117N7GT — Combined Half-Wave Rectifier and Beam Power Amplifier

The 117N7GT (also written as 117N7-GT) is a multi-unit vacuum tube containing a half-wave rectifier and a beam power amplifier within a single glass envelope. It is specifically designed with a high-voltage heater for direct connection across a 117-volt AC or DC power line, eliminating the need for a power transformer or series dropping resistor in compact line-operated equipment.

Physical Construction and Pinout

The 117N7GT is constructed in a T-9 glass bulb with an intermediate shell 8-pin octal base. The tube features coated unipotential cathodes and is designed to operate in any mounting position.

The physical dimensions of the device are:

• Maximum overall length: 3 7/16 inches
• Maximum seated height: 2 7/8 inches
• Maximum diameter: 1 3/16 inches

The base connections follow the 8AV pinout configuration, viewed from the bottom of the tube:

• Pin 1: No Connection (NC)
• Pin 2: Heater (H)
• Pin 3: Plate, Beam Power Amplifier ($P_A$)
• Pin 4: Screen Grid, Beam Power Amplifier ($G_2$)
• Pin 5: Control Grid, Beam Power Amplifier ($G_1$)
• Pin 6: Plate, Rectifier ($P_R$)
• Pin 7: Heater (H)
• Pin 8: Common Cathode ($K_R$ and $K_A$ internally connected)

Both the rectifier cathode ($K_R$) and the beam power amplifier cathode ($K_A$) share the connection at Pin 8.

Heater and Rectifier Unit Ratings

The heater is designed to operate directly from standard mains voltages:

• Heater Voltage (AC or DC): 117 Volts
• Heater Current: 0.09 Ampere

The rectifier unit is a half-wave high-vacuum rectifier. Its maximum ratings are interpreted according to the RMA Standard M8-210:

• Maximum Peak Inverse Plate Voltage: 350 Volts
• Maximum Peak Plate Current: 450 mA
• Maximum DC Heater-Cathode Potential: 175 Volts
• Tube Voltage Drop (at 150 mA DC): 16 Volts

When used with a condenser-input filter, the rectifier is subject to the following operating limits:

• Maximum AC Plate Voltage (RMS): 117 Volts
• Maximum DC Output Current: 75 mA
• Minimum Total Effective Plate Supply Impedance: 15 Ohms

If a filter-input condenser larger than 40 µF is employed, additional plate-supply impedance must be added to the circuit to limit the peak plate current to the rated 450 mA maximum.

Beam Power Amplifier Performance and Operating Points

The beam power amplifier section is designed for Class A1 audio power amplification.

Maximum Ratings (RMA Standard M8-210)

• Maximum Plate Voltage: 117 Volts
• Maximum Screen Voltage: 117 Volts
• Maximum Plate Dissipation: 5.5 Watts
• Maximum Screen Dissipation: 1.0 Watt

Typical Operating Conditions and Characteristics

Under typical Class A1 operating conditions, the amplifier unit provides the following electrical characteristics:

• Plate Voltage: 100 Volts
• Screen Voltage: 100 Volts
• Grid Voltage: -6 Volts
• Peak AF Grid Voltage: 6 Volts
• Zero-Signal Plate Current: 51 mA
• Zero-Signal Screen Current: 5 mA
• Plate Resistance (Approximate): 16,000 Ohms
• Transconductance: 7,000 µmhos
• Load Resistance: 3,000 Ohms
• Total Harmonic Distortion: 6%
• Maximum Signal Power Output: 1.2 Watts

Grid Circuit Resistance Limits

To ensure stable operation and prevent thermal runaway, the resistance in the control grid circuit must be limited based on the biasing method:

• Maximum Grid Resistance (Fixed Bias): 0.25 Megohm
• Maximum Grid Resistance (Cathode Bias): 1.0 Megohm

The input coupling network should be designed so that it does not introduce excessive resistance into the grid circuit.

Comparison with Related Types

The 117N7GT is part of a family of 117-volt combination rectifier-amplifier tubes. It can be compared with the 117P7-GT and 117L7/M7-GT:

• 117P7-GT: This type shares the same 8AV pinout and physical envelope as the 117N7GT, but its amplifier section is designed for different operating conditions, resulting in lower power output. At a plate and screen voltage of 105 V and a grid bias of -5.2 V, the 117P7-GT has a transconductance of 5,300 µmhos, a plate resistance of 17,000 Ohms, and delivers 0.85 Watts into a 4,000 Ohm load.
• 117L7/M7-GT: This tube has identical electrical characteristics to the 117P7-GT (delivering 0.85 Watts under the same operating conditions) but utilizes a different base connection layout (8AO instead of 8AV).

All three types share the same heater rating (117 V, 0.09 A) and a rectifier tube voltage drop of 16 V at 150 mA DC.

Technical Summary

The 117N7GT is an octal-based, multi-unit vacuum tube combining a half-wave rectifier and a beam power amplifier in a single T-9 glass envelope. Its high-voltage heater (117 V, 0.09 A) is optimized for direct line-voltage operation. The rectifier unit can deliver up to 75 mA of DC output current with a maximum peak inverse voltage of 350 V, while the beam power amplifier unit is capable of delivering 1.2 Watts of audio output power into a 3,000 Ohm load under Class A1 operating conditions. The integration of these two functions, along with a shared cathode connection on an 8-pin octal base, makes the 117N7GT an efficient solution for line-powered audio and receiver applications.