117L7 — Half-Wave Rectifier and Beam Power Amplifier
The 117L7 (also designated as 117L7/M7-GT) is a multi-unit vacuum tube combining a half-wave high-vacuum rectifier and a beam power amplifier within a single glass envelope. Designed primarily for compact, transformerless receiver applications, the tube features a high-voltage heater that can be operated directly from a standard 117-volt AC or DC power line.
Construction, Heater, and Base Pinout
The 117L7 is constructed with an indirectly heated, coated unipotential cathode system. The heater is designed to operate at a nominal line voltage of 117 V with a current of 0.09 A.
Heater Specifications
- Heater Voltage: 117 V
- Heater Current: 0.09 A
- Calculated Heater Power: 10.53 W (calculated as 117 V * 0.09 A)
- Cathode Type: Indirectly heated, unipotential
Base and Pin Connections
The tube utilizes an 8-pin octal base designated as 8AO and is housed in a glass envelope corresponding to outline drawing 9-15. The pin connections, viewed from the bottom of the base, are assigned as follows:
The 8AO base diagram explicitly depicts the beam-forming plates of the amplifier section connected internally to the amplifier's cathode at Pin 8. This construction distinguishes it from the suppressor-grid construction shown in the 8AV base diagram of related types.
Beam Power Amplifier Section
The amplifier portion of the 117L7 is a beam power tetrode designed for Class A audio power amplification. It operates at moderate plate and screen voltages, making it suitable for line-voltage-powered equipment.
Typical Class A Operating Characteristics
Under typical operating conditions, the beam power amplifier section exhibits the following electrical parameters:
- Plate Voltage: 105 V
- Screen Voltage (Grid 2): 105 V
- Control Grid Voltage (Grid 1): -5.2 V
- Zero-Signal Plate Current: 43 mA
- Zero-Signal Screen Current: 4 mA
- Plate Resistance (Rp): Approximately 17,000 Ohms
- Transconductance (Gm): 5,300 micromhos
- Load Resistance for Rated Output: 4,000 Ohms
- Rated Power Output: 0.85 W
Power Dissipation and Efficiency Calculations
At the typical zero-signal operating point, the power dissipation of the amplifier electrodes can be calculated as follows:
- Zero-Signal Plate Dissipation: 105 V * 0.043 A = 4.515 W
- Zero-Signal Screen Dissipation: 105 V * 0.004 A = 0.42 W
- Total Zero-Signal Electrode Dissipation: 4.515 W + 0.42 W = 4.935 W
- Plate Efficiency at Rated Output: 0.85 W / 4.515 W = 18.8%
Half-Wave Rectifier Section
The rectifier portion of the 117L7 is an independent half-wave high-vacuum diode designed to supply DC power to the receiver circuitry.
Rectifier Operating Ratings and Limits
- Maximum RMS Supply Voltage: 117 V
- Maximum DC Output Current: 75 mA
- Maximum Peak Inverse Voltage: 350 V
- Maximum Peak Current: 450 mA
- Tube Voltage Drop: 16 V measured at a DC current of 150 mA
Comparison with Related Types
The 117L7 belongs to a family of 117-volt combination rectifier-amplifier tubes. It shares close similarities with the 117P7-GT and 117N7-GT, though they differ in base designations, internal construction, or electrical performance.
117P7-GT Comparison
The 117P7-GT is electrically identical to the 117L7/M7-GT in both its amplifier and rectifier sections, sharing the same operating points, voltage drop, and output ratings. However, it utilizes the 8AV base connection instead of the 8AO base. In the 8AV base diagram, the beam-forming plates are represented as a conventional suppressor grid (Grid 3) connected to the cathode at Pin 8, reflecting a minor constructional variation.
117N7-GT Comparison
The 117N7-GT also utilizes the 8AV base and operates with a 117 V, 0.09 A heater. While its rectifier section shares the identical ratings of the 117L7 (75 mA max DC output, 350 V peak inverse, 117 V RMS supply, and a 16 V drop at 150 mA), its beam power amplifier section is designed for higher power output at slightly different operating conditions:
- Plate and Screen Voltages: 100 V
- Control Grid Voltage: -6.0 V
- Zero-Signal Plate Current: 51 mA
- Zero-Signal Screen Current: 5 mA
- Plate Resistance (Rp): Approximately 16,000 Ohms
- Transconductance (Gm): 7,000 micromhos
- Load Resistance for Rated Output: 3,000 Ohms
- Rated Power Output: 1.2 W (a 41% increase over the 117L7's 0.85 W output)
Technical Summary
The 117L7 is a versatile, space-saving vacuum tube that combines a half-wave rectifier and a beam power amplifier in a single envelope with an 8AO octal base. Its high-voltage heater (117 V, 0.09 A) allows direct operation from AC or DC mains, eliminating the need for a power transformer or series dropping resistors. The rectifier section provides up to 75 mA of DC current with a 16 V tube drop at 150 mA, while the beam power amplifier section delivers 0.85 W of audio output into a 4,000 Ohm load under typical Class A conditions. Through its integrated design, the 117L7 served as a foundational component in compact, cost-effective line-powered electronic equipment.
