13P1P — Low-Voltage Output Pentode

An indirectly heated, low-voltage output pentode designed for low-frequency power amplification in compact or battery-operated equipment.

Overview

The 13P1P is a specialized power output pentode designed for low-frequency (audio) power amplification. It is characterized by its ability to operate at exceptionally low anode and screen grid voltages, with a nominal operating point of only 26 V. This makes the tube highly suitable for low-voltage applications where conventional high-voltage power supplies are impractical or undesirable.

The tube features an indirectly heated oxide cathode, a high transconductance of 7.5 mA/V, and a relatively low internal plate resistance of 1500 Ω. It is capable of delivering a minimum output power of 220 mW under nominal operating conditions.

Construction, Heater, and Pin Connections

The 13P1P is constructed with an indirectly heated oxide cathode (旁热式氧化物阴极). The heater is designed to be powered by either an AC or DC source.

Heater Characteristics and Limits

Heater Power Calculation: $\text{Power} = \text{Voltage} \times \text{Current} = 13\text{ V} \times 0.75\text{ A} = 9.75\text{ W}$

Physical Dimensions

The tube is housed in a glass envelope with the following physical limits:

• Maximum Overall Length: 90 mm
• Maximum Seated Height: 76 mm
• Maximum Bulb Diameter: 34 mm

Pinout and Electrode Connections

The tube utilizes an eight-pin base configuration. The pin connections, viewed from the underside of the base, are assigned as follows:

The suppressor grid (Grid 3) is internally tied to the plate (anode) at Pin 3, simplifying external circuit wiring.

Electrical Characteristics and Typical Operating Point

The 13P1P is optimized for low-voltage operation. Its typical Class A operating parameters are defined at a nominal supply voltage of 26 V:

Interelectrode Capacitances

The internal capacitances of the tube electrodes are specified as follows:

• Input Capacitance ($C_{in}$): 15.5 ± 2.5 pF
• Output Capacitance ($C_{out}$): 10.5 ± 2.5 pF
• Feedback / Transfer Capacitance ($C_{gp}$): ≤ 2.5 pF

Maximum Ratings

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

Performance Curves and Circuit Design

The electrical behavior of the 13P1P is further illustrated by its characteristic curves, which assist in selecting appropriate load lines and bias points.

Plate Characteristics ($I_a$ vs. $U_a$)

The plate family curves show the relationship between plate current ($I_a$) and plate voltage ($U_a$) for various control grid voltages ($U_{g1}$) ranging from $+5\text{ V}$ down to $-14\text{ V}$, with the screen grid voltage ($U_{[g2](/tubes/G2)}$) fixed at 26 V.

• At the nominal bias of $U_{g1} = 0\text{ V}$ and $U_a = 26\text{ V}$, the plate current is approximately 42 mA.
• Driving the control grid positive (up to $+5\text{ V}$) increases the plate current significantly, exceeding 80 mA, though this operating region introduces grid current.
• Applying negative bias reduces the plate current, with cutoff occurring near $-14\text{ V}$ under low plate voltage conditions.

An additional set of curves details the plate current at $U_{g1} = 0\text{ V}$ for different screen grid voltages ($U_{[g2](/tubes/G2)} = 16\text{ V}, 26\text{ V}, 36\text{ V}, 46\text{ V}$). This highlights that the screen grid voltage exerts strong control over the overall current capacity of the tube, with higher screen voltages shifting the entire plate current curve upward.

Transfer Characteristics ($I_a, I_{g2}$ vs. $U_{g1}$)

The transfer curves plot plate current ($I_a$) and screen grid current ($I_{[g2](/tubes/G2)}$) against control grid voltage ($U_{g1}$) at a constant plate voltage of 26 V, for screen voltages of 16 V, 26 V, 36 V, and 46 V.

• The slope of these curves represents the transconductance ($g_m$). At higher screen voltages (such as 46 V), the transconductance is higher, and the plate current rises more steeply as $U_{g1}$ approaches 0 V.
• The screen grid current ($I_{[g2](/tubes/G2)}$) remains a small fraction of the plate current across the operating range, typically staying well below the 4 mA limit at the nominal 26 V operating point.

Output Power and Distortion vs. Load Resistance

The relationship between output power ($P_{sc}$), distortion ($K_f$), and load resistance ($R_a$) is documented at $U_f = 13\text{ V}$, $U_a = 26\text{ V}$, and $U_{[g2](/tubes/G2)} = 26\text{ V}$:

• Output Power ($P_{sc}$): The output power rises as the load resistance increases from 200 Ω, reaching a peak of approximately 300 mW at a load resistance ($R_a$) of around 900 to 1000 Ω.
• Distortion ($K_f$): The distortion factor reaches a minimum of approximately 6% at a load resistance of 600 Ω, where the output power is about 260 mW. As the load resistance increases beyond 600 Ω, the distortion rises, reaching approximately 12% at 800 Ω and continuing to increase to around 20% as the load approaches 1200 Ω.
• Design Trade-off: For optimum performance, a designer must balance output power against distortion. A load resistance of 600 Ω provides the lowest distortion (6%) with a slight reduction in output power (260 mW), whereas a load of 900 Ω maximizes power output (300 mW) at the expense of higher distortion (around 16%).

Technical Summary

The 13P1P is an indirectly heated, low-voltage output pentode designed for low-frequency power amplification. Featuring a 13 V heater and optimized for operation at a nominal plate and screen voltage of 26 V, it provides a unique solution for low-voltage tube circuitry. With a transconductance of $7.5\text{ mA/V}$, an internal resistance of 1500 Ω, and a maximum plate dissipation of 6 W, the 13P1P can deliver over 220 mW of audio output power into a typical load of 600 to 1000 Ω. Its pinout integrates the suppressor grid internally with the plate on Pin 3, simplifying circuit layout.