14N7 — Medium-Mu Twin Triode Vacuum Tube

Overview

The 14N7 is a loctal (lock-in) double triode vacuum tube designed for use as a voltage amplifier or phase inverter. It contains two independent triode sections within a single T-9 glass envelope, sharing a common heater but having separate cathode, grid, and plate connections for each section. The tube is characterized by its lock-in construction, which provides robust mechanical support and secure retention in its socket, making it suitable for applications requiring high reliability.

The 14N7 is closely related to the 7N7; the two types share identical electrical characteristics, ratings, and physical dimensions, differing only in their heater ratings. While the 7N7 operates with a 6.3 V heater, the 14N7 is designed for a nominal 12.6 V heater supply, making it suitable for series-string heater circuits or equipment with higher-voltage supply rails.

Construction, Heater, and Base Configuration

The 14N7 features an indirectly heated, coated unipotential cathode structure. The two triode units are electrically independent, allowing them to be operated in separate circuits or cascaded stages, provided that the heater-to-cathode voltage limits are not exceeded.

The tube is housed in a T-9 glass bulb with an 8-pin lock-in (loctal) base, designated as the 8AC basing arrangement. The physical dimensions of the tube are as follows:

• Maximum overall length: 3-5/32 inches
• Maximum seated height: 2-5/8 inches
• Maximum bulb diameter: 1-9/32 inches
• Maximum base diameter: 1-3/16 inches

The tube can be mounted and operated in any position.

The 8AC base connections, viewed from the bottom of the tube, are assigned as follows:

• Pin 1: Heater
• Pin 2: Grid (Triode II)
• Pin 3: Plate (Triode II)
• Pin 4: Cathode (Triode II)
• Pin 5: Cathode (Triode I)
• Pin 6: Plate (Triode I)
• Pin 7: Grid (Triode I)
• Pin 8: Heater

Heater Characteristics

The heater is designed for either AC or DC operation. The electrical specifications for the heater of the 14N7, compared with the 7N7, are:

The calculated heater power for both tubes is identical (12.6 V * 0.3 A = 3.78 W and 6.3 V * 0.6 A = 3.78 W), reflecting their shared internal structure.

Electrical Characteristics and Ratings

The maximum ratings for the 14N7 are interpreted according to the RMA Standard MB-210. These limits represent the absolute boundaries within which the tube must be operated to ensure reliable performance and longevity.

Maximum Ratings (Per Section)

• Maximum Plate Voltage: 300 V
• Maximum Plate Dissipation: 2.5 W
• Maximum Heater-Cathode Voltage: 90 V
• Minimum Grid Voltage: 0 V

Typical Class A1 Amplifier Operation

Under typical Class A1 operating conditions, each triode section exhibits the following characteristics:

At these operating points, the relationship between amplification factor, transconductance, and plate resistance is maintained according to the standard triode formula: mu = gm * rp. For Operating Point II, this is illustrated by: 0.0026 S * 7,700 Ω = 20.02, which closely matches the nominal amplification factor of 20.

Phase Inverter Operation

The 14N7 is specifically designed to function as a phase inverter, utilizing its two independent triode sections to provide balanced, out-of-phase signals. Typical operating conditions for phase inverter service are documented at two different plate supply voltages:

These operating points demonstrate that with a higher plate supply voltage of 250 V and a larger plate load resistor of 50,000 Ω, the stage can deliver a significantly higher maximum output voltage of 65 V RMS compared to the 20 V RMS achieved with a 100 V supply.

Interelectrode Capacitances

Direct interelectrode capacitances are measured with an RMA standard shield (#308) connected to the cathode. Because the two triode units are housed in close proximity within the same envelope, small capacitances exist between the sections in addition to the internal capacitances of each individual unit.

Individual Section Capacitances

• Grid to Plate (G to P):
  • Triode I: 3.0 pF
  • Triode II: 3.0 pF
• Input (Grid to Heater and Cathode):
  • Triode I: 3.4 pF
  • Triode II: 2.9 pF
• Output (Plate to Heater and Cathode):
  • Triode I: 2.0 pF
  • Triode II: 2.4 pF

Inter-Section Capacitances

• Grid to Grid (1G to 2G): 0.4 pF
• Plate to Plate (1P to 2P): 0.34 pF
• Grid to Plate (1G to 2P): 0.08 pF
• Grid to Plate (2G to 1P): 0.06 pF

These small but measurable inter-section capacitances indicate that while the triodes are galvanically isolated, high-frequency signals can couple between the two stages. This must be accounted for in high-gain or high-frequency amplifier designs.

Characteristic Curves and Design Interpretation

The electrical behavior of each section of the 14N7 is illustrated by its characteristic curves, which show the relationship between plate voltage, grid voltage, and plate current.

Plate Characteristics

The plate family of curves plots plate current in milliamperes against plate voltage for various negative grid bias voltages.

• At zero grid bias, the plate current rises steeply with plate voltage, reaching approximately 30 mA at a plate voltage of 160 V.
• As the grid bias is made more negative, the curves shift to the right. For example, at a grid bias of -6 V, a plate voltage of approximately 210 V is required to achieve a plate current of 10 mA.
• The curves are relatively linear and parallel over the typical operating region, indicating low distortion when the tube is operated within its rated limits.

Transfer and Parameter Curves

The transfer characteristics show how plate current, plate resistance, and amplification factor vary with grid voltage at fixed plate voltages of 100 V and 250 V.

• At Plate Voltage = 100 V:
  • With grid voltage at 0 V, the plate current is approximately 11 mA, and the plate resistance is at its minimum of about 7,000 Ω.
  • As the grid voltage is made negative, the plate current decreases, and the plate resistance increases. At a grid bias of -5 V, the plate current drops below 1 mA, and the plate resistance rises sharply toward 50,000 Ω.
  • The amplification factor remains remarkably stable near 20 across most of the useful operating range, only dropping slightly as the tube approaches cutoff.
• At Plate Voltage = 250 V:
  • At the typical operating bias of -8 V, the plate current is 9 mA, and the plate resistance is approximately 7,700 Ω.
  • Cutoff is reached near a grid bias of -16 V, where the plate current approaches zero and the plate resistance increases rapidly.
  • The amplification factor remains highly constant at approximately 20 throughout the active region, ensuring consistent voltage gain.

Technical Summary

The 14N7 is a loctal-base, medium-mu twin triode vacuum tube designed for voltage amplification and phase inverter service. It features two independent triode sections with separate unipotential cathodes, housed in a T-9 glass envelope with an 8-pin lock-in base (8AC). The 14N7 operates with a 12.6 V, 0.3 A heater, distinguishing it from its 6.3 V counterpart, the 7N7.

Each triode section provides an amplification factor of 20, a transconductance of 2,600 to 3,000 µmhos, and a plate resistance of 6,700 to 7,700 Ω under typical Class A1 operating conditions. With a maximum plate voltage rating of 300 V and a plate dissipation limit of 2.5 W per section, the 14N7 offers a reliable and mechanically secure solution for general-purpose audio and low-frequency electronic circuits.