18004 — Low-Mu Power Triode

The 18004 (also known under the Philips "Miniwatt" brand) is a directly heated power triode designed specifically for use as an output valve in telephone equipment. It is characterized by a low amplification factor, low internal resistance, and robust construction suitable for telecommunications applications.

Mechanical and Physical Specifications

The 18004 is constructed with a glass bulb envelope and has the following physical dimensions:

• Maximum Diameter: 48 mm
• Maximum Height: 118 mm (excluding pins)
• Maximum Pin Length: 9.5 mm

The tube features a standard 4-pin base. The pinout configuration is designated as follows:

• Filament ($f$, $f$)
• Anode ($a$)
• Grid ($g$)

Filament and Heating

The 18004 utilizes a directly heated filament designed for parallel DC supply operation.

• Filament Voltage ($V_f$): 4.4 V
• Filament Current ($I_f$): 0.97 A
• Heating Type: Direct by D.C., parallel supply

The calculated power consumption of the filament is approximately 4.27 W (derived from 4.4 V multiplied by 0.97 A).

Interelectrode Capacitances

The internal capacitances between the electrodes of the 18004 are:

• Anode Capacity ($C_a$): 2.5 pF
• Grid Capacity ($C_g$): 2.9 pF
• Anode-to-Grid Capacity ($C_{ag}$): 4.5 pF

Typical Electrical Characteristics

At the standard test operating point, the 18004 exhibits the following electrical characteristics:

• Anode Voltage ($V_a$): 130 V
• Grid Voltage ($V_g$): -25 V
• Anode Current ($I_a$): 22 mA
• Mutual Conductance ($S$): 1.0 mA/V
• Amplification Factor ($\mu$): 2.3
• Internal Resistance ($R_i$): 2.3 k̤

The relationship between these parameters aligns with the standard triode formula, where the amplification factor ($\mu$) is approximately equal to the product of the mutual conductance ($S$) and the internal resistance ($R_i$): $1.0\text{ mA/V} \times 2.3\text{ k}\Omega = 2.3$

Operating Conditions as an Output Valve

When operated as a class-A output stage in telephone equipment, the recommended operating conditions are:

• Anode Voltage ($V_a$): 130 V
• Grid Voltage ($V_g$): -25 V
• Anode Load Resistance ($R_a$): 2.1 k̤
• Anode Current ($I_a$): 22 mA
• Output Power ($W_o$): 0.2 W
• Total Distortion ($d_{\text{tot}}$): Less than 5%

Limiting Values (Absolute Maxima)

To ensure reliable operation and prevent damage, the following absolute maximum ratings must be observed:

• Cold Anode Voltage ($V_{a0}$): Max. 300 V
• Anode Voltage ($V_a$): Max. 150 V
• Anode Dissipation ($W_a$): Max. 3.5 W
• Cathode Current ($I_k$): Max. 35 mA
• Grid Current Start Point ($V_g$ at $I_g = +0.3\ \mu\text{A}$): Max. -1.3 V
• Grid Resistance ($R_g$): Max. 1 M̤

Analysis of Characteristic Curves

The electrical behavior of the 18004 is further described by its characteristic curves:

• Anode Characteristics ($I_a$ vs. $V_a$): The family of curves plotted for grid voltages ($V_g$) from 0 V down to -60 V in 10 V steps shows typical triode behavior up to an anode current of 60 mA and an anode voltage of 200 V.
• Transfer Characteristics ($I_a$ vs. $V_{g1}$): Plotted at a filament voltage of 4.4 V and an anode voltage of 130 V, the transfer curve shows a smooth, quadratic-like rise. The curve indicates a cutoff region between -70 V and -80 V, rising to approximately 50 mA as the grid voltage approaches 0 V.

Technical Summary

The 18004 is a specialized, directly heated low-mu power triode designed for telephone equipment output stages. Operating with a 4.4 V, 0.97 A filament, it delivers 0.2 W of output power with less than 5% total distortion under typical operating conditions (130 V anode, -25 V grid bias). With an internal resistance of 2.3 k̤ and a maximum anode dissipation of 3.5 W, the 18004 provides robust and stable performance in low-frequency telecommunications applications.