12BW4 — Miniature Full-Wave Vacuum Rectifier

A miniature, cathode-type full-wave rectifier tube designed for power supply applications requiring relatively high output current capabilities.

Overview

The 12BW4 is a miniature double-diode vacuum tube designed for full-wave rectifier service. It features a coated unipotential cathode and is housed in a T-6 1/2 glass envelope with a 9-pin miniature button base. The tube is engineered to deliver high output currents in compact electronic equipment. Except for its heater ratings, the 12BW4 is electrically identical to the 6BW4.

Construction, Physical Dimensions, and Pinout

The 12BW4 is constructed with a coated unipotential cathode, which allows for a high heater-to-cathode voltage rating and isolates the heater circuit from the high-voltage DC output. The tube is enclosed in a standard T-6 1/2 glass bulb and utilizes a 9-pin miniature button base (designated 9DJ). It is designed to operate reliably in any mounting position.

Physical Dimensions

• Maximum overall length: 2 5/8 inches
• Maximum seated height: 2 3/8 inches
• Maximum diameter: 7/8 inch

Pin Connections (9DJ Base)

Viewed from the underside of the base, the 9-pin connection layout is as follows:

• Pin 1: Plate 2 ($P_2$)
• Pin 2: No Connection (NC)
• Pin 3: No Connection (NC)
• Pin 4: Heater (H)
• Pin 5: Heater (H)
• Pin 6: No Connection (NC)
• Pin 7: Plate 1 ($P_1$)
• Pin 8: No Connection (NC)
• Pin 9: Cathode (K)

Heater Characteristics

The heater is designed for operation from either AC or DC power supplies.

• Nominal heater voltage: 12.6 Volts
• Nominal heater current: 450 mA (0.45 A)
• Heating method: Indirectly heated, coated unipotential cathode

Heater Power Calculation: Using the nominal heater voltage ($V_h = 12.6\text{ V}$) and heater current ($I_h = 0.45\text{ A}$), the nominal heater power consumption is calculated as: $\text{Heater Power} = V_h \times I_h = 12.6\text{ V} \times 0.45\text{ A} = 5.67\text{ Watts}$

Maximum Ratings

The maximum ratings for the 12BW4 are established under the Design Center System for rectifier service. These limits must not be exceeded under the worst-case operating conditions:

Typical Operating Conditions

The 12BW4 can be configured with either a capacitor-input filter or a choke-input filter. Typical operating parameters for full-wave rectifier service are detailed below:

*Note: AC plate voltage is measured under no-load conditions.

Performance and Rating Charts Analysis

The safe operating boundaries of the 12BW4 are defined by three distinct rating charts, which govern the relationships between plate supply voltage, output current, rectification efficiency, and circuit resistance.

Rating Chart I: DC Output Current vs. AC Plate Supply Voltage

This chart defines the maximum allowable DC output current per plate as a function of the unloaded AC plate supply voltage (RMS) per plate:

• Choke Input Filter: Under choke-input conditions, the maximum operating boundary allows a constant DC output current of 62.5 mA per plate from 0 V up to approximately 320 V RMS. Above this voltage, the maximum current limit decreases linearly, reaching 50 mA per plate at the maximum rated AC plate supply voltage of 450 V RMS.
• Capacitor Input Filter: Under capacitor-input conditions, the maximum operating boundary allows 62.5 mA per plate up to approximately 60 V RMS. Beyond this point, the maximum allowable DC output current decreases steadily as the AC plate supply voltage increases, dropping to approximately 30 mA per plate at 450 V RMS.

Rating Chart II: Rectification Efficiency and Steady-State Limits

For capacitor-input filters, the maximum DC output current is also limited by the steady-state peak plate current rating of 350 mA per plate. This relationship is plotted against the rectification efficiency, which is calculated as: $\text{Rectification Efficiency} = \frac{\text{DC Output Voltage at Filter Input}}{\text{Peak AC Input Voltage Each Plate (Without Load)}}$

The permissible operating region allows a maximum DC output current of 62.5 mA per plate for rectification efficiencies up to approximately 0.55. For efficiencies higher than 0.55, the maximum allowable DC output current decreases rapidly, reaching 0 mA as the rectification efficiency approaches 1.00.

Rating Chart III: Minimum Effective Plate Supply Resistance

To prevent damage from transient surge currents during startup, a minimum effective plate supply resistance must be present in capacitor-input circuits. This chart defines the minimum resistance required per plate to limit the transient peak plate current to 2.0 Amperes:

• For unloaded AC plate supply voltages below approximately 215 V RMS, no additional external plate supply resistance is required (0 $\Omega$).
• For voltages above 215 V RMS, the minimum required effective plate supply resistance increases linearly with the AC plate supply voltage, reaching approximately 160 $\Omega$ at the maximum rated AC plate supply voltage of 450 V RMS.

Technical Summary

The 12BW4 is a miniature 9-pin full-wave vacuum rectifier featuring relatively high output current capabilities. Operating with a 12.6 V, 450 mA heater, it is identical to the 6BW4 except for its heater ratings. With a maximum peak inverse plate voltage of 1275 V and a maximum heater-cathode voltage of 450 V (heater negative, DC), it is highly versatile for power supply designs. When configured as a full-wave rectifier, it can deliver a DC output current of 100 mA, yielding a DC output voltage of 330 V with a 40 $\mu\text{F}$ capacitor-input filter, or 360 V with a 10 Henry choke-input filter, while exhibiting a tube voltage drop of 40 V.