18014 — Telephone Equipment Pentode

Overview

The 18014 is an indirectly heated pentode designed specifically for use in professional telephone equipment. It is engineered to perform reliably as either a pre-amplifier or an output valve in telecommunication systems. The tube features a robust construction with a metallized shield envelope, a 4 V heater, and a high mutual conductance of 8 mA/V, making it highly suitable for stable, low-distortion amplification.

Construction, Heater, and Physical Dimensions

The 18014 is constructed with an indirectly heated cathode designed for parallel supply using either alternating current (A.C.) or direct current (D.C.). The external envelope features a metallized shield coating (indicated by the connection $m$) to minimize external noise and electromagnetic interference, which is critical in sensitive telephone pre-amplifier stages.

The physical dimensions of the tube are as follows:

• Maximum Diameter: 46 mm
• Maximum Height (excluding pins): 126 mm
• Base Pin Length: Maximum 18 mm

Heater Characteristics

The heater is designed for low-voltage, high-current operation:

• Heater Voltage ($V_f$): 4 V
• Heater Current ($I_f$): 1.6 A
• Heating Method: Indirect (A.C. or D.C., parallel supply)

Pinout and Connections

The 18014 has been documented with slight variations in its pinout configurations and electrode terminations. In some configurations, the control grid ($g_1$) is terminated at a top cap, while in others, it is assigned to a base pin. The suppressor grid ($g_3$) is internally connected to the cathode ($k$).

Configuration A (Top Cap Control Grid)

In this layout, the control grid is located on the top cap of the envelope:

• Top Cap: Control Grid ($g_1$)
• Pin 2: Cathode and Suppressor Grid ($k, g_3$)
• Pin 3: Heater ($f$)
• Pin 4: Heater ($f$)
• Pin 5: Screen Grid ($g_2$)
• Pin 7: Metallization/Shield ($m$)
• Pin 8: Anode ($a$)
• Pin 10: Cathode and Suppressor Grid ($k, g_3$)

Configuration B (Base-Terminated Control Grid)

An alternative documentation layout lists the connections as follows:

• Pin 1: Control Grid ($g_1$)
• Pin 3: Heater ($f$)
• Pin 4: Heater ($f$)
• Pin 5: Screen Grid ($g_2$)
• Pin 6: Metallization/Shield ($m$)
• Pin 8: Anode ($a$)
• Pin 10: Cathode and Suppressor Grid ($k, g_3$)

Interelectrode Capacitances

The interelectrode capacitances of the 18014 reflect its design as a high-frequency telephone pentode, featuring low feedback capacitance to prevent instability:

• Anode Capacitance ($C_a$): 13 pF
• Control Grid Capacitance ($C_{g1}$): 15 pF
• Feedback Capacitance ($C_{ag1}$): < 0.6 pF
• Grid-to-Heater Capacitance ($C_{g1f}$): < 2 pF

Typical Electrical Characteristics

At a standard operating point, the 18014 exhibits the following typical characteristics:

• Anode Voltage ($V_a$): 200 V
• Screen Grid Voltage ($V_{[g2](/tubes/G2)}$): 200 V
• Cathode Bias Resistor ($R_k$): 125 $\Omega$
• Anode Current ($I_a$): 35 mA
• Screen Grid Current ($I_{[g2](/tubes/G2)}$): 4.6 mA
• Mutual Conductance ($S$): 8 mA/V
• Amplification Factor ($\mu_{g2g1}$): 20 (from screen grid to control grid)
• Internal Resistance ($R_i$): 50 k$\Omega$

Operating Conditions

As a Pre-Amplifier

When configured as a pre-amplifier, the tube operates under the typical characteristics with an anode load resistor ($R_a$) of 8 k$\Omega$.

• Anode Voltage ($V_a$): 200 V
• Screen Grid Voltage ($V_{[g2](/tubes/G2)}$): 200 V
• Cathode Resistor ($R_k$): 125 $\Omega$
• Anode Load Resistor ($R_a$): 8 k$\Omega$
• Anode Current ($I_a$): 35 mA
• Screen Grid Current ($I_{[g2](/tubes/G2)}$): 4.6 mA
• Mutual Conductance ($S$): 8 mA/V
• Internal Resistance ($R_i$): 50 k$\Omega$
• Voltage Gain ($V_o/V_i$): 4 Nepers (N)
• Maximum Control Grid Resistor ($R_{g1}$):
  • Automatic Bias: 1 M$\Omega$
  • Fixed Bias: 0.5 M$\Omega$

Calculation Note: The voltage gain is specified as 4 Nepers. Converting this unit to decibels yields a voltage gain of approximately 34.74 dB (calculated as $4 \text{ N} \times 8.686 \text{ dB/N}$).

As an Output Valve

When used as an output stage, the operating voltages and currents remain identical to the pre-amplifier configuration, but the grid resistor limits are tightened to prevent thermal runaway under power conditions, and the power output is specified:

• Anode Voltage ($V_a$): 200 V
• Screen Grid Voltage ($V_{[g2](/tubes/G2)}$): 200 V
• Cathode Resistor ($R_k$): 125 $\Omega$
• Anode Load Resistor ($R_a$): 8 k$\Omega$
• Anode Current ($I_a$): 35 mA
• Screen Grid Current ($I_{[g2](/tubes/G2)}$): 4.6 mA
• Mutual Conductance ($S$): 8 mA/V
• Internal Resistance ($R_i$): 50 k$\Omega$
• Output Power ($W_o$): 0.8 W
• Total Distortion ($d_{\text{tot}}$): < 3%
• Maximum Control Grid Resistor ($R_{g1}$):
  • Automatic Bias: 0.3 M$\Omega$
  • Fixed Bias: 0.15 M$\Omega$

Absolute Maximum Ratings

The absolute maximum ratings define the safe operating envelope of the 18014. Exceeding these limits may cause permanent damage or severely degrade the tube's lifespan.

Technical Summary

The 18014 is a specialized, high-reliability pentode designed for telephone equipment applications. Operating with a 4 V, 1.6 A heater, it provides a mutual conductance of 8 mA/V and an internal resistance of 50 k$\Omega$ at its typical 200 V operating point. It is highly versatile, serving as a pre-amplifier with a gain of 4 Nepers (~34.7 dB) or as an output valve delivering 0.8 W of power with less than 3% total distortion. With its metallized shield and low feedback capacitance (< 0.6 pF), the 18014 ensures stable and low-noise performance in demanding telecommunication circuits.