1B2P — Diode-Pentode Vacuum Tube

A miniature, directly heated glass diode-pentode designed for low-frequency voltage pre-amplification and signal detection in compact or battery-operated equipment.

Overview

The 1B2P (also designated in Cyrillic as 1Б2П) is a specialized miniature glass diode-pentode. It combines a single diode detector and a high-gain pentode amplifier section within a single envelope, sharing a common directly heated filament. The tube is primarily designed for simultaneous signal detection and low-frequency voltage pre-amplification.

The 1B2P is constructed in a miniature glass envelope (type 2П) with a mass of approximately 10 grams. It is designed to plug into a standard RSH4 socket. The tube is electrically equivalent to the 1AF34 designation.

Construction, Filament, and Pinout

The 1B2P features a directly heated, oxide-coated cathode (filament) designed for low-power battery operation. The filament is highly efficient, requiring a nominal voltage of only 1.2 V. The physical layout of the tube utilizes a 6-pin arrangement on a miniature glass base (type 2П).

The pin connections, viewed from the bottom of the tube, are assigned as follows:

The internal connection of the suppressor grid (Grid 3) to the negative side of the filament (Pin 1) simplifies external circuitry and ensures proper electron suppression within the pentode section.

Electrical Characteristics and Typical Operation

The 1B2P is characterized by its extremely low power consumption, making it ideal for portable applications. Under typical operating conditions, the tube exhibits the following electrical parameters:

Filament Ratings

• Nominal Filament Voltage ($U_н$): 1.2 V
• Filament Current ($I_н$): 30 mA (with an allowable range of 27 mA to 33 mA)

Typical Operating Point

The standard operating parameters for evaluating the tube's performance are established with a filament voltage of 1.2 V, a pentode anode voltage of 60 V, a screen grid (Grid 2) voltage of 45 V, and the control grid (Grid 1) at 0 V bias. The diode anode is tested at a reference voltage of 1.2 V.

Under reduced filament voltage conditions ($U_н = 0.95\text{ V}$), the pentode transconductance remains at or above $0.25\text{ mA/V}$.

Lifetime and Reliability

The 1B2P is rated for a minimum operating lifetime of 1500 hours. The criteria for determining the end of the tube's useful life are defined by the following performance thresholds:

• Diode Anode Current: $\ge 3\text{ \mu A}$
• Pentode Transconductance: $\ge 0.25\text{ mA/V}$

Interelectrode Capacitances

Due to its miniature construction, the 1B2P maintains low internal capacitances, which helps prevent unwanted feedback and high-frequency roll-off in pre-amplifier stages:

• Input Pentode Capacitance ($C_{вх}$): 1.85 pF
• Output Pentode Capacitance ($C_{вых}$): 2.1 pF
• Transfer Pentode Capacitance ($C_{прох}$): 0.27 pF
• Diode Anode to Cathode Capacitance ($C_{а.д - к}$): 0.3 pF

Maximum Operating Ratings

To ensure reliable operation and prevent premature failure, circuit designs must adhere to the absolute maximum ratings. Operating the tube beyond these limits can cause permanent damage.

The high cold-start voltage ratings (250 V) for the anode and screen grid allow the tube to safely withstand initial power-supply surges before the filament reaches operating temperature and begins drawing current.

Analysis of Characteristic Curves

The behavior of the 1B2P under varying voltage conditions is described by its characteristic curves:

• Diode Characteristic ($I_{а.д}$ vs $U_{а.д}$): With a nominal filament voltage of 1.2 V, the diode section exhibits a non-linear current rise starting from 0 V. As the diode anode voltage is increased, the current rises smoothly, reaching approximately 0.9 mA at an anode potential of 12 V.
• Pentode Transfer Characteristic ($I_{а.п}$ vs $U_{с1}$): At $U_н = 1.2\text{ V}$, $U_{а.п} = 60\text{ V}$, and $U_{с2} = 45\text{ V}$, the control grid cutoff voltage is located between -2.5 V and -3 V. As the grid bias becomes less negative, the anode current rises to its nominal value of 0.9 mA at $U_{с1} = 0\text{ V}$, and continues to rise slightly above 1.0 mA if a small positive bias of +0.5 V is applied.
• Pentode Plate Characteristics ($I_{а.п}$ vs $U_{а.п}$): Under standard operating conditions ($U_н = 1.2\text{ V}$, $U_{с2} = 45\text{ V}$), the plate curves display a classic pentode knee between 10 V and 15 V. Above this knee, the curves are highly flat and horizontal; at $U_{с1} = 0\text{ V}$, the anode current remains virtually constant at approximately 0.9 mA across a wide plate voltage range from 20 V to 80 V, demonstrating high dynamic plate resistance.

Technical Summary

The 1B2P is a highly efficient, directly heated diode-pentode designed for low-power detection and pre-amplification. Operating on a nominal 1.2 V, 30 mA filament, it integrates a detector diode and a high-impedance pentode ($R_i = 1\text{ M\Omega}$) with a transconductance of 0.55 mA/V into a single 10-gram miniature glass package. With its low power requirements, robust cold-start ratings, and reliable 1500-hour service life, the 1B2P serves as a compact solution for low-frequency signal processing in portable vacuum tube electronics.