18040 — Special Quality Pentode

The 18040 (also designated as the SQ 18040) is a Special Quality (SQ) pentode designed primarily for high-reliability telephone equipment. Engineered for long life and stable performance, the tube is highly versatile, serving as either a voltage pre-amplifier or a power output valve in telecommunication systems. It features an indirectly heated cathode, a loctal base, and robust electrical characteristics tailored for demanding professional applications.

Mechanical Design and Pinout

The 18040 is constructed with a glass envelope and a standard eight-pin loctal base. The physical dimensions of the tube are tightly controlled to ensure compatibility and mechanical stability:

• Maximum overall height: 85 mm
• Maximum envelope height (excluding pins/base sleeve): 70 mm
• Maximum diameter: 33 mm

The loctal base configuration provides secure retention and reliable electrical contact. The pin connections, viewed from the bottom of the base, are assigned as follows:

Heater and Power Supply Requirements

The 18040 utilizes an indirectly heated, coated unipotential cathode. The heater is designed to operate from either an AC or DC supply and can be integrated into series or parallel heater strings. The nominal heater ratings and strict operational tolerances are:

• Nominal heater voltage ($V_f$): 18 V
• Nominal heater current ($I_f$): 0.2 A
• Maximum current deviation: $\pm 0.01$ A at $V_f = 18$ V

To maintain the "Special Quality" reliability standards, the power supply must adhere to the following absolute limits:

• Parallel Supply: The maximum permissible variation of the heater voltage ($V_f$) is restricted to $\pm 5\%$.
• Series Supply: The maximum permissible deviation of the heater current ($I_f$) due to supply voltage fluctuations and component tolerances is restricted to $\pm 1.5\%$.

Electrical Characteristics and Capacitances

Under typical operating conditions, the 18040 exhibits high transconductance and moderate internal plate resistance. The standard test conditions and typical parameters are defined below:

Typical Operating Conditions

• Anode voltage ($V_a$): 210 V
• Suppressor grid voltage ($V_{g3}$): 0 V
• Screen grid voltage ($V_{[g2](/tubes/G2)}$): 210 V
• Cathode bias resistor ($R_k$): $120\ \Omega$

Typical Parameters

• Anode current ($I_a$): $20 \pm 3.5$ mA
• Screen grid current ($I_{[g2](/tubes/G2)}$): $5.3 \pm 1.5$ mA
• Mutual conductance ($S$): $11 \pm 2.0$ mA/V
• Amplification factor (screen to control grid, $\mu_{g2g1}$): 37
• Internal resistance ($R_i$): 0.3 M$\Omega$ (minimum 0.2 M$\Omega$)
• Equivalent noise resistance ($R_{eq}$):
  • High Frequency (H.F.): Maximum 1.2 k$\Omega$
  • Audio/Low Frequency (A.F. / B.F. / N.F.): Maximum 12 k$\Omega$

Interelectrode Capacitances

The tube's internal shielding minimizes parasitic capacitances, which is critical for high-frequency and pre-amplifier stability. The capacitances are measured as:

• Anode capacitance ($C_a$): $7.5 \pm 1.0$ pF
• Control grid capacitance ($C_{g1}$): $14.5 \pm 1.0$ pF
• Anode to control grid capacitance ($C_{ag1}$): $< 0.02$ pF
• Control grid to heater capacitance ($C_{g1f}$): $< 0.02$ pF
• Cathode to heater capacitance ($C_{kf}$): $4.5 \pm 1.5$ pF
• Control grid capacitance in operating state (measured at $I_k = 25.3$ mA): 18.5 pF

Circuit-Specific Operating Examples

The 18040 is optimized for two distinct operating modes within telephone and line transmission equipment: as a voltage pre-amplifier and as a power output stage.

Pre-Amplifier Service

When configured as a pre-amplifier, the tube is biased for lower current consumption and higher input impedance:

• Anode voltage ($V_a$): 210 V
• Suppressor grid voltage ($V_{g3}$): 0 V
• Screen grid voltage ($V_{[g2](/tubes/G2)}$): 210 V
• Cathode resistor ($R_k$): $185\ \Omega$
• Anode load resistor ($R_a$): 20 k$\Omega$
• Anode current ($I_a$): 15 mA
• Screen grid current ($I_{[g2](/tubes/G2)}$): 4 mA
• Mutual conductance ($S$): 10 mA/V
• Internal resistance ($R_i$): 0.4 M$\Omega$
• Voltage gain ($g$): 5.15 N (Nepers)

Output Valve Service

In output stage configurations, the tube is biased to maximize power transfer into the load while maintaining acceptable distortion levels:

• Anode voltage ($V_a$): 210 V
• Suppressor grid voltage ($V_{g3}$): 0 V
• Screen grid voltage ($V_{[g2](/tubes/G2)}$): 210 V
• Cathode resistor ($R_k$): $120\ \Omega$
• Anode load resistor ($R_a$): 15 k$\Omega$
• Anode current ($I_a$): 20 mA
• Screen grid current ($I_{[g2](/tubes/G2)}$): 5.3 mA
• Mutual conductance ($S$): 11 mA/V
• Internal resistance ($R_i$): 0.3 M$\Omega$
• Output power ($W_o$): 1.0 W
• Total distortion ($d_{tot}$): 5%

Absolute Maximum Ratings

To ensure the long-term reliability and survival of the device, the following absolute maximum limits must not be exceeded under any operating conditions:

Maximum Control Grid Resistor ($R_{g1}$) Limits

The maximum allowable resistance in the control grid circuit depends on the application and the biasing method:

• Pre-amplifier, automatic bias: 1 M$\Omega$
• Pre-amplifier, fixed bias: 0.5 M$\Omega$
• Output tube, automatic bias: 0.3 M$\Omega$
• Output tube, fixed bias: 0.15 M$\Omega$

Dynamic Behavior and Curve Interpretation

Analysis of the tube's characteristic curves reveals several key aspects of its dynamic performance:

• Transfer Characteristics: At a constant anode and screen grid voltage of 210 V, the mutual conductance ($S$) rises almost linearly from near zero at a control grid bias of $-7$ V up to 15 mA/V as the bias approaches 0 V. Concurrently, the anode current ($I_a$) curves upward sharply, reaching approximately 55 to 60 mA at $V_{g1} = 0$ V.
• Anode Characteristics: The family of $I_a$ versus $V_a$ curves displays classic pentode knee behavior, with the knee occurring below $V_a = 50$ V. The safe operating region at higher anode voltages is strictly bounded by the $W_a = 4.5$ W anode dissipation curve.
• Distortion and Power Output: In output configuration ($R_a = 15$ k$\Omega$, $R_k = 120\ \Omega$), total harmonic distortion rises progressively with output power. The distortion is 5% at 1.0 W of output power, reaching approximately 10% at 1.7 W, and rising to 20% to 22% near the maximum output power of 2.2 W.
• Output Power vs. Load Resistance: The output power peaks for various distortion limits (1%, 2.5%, 5%, and 10%) across a range of load resistances from 0.1 k$\Omega$ to 100 k$\Omega$. The optimal load resistance for maximizing power output while minimizing distortion lies in the range of 10 k$\Omega$ to 15 k$\Omega$.
• Input Capacitance Variation: The change in input capacitance ($\Delta C_i$) is highly non-linear with respect to control grid bias. It decreases from approximately 5.2 pF at $V_{g1} = -2$ V down to a baseline of 0.5 to 0.7 pF at $V_{g1} = -8$ V.

End of Life Criteria

For maintenance and performance monitoring in telephone systems, the 18040 is considered to have reached its end of life when its parameters degrade below the following thresholds:

• Anode current ($I_a$): $< 13$ mA (under typical operating conditions)
• Screen grid current ($I_{[g2](/tubes/G2)}$): $< 3$ mA
• Mutual conductance ($S$): $< 7.5$ mA/V

Technical Summary

The 18040 is a high-reliability, Special Quality loctal pentode designed for telephone pre-amplifier and output stage applications. Operating with an 18 V, 0.2 A heater, it provides a mutual conductance of 11 mA/V, an amplification factor ($\mu_{g2g1}$) of 37, and an anode dissipation rating of 4.5 W. Its robust mechanical construction, strict heater tolerances, and well-defined end-of-life criteria make it an exemplary choice for professional telecommunications equipment requiring long-term operational stability.