured without an external shield. The coupling capacitances between the separate units are kept low by internal shielding.

Triode Unit No. 1 (High-Mu Triode)

• Grid to Plate: 1.9 pF
• Input (Grid to Cathodes, Heater, Pentode Grid No. 3, and Internal Shield): 3.0 pF
• Output (Plate to Cathodes, Heater, Pentode Grid No. 3, and Internal Shield): 2.2 pF

Triode Unit No. 2 (Medium-Mu Triode)

• Grid to Plate: 3.6 pF
• Input (Grid to Cathodes, Heater, Pentode Grid No. 3, and Internal Shield): 2.4 pF
• Output (Plate to Cathodes, Heater, Pentode Grid No. 3, and Internal Shield): 3.8 pF

Pentode Unit

• Grid No. 1 to Plate: 0.12 pF
• Input (Grid No. 1 to Cathodes, Heater, Pentode Grid No. 2, Pentode Grid No. 3, and Internal Shield): 10.0 pF
• Output (Plate to Cathodes, Heater, Pentode Grid No. 2, Pentode Grid No. 3, and Internal Shield): 4.5 pF

Inter-Unit Coupling

• Pentode Plate to Triode No. 2 Plate: 0.045 pF maximum
• Triode No. 1 Plate to Triode No. 2 Plate: 0.06 pF maximum

Maximum Ratings (Design-Maximum System)

The maximum ratings for each section of the 15AF11 are established under the Design-Maximum Rating System. These limits must not be exceeded under the worst-case operating conditions.

Electrical Characteristics and Typical Operating Points

The three units within the 15AF11 exhibit distinct electrical characteristics optimized for their respective roles. Typical Class A1 amplifier operating points are detailed below.

Triode Unit No. 1 (High-Mu AGC Keyer)

• Plate Voltage: 200 V
• Grid-No. 1 Voltage: -2.0 V
• Amplification Factor: 68
• Plate Resistance (Approximate): 12,400 Ω
• Transconductance: 5500 µmhos
• Plate Current: 7.0 mA
• Grid-No. 1 Voltage for Cutoff (at $I_b = 10$ µA): -5.5 V

Triode Unit No. 2 (Medium-Mu Sync Separator)

• Plate Voltage: 200 V
• Cathode-Bias Resistor: 220 Ω
• Amplification Factor: 41
• Plate Resistance (Approximate): 9400 Ω
• Transconductance: 4400 µmhos
• Plate Current: 9.2 mA
• Grid-No. 1 Voltage for Cutoff (at $I_b = 10$ µA): -6.5 V

Pentode Unit (Sharp-Cutoff Video Amplifier)

The pentode section can be operated under different conditions depending on the circuit requirements. Two typical operating points are documented:

Related Designations

The 15AF11 belongs to a family of identical multi-unit tubes that differ only in their heater configurations to accommodate different power supply arrangements:

• 6AF11: Designed for parallel heater circuits, requiring a nominal heater supply of 6.3 V at 1.05 A.
• 10AF11: Designed for series heater strings, requiring a nominal heater supply of 9.8 V at 0.6 A.
• 12AF11: Designed for series heater strings, requiring a nominal heater supply of 12.6 V at 0.45 A.

Aside from these heater variations, the 6AF11, 10AF11, 12AF11, and 15AF11 share identical physical dimensions, basing configurations, interelectrode capacitances, maximum ratings, and electrical characteristics.

Technical Summary

The 15AF11 is a versatile duodecar/compactron multi-unit tube containing a high-mu triode, a medium-mu triode, and a sharp-cutoff pentode in a single T-9 envelope. Operating with a 14.7 V, 0.45 A series-string heater, it provides a highly integrated solution for television receiver designs. By combining an AGC keyer (Triode 1, $\mu = 68$), a sync separator (Triode 2, $\mu = 41$), and a high-transconductance video amplifier (Pentode, $g_m = 11,000$ µmhos) into a single 12-pin envelope, the 15AF11 minimizes component count and chassis space while maintaining high performance.