14W7 — Sharp-Cutoff RF Pentode

The 14W7 is a sharp-cutoff radio frequency (RF) pentode designed for voltage amplification in radio receiver circuits. It features a coated unipotential cathode, a lock-in eight-pin base, and is housed in a T-9 glass envelope. The tube is characterized by high transconductance, low grid-to-plate capacitance, and versatile screen-grid operating configurations that allow for either a sharp cut-off or an extended cut-off characteristic suitable for automatic volume control (AVC) or variable-bias gain control.

Physical Specifications and Basing

The 14W7 is constructed with a compact, mechanically robust lock-in design. Its physical dimensions and mounting requirements are as follows:

• Bulb Type: T-9
• Base: Lock-In 8-pin
• Maximum Diameter: 1 3/16 inches
• Maximum Overall Length: 2 25/32 inches
• Maximum Seated Height: 2 1/4 inches
• Mounting Position: Any

The tube utilizes the 8BJ (also designated 8BJ-L-5) basing arrangement. The pin connections, viewed from the bottom of the base, are:

Both Pin 4 and Pin 7 are internally connected to the cathode. Pin 5 connects the internal shield and Grid #4. Grids #2 and #3 are internally tied to Pin 3.

Heater Characteristics and Maximum Ratings

The 14W7 is designed to operate under standard mobile or receiver power systems, with heater specifications defined for both nominal and typical operating conditions:

• Nominal Heater Voltage (AC or DC): 14.0 V
• Nominal Heater Current: 0.240 A
• Typical Operating Heater Voltage: 12.6 V
• Typical Operating Heater Current: 0.225 A

To maintain device longevity and performance, the tube must be operated within the following maximum ratings:

• Maximum Plate Voltage: 300 V
• Maximum Screen Voltage: 150 V
• Maximum Plate and Screen Dissipation (Total): 4.0 W
• Maximum Screen Dissipation: 0.8 W

Typical Operating Conditions and Characteristics

The 14W7 can be operated under two distinct typical configurations, depending on whether a fixed screen supply or a series screen resistor is utilized. Both conditions operate with a plate voltage of 300 V, a suppressor voltage of 0 V, and a minimum cathode bias resistor of 160 ohms.

Operating Configuration Analysis

• Condition I (Fixed Screen Supply): This configuration utilizes a direct, fixed 150 V screen supply. Under this condition, the control grid voltage required to cut off cathode current is −6 V. The maximum allowable d-c resistance of the grid circuit in this configuration is 0.25 megohm.
• Condition II (Series Screen Resistor): This configuration uses a 300 V screen supply connected through a 40,000 ohm series screen resistor. This arrangement provides an extended cut-off characteristic, requiring −14 V on the control grid to achieve cathode current cut-off. Because of this extended cut-off, Condition II is recommended when stage gain is controlled by varying the grid bias. Additionally, when using a series screen resistor and full cathode bias, the maximum allowable d-c resistance in the grid circuit is increased to 0.5 megohm.

Screen Voltage Regulation Requirement

When a screen supply voltage exceeding 150 V is employed, a series screen resistor must be incorporated. This resistor limits the voltage at the screen grid to 150 V when the plate current is at its rated value of 10.0 mA.

Calculation: With a 300 V supply and a target screen voltage of 150 V, the voltage drop across the series resistor must be: 300 V - 150 V = 150 V At the rated screen current of 3.9 mA (0.0039 A), the required resistance is: R = 150 V / 0.0039 A = 38,462 ohms The specified standard value of 40,000 ohms closely matches this requirement, ensuring the screen voltage does not exceed its 150 V maximum rating under full-load conditions.

Direct Interelectrode Capacitances

The capacitances of the 14W7 are measured using an RMA tube shield (type MB-308) connected directly to the cathode. The typical values are:

• Grid to Plate Capacitance: 0.0025 pF (maximum)
• Input Capacitance (Grid #1 to Heater, Cathode, Grid #2, Grid #3, and Grid #4): 9.5 pF
• Output Capacitance (Plate to Heater, Cathode, Grid #2, Grid #3, and Grid #4): 7.0 pF

The extremely low grid-to-plate capacitance (0.0025 pF max) is a key characteristic of this sharp-cutoff pentode, minimizing Miller effect feedback and enabling stable high-frequency RF amplification.

Technical Summary

The 14W7 is a high-performance, lock-in base, sharp-cutoff RF pentode designed for stable voltage amplification in radio-frequency applications. Operating with a typical 12.6 V heater, it delivers a high transconductance of 5,800 µmhos and a plate resistance of 0.3 megohm at a plate voltage of 300 V. Its design allows for two distinct operating modes: a fixed-screen mode (Condition I) with a sharp −6 V cut-off, and a series-resistor screen mode (Condition II) providing an extended −14 V cut-off for effective variable-bias gain control. With its robust lock-in construction, low interelectrode capacitance (0.0025 pF max grid-to-plate), and flexible biasing options, the 14W7 is a highly capable pentode for RF receiver and amplifier designs.