14C5 — Beam Power Amplifier Vacuum Tube

An indirectly heated beam power amplifier with a lock-in base, designed for high power sensitivity and high power output in the output stage of receivers.

Overview

The 14C5 is a beam power amplifier designed for service in the output stage of AC and storage battery operated receivers. It is engineered to deliver high power sensitivity and high power output while operating at comparatively low supply voltages. The tube features a coated unipotential cathode and is housed in a T-9 glass bulb with an 8-pin lock-in base.

The electrical characteristics and ratings of the 14C5 are identical to those of the 7C5, with the sole distinction being their heater requirements. While the 7C5 operates with a nominal heater supply of 6.3 V at 0.45 A, the 14C5 is designed for a 12.6 V heater supply at 0.225 A. Both configurations consume the same nominal heater power of approximately 2.835 W (calculated as 12.6 V multiplied by 0.225 A, and 6.3 V multiplied by 0.45 A, respectively).

Construction, Heater, and Base Pinout

The 14C5 utilizes a coated unipotential cathode and a T-9 glass bulb structure. It is designed to be mounted in any position. The physical dimensions of the tube are as follows:

The tube uses a 6AA base connection diagram with an 8-pin lock-in base. The pin assignments, viewed from the bottom of the base, are:

The heater is designed for operation on either AC or DC. The heater specifications for the 14C5 and its sister type, the 7C5, are:

Maximum Ratings

The maximum ratings for the 14C5 are established according to the RMA Standard M8-210 convention. These limits represent the absolute boundaries within which the tube must be operated to ensure reliable performance and longevity:

Typical Operating Conditions

The 14C5 can be operated as a single-tube Class A1 amplifier or in a push-pull Class AB1 configuration.

Class A1 Amplifier (Single Tube)

Three typical operating points are documented for single-tube Class A1 service, demonstrating the tube's performance across different plate and screen voltage combinations:

Class AB1 Push-Pull Amplifier (Two Tubes)

For higher power output, two tubes can be operated in a Class AB1 push-pull configuration. The values below represent the combined requirements and performance for two tubes:

Grid #1 Circuit Resistance Limits

To prevent thermal runaway and maintain stable bias, the maximum resistance in the control grid (Grid #1) circuit must be strictly limited based on the biasing method:

• Fixed Bias: 0.1 MΩ maximum
• Cathode Bias: 0.5 MΩ maximum

Performance Curves and Analysis

The operational behavior of the 14C5 is characterized by several key performance curves:

Plate Characteristics

The plate current ($I_b$) versus plate voltage ($E_b$) curves, plotted at a constant screen grid voltage ($E_{c2}$) of 250 V, illustrate the classic knee behavior of a beam power tube. At low plate voltages, the plate current rises rapidly before flattening out into a relatively flat, high-impedance region. The curves are plotted for control grid voltages ($E_{[c1](/tubes/C1)}$) ranging from 0 V down to −30 V. The sharp knee minimizes the region of low plate voltage where distortion increases, enabling high power output and high efficiency.

Power Output and Distortion vs. Input Signal

Under typical Class A1 conditions ($E_b = 250$ V, $E_{c2} = 250$ V, $E_{[c1](/tubes/C1)} = -12.5$ V, and a load resistance $R_L = 5000$ Ω), the relationship between the input signal voltage, power output, and harmonic distortion reveals the following:

• Power Output ($P_o$): Increases progressively with the RMS signal voltage ($E_{sig}$), reaching approximately 4.5 W at the full drive of 8.8 V RMS (which corresponds to the 12.5 V peak grid bias).
• Harmonic Distortion: The second harmonic distortion rises gradually and peaks around 4.5% before slightly declining at maximum output. The third harmonic distortion remains very low at low power levels but rises more sharply as the tube approaches full output, crossing the second harmonic curve near the maximum power output point.
• Currents: The plate current ($I_b$) rises slightly from its zero-signal value of 45 mA to about 47 mA at maximum signal, while the screen grid current ($I_{c2}$) increases from 4.5 mA to 7 mA.

Load Resistance Effects

When operating at the same 250 V bias point with a constant input signal of 8.8 V RMS, varying the load resistance ($R_L$) from 1,000 Ω to 12,000 Ω shows distinct performance trade-offs:

• Power Output ($P_o$): Peaks at a broad maximum of approximately 4.5 W near the rated load of 5,000 Ω.
• Distortion Profile: Total harmonic distortion is highly dependent on the load resistance. The second harmonic distortion decreases steadily as the load resistance increases, passing through a minimum near 6,500 Ω. Conversely, the third harmonic distortion is low at low load resistances but rises steeply as the load exceeds 5,000 Ω. The rated load of 5,000 Ω represents an optimal compromise where the total harmonic distortion is kept to approximately 8% while maximizing power output.
• Currents: As the load resistance increases, the average plate current ($I_b$) decreases from over 50 mA down to approximately 35 mA, while the screen grid current ($I_{c2}$) rises from about 3 mA to over 10 mA. This highlights the importance of maintaining the proper load impedance to prevent excessive screen grid dissipation.

Technical Summary

The 14C5 is a high-sensitivity beam power amplifier featuring an 8-pin lock-in base and a 12.6 V, 0.225 A heater. It is electrically identical to the 6.3 V 7C5, sharing the same maximum ratings of 315 V plate voltage, 285 V screen voltage, and 12 W plate dissipation.

In single-tube Class A1 service, the 14C5 can deliver up to 5.5 W of audio power, while a push-pull pair in Class AB1 can produce up to 14 W of output with low distortion. Its high power sensitivity and robust performance at moderate operating voltages make it an exceptionally efficient choice for the audio output stages of radio receivers and similar amplification applications.