14A5 — Beam Power Amplifier
An indirectly heated lock-in beam power pentode designed for Class A audio power amplification in radio receivers and electronic equipment.
Overview
The 14A5 is a beam power amplifier (also classified as a beam pentode) featuring a lock-in construction. It is designed with a coated unipotential cathode and a heater rated for 12.6 V operation.
In vacuum tube design, corresponding 7-volt and 14-volt types often share identical electrical characteristics aside from their heater ratings. However, the 14A5 is a notable exception to this rule, possessing a distinct set of operating characteristics and ratings that do not directly mirror a corresponding 7-volt counterpart. The tube is primarily applied as a Class A1 audio power amplifier, capable of delivering moderate power output with low distortion.
Construction, Physical Dimensions, and Base Connection
The 14A5 is constructed using a T-9 glass bulb and an 8-pin lock-in base, which corresponds to the 6AA basing diagram. The lock-in design provides a secure mechanical connection and integrated shielding properties via the metal base shell and center guide pin. The tube is designed to operate reliably in any mounting position.
Physical Dimensions
- Maximum Overall Length: 2-25/32 inches
- Maximum Seated Height: 2-1/4 inches
- Maximum Bulb Diameter: 1-3/16 inches
Pinout Configuration
When viewed from the bottom of the lock-in base, the pin connections are assigned as follows:
Heater and Electrical Ratings
The heater of the 14A5 is designed to operate from either an AC or DC supply. The maximum ratings are established under the RMA Standard M8-210 system, though some variation exists in general reference tables regarding absolute limits.
Heater Specifications
- Heater Voltage: 12.6 V
- Heater Current: 150 mA (0.15 A)
- Heater Supply: AC or DC
Maximum Ratings
The absolute and design-maximum limits for the electrodes are defined as follows:
- Maximum Plate Voltage: 250 V to 300 V
- Maximum Grid #2 (Screen) Voltage: 250 V to 300 V
- Maximum Heater-Cathode Voltage: 90 V
- Maximum Plate Dissipation: 7.5 W
- Maximum Grid #2 Dissipation: 1.5 W
- Maximum Grid #1 Circuit Resistance (Fixed Bias): 1.0 MΩ
- Maximum Grid #1 Circuit Resistance (Self Bias): 0.5 MΩ
The variation in maximum plate and screen voltages (ranging from 250 V in general tables to 300 V under specific RMA standard interpretations) highlights the importance of operating the tube within conservative design limits to ensure longevity.
Typical Operating Conditions and Characteristics
The 14A5 is optimized for Class A₁ audio amplification. Under typical operating conditions with a 250 V supply on both the plate and screen grid, the tube exhibits the following electrical characteristics:
Operating Analysis
When biased at -12.5 V, the control grid (Grid #1) can accept a peak audio-frequency signal voltage of 12.5 V without driving the grid positive, preventing the onset of grid current. Under zero-signal conditions, the plate draws 30 mA and the screen grid draws 3.5 mA.
When driven to full output by the maximum peak signal, the plate current rises slightly to 32 mA, and the screen grid current increases to 5.5 mA. The plate resistance is approximately 70,000 Ω, which, combined with a transconductance of 3,000 µmhos, allows the tube to deliver 2.8 W of audio power into a 7,500 Ω load. At this maximum output level, the total harmonic distortion is limited to a moderate 7 percent.
Technical Summary
The 14A5 is an 8-pin lock-in beam power amplifier featuring a 12.6 V, 150 mA heater and a coated unipotential cathode. It is designed for Class A₁ audio power output service, where it can deliver 2.8 W of output power into a 7,500 Ω load with 7% total harmonic distortion when operated at a plate and screen potential of 250 V.
Its lock-in construction (basing 6AA) provides robust mechanical stability and integrated shielding, making it suitable for compact receiver designs. Because the 14A5 is an exception to the typical parameter matching between corresponding 7-volt and 14-volt series, designers must rely strictly on its unique ratings and operating points rather than assuming equivalence with related designations.
