14H7 — High-Slope Semi-Remote-Cutoff RF Pentode
The 14H7 is a loctal-base, indirectly heated, semi-remote-cutoff (variable-mu) pentode designed for radio frequency (RF) and intermediate frequency (IF) amplification. It is particularly suited for use in AC-DC operated radio receivers employing series-connected heater strings. The tube features a unipotential cathode, triple-grid construction, and an internal shield.
Construction, Heater, and Pinout
The 14H7 is constructed in a T-9 glass envelope with a locking-in 8-pin (loctal) base. Its physical dimensions include a maximum diameter of 1-3/16 inches, a maximum seated height of 2-3/32 inches, and a maximum overall length of 2-5/8 inches.
Heater Specifications
The heater of the 14H7 is designed for AC or DC operation. The published specifications for the heater vary slightly across different documentation sources:
- Typical Operating Conditions: 12.6 V at 0.15 A
- Alternative Ratings: 12.6 V at 0.16 A
- Nominal Ratings: 14.0 V at 0.16 A
These variations reflect the tube's adaptability to 150 mA series-string heater circuits.
Pin Connections
The 14H7 utilizes the 8V loctal basing diagram. When viewed from the bottom of the tube, the pin connections are:
The center of the base features a metal locking key.
Electrical Characteristics and Operating Points
The 14H7 is characterized by high transconductance (high slope) and high plate resistance. Under Class A (or Class A1) amplifier service, typical operating conditions are documented for two distinct voltage levels:
Low-Voltage Operating Point
- Plate Voltage: 100 V
- Screen Voltage: 100 V
- Control Grid Voltage: -1 V (or established via a cathode bias resistor of 180 Ι in some configurations)
- Suppressor and Internal Shield Voltage: 0 V
- Plate Current: 7.5 mA to 8.2 mA
- Screen Current: 2.6 mA to 3.3 mA
- Plate Resistance (approximate): 0.25 MΙ to 0.35 MΙ
- Transconductance: 3,800 Ιmhos to 4,000 Ιmhos
- Control Grid Voltage for Cutoff (approximate, for Gm = 35 Ιmhos): -12 V
High-Voltage Operating Point
- Plate Voltage: 250 V
- Screen Voltage: 150 V
- Control Grid Voltage: -2.5 V
- Suppressor and Internal Shield Voltage: 0 V
- Plate Current: 9.5 mA to 10.0 mA
- Screen Current: 3.2 mA to 3.5 mA
- Plate Resistance (approximate): 0.8 MΙ (800,000 Ι)
- Transconductance: 3,800 Ιmhos to 4,000 Ιmhos
- Control Grid Voltage for Cutoff (approximate, for Gm = 35 Ιmhos): -19 V
The semi-remote-cutoff characteristic is demonstrated by the grid bias required to reduce transconductance to 35 Ιmhos, which increases from -12 V to -19 V as screen voltage is raised from 100 V to 150 V.
Maximum Ratings
The maximum ratings for the 14H7 are established to ensure reliable operation and prevent thermal or electrical breakdown:
Interelectrode Capacitances
The direct interelectrode capacitances of the 14H7 are measured with an external shield connected to the cathode:
- Input Capacitance (Control Grid to all other electrodes except plate): 8.0 ΙΙF (pF)
- Output Capacitance (Plate to all other electrodes except control grid): 7.0 ΙΙF (pF)
- Control Grid to Plate Capacitance: 0.004 ΙΙF to 0.007 ΙΙF maximum
The extremely low grid-to-plate capacitance is critical for preventing self-oscillation in high-gain RF and IF amplifier stages.
Related Designations
The 14H7 is closely related to the 7H7 vacuum tube. Manufacturers note that for further technical information, reference should be made to the 7H7, indicating that the 14H7 shares identical electrical characteristics and design features with the 7H7, with the exception of the heater configuration.
Technical Summary
The 14H7 is a loctal-base, high-slope, semi-remote-cutoff RF pentode designed for RF and IF amplification in AC-DC receivers. It features a unipotential cathode, a triple-grid structure with an internal shield, and a locking 8-pin base. Depending on the application, its heater is rated at 12.6 V / 0.15 A (or up to 14.0 V / 0.16 A). The tube delivers a high transconductance of 3,800 to 4,000 Ιmhos and a high plate resistance of 0.25 to 0.8 MΙ under typical Class A operating conditions. Its very low grid-to-plate capacitance of 0.004 to 0.007 pF ensures stable operation in high-frequency circuits.
