1805 — Full-Wave High-Vacuum Rectifier

A directly heated, full-wave high-vacuum rectifier designed for A.C. mains receivers, providing reliable high-voltage rectification with flexible operating limits.

Overview

The 1805 is a high-vacuum rectifier tube designed primarily for full-wave rectification in A.C. mains radio receivers. It features a directly heated filament and is housed in a glass envelope. While primarily documented and utilized as a full-wave (biplate) rectifier, historical variants also outline its application in half-wave (monoplate) configurations.

The tube is engineered to deliver rectified direct current to receiver circuits, with its maximum current output dynamically rated based on the applied alternating current (A.C.) transformer voltage. At its maximum rated input voltage of $2 \times 500\text{ V}_{\text{eff}}$, it can safely deliver a continuous D.C. output of$60\text{ mA}$.

Mechanical Specifications and Base Pinout

The 1805 is constructed with a glass envelope and is fitted with either an A35 base (with an S III connection reference) or a standard 4-pin base. The physical dimensions of the tube vary slightly depending on the manufacturing era and envelope design:

• Maximum Overall Length/Height (excluding pins): 110 mm to 116 mm
• Maximum Bulb Diameter: 48 mm to 53 mm
• Pin Length: Maximum 18 mm

The pin connections for the 4-pin base configuration, viewed from the bottom of the tube, are assigned as follows:

Filament Ratings

The 1805 utilizes a directly heated filament designed for parallel supply from an alternating current (A.C.) source. The electrical requirements for the filament are:

• Filament Voltage ($V_f$): 4.0 V
• Filament Current ($I_f$): Approximately 1.0 A
• Heating Method: Direct, A.C. parallel supply

Because the filament is directly heated, the filament supply also serves as the electrical connection point for the positive D.C. output of the rectifier circuit.

Operating Characteristics and Limiting Values

The maximum permissible rectified output current ($I_o$) of the 1805 is inversely related to the applied secondary transformer voltage ($V_{tr}$). Operating the tube at lower transformer voltages allows for higher current delivery without exceeding the thermal and emission limits of the structure.

The standard operating limits and maximum ratings are structured as follows:

Historical Variant Ratings

Depending on the specific production date and variant, different maximum ratings were established:

• Full-Wave Variant (Late 1933): Rated for a maximum A.C. anode voltage of $2 \times 300\text{ V}$with a maximum rectified current of$100\text{ mA}$.
• Full-Wave Variant (Mid 1933): Rated for a maximum A.C. anode voltage of $2 \times 400\text{ V}$with a maximum rectified current of$75\text{ mA}$.
• Half-Wave Variant (Mid 1933): Documented with a maximum A.C. anode voltage of $2 \times 300\text{ V}$and a maximum rectified current of$100\text{ mA}$.

Performance Curves and Rectification Characteristics

Anode Conduction Characteristics

The relationship between individual anode current ($I_a$) and anode voltage ($V_a$) shows a smooth diode conduction curve starting at $0\text{ V}$. The internal resistance of the diode determines the voltage drop across the tube during conduction. Two distinct sets of curve measurements describe this behavior:

• First Characteristic Set:

  • At $V_a \approx 37\text{ V}$, the anode current reaches $40\text{ mA}$.
  • At $V_a \approx 70\text{ V}$, the anode current reaches $100\text{ mA}$.
  • At $V_a \approx 90\text{ V}$, the anode current reaches $140\text{ mA}$.

• Second Characteristic Set:

  • At $V_a = 60\text{ V}$, the anode current is approximately $50\text{ mA}$.
  • At $V_a = 80\text{ V}$, the anode current is approximately $70\text{ mA}$.
  • Above $70\text{ mA}$, the characteristic transitions to a dashed line, indicating peak or non-continuous operation, reaching $140\text{ mA}$ at $V_a \approx 140\text{ V}$.

Rectification and Output Voltage Drop

When operating in a practical rectifier circuit with a filter capacitor ($C = 16\ \mu\text{F}$), the output voltage ($V_o$) drops as the load current ($I_o$) increases. This drop is heavily influenced by the total protective source resistance ($R_t$) of the transformer circuit.

The following output voltage behaviors are observed across different transformer voltages and source resistances ($R_t = 100\ \Omega, 200\ \Omega, 400\ \Omega, \text{ and } 800\ \Omega$):

• At $V_{tr} = 2 \times 500\text{ V}_{\text{eff}}$:

  • The no-load voltage ($I_o = 0\text{ mA}$) is approximately $700\text{ V}$.
  • At the maximum rated load of $I_o = 60\text{ mA}$, the output voltage ($V_o$) drops to approximately $610\text{ V}$ with a low source resistance ($R_t = 100\ \Omega$), and down to approximately $540\text{ V}$ (or $530\text{ V}$ in alternative curve sets) with a high source resistance ($R_t = 800\ \Omega$).

• At $V_{tr} = 2 \times 400\text{ V}_{\text{eff}}$:

  • The no-load voltage ($I_o = 0\text{ mA}$) is approximately $560\text{ V}$.
  • At a load of $I_o = 75\text{ mA}$, the output voltage ranges from approximately $470\text{ V}$ ($R_t = 100\ \Omega$) down to approximately $410\text{ V}$ ($R_t = 800\ \Omega$). Under a $60\text{ mA}$ load, the output voltage is approximately $400\text{ V}$.

• At $V_{tr} = 2 \times 300\text{ V}_{\text{eff}}$:

  • The no-load voltage ($I_o = 0\text{ mA}$) is approximately $420\text{ V}$.
  • At the maximum rated load of $I_o = 100\text{ mA}$, the output voltage ranges from approximately $320\text{ V}$ ($R_t = 100\ \Omega$) down to approximately $260\text{ V}$ ($R_t = 800\ \Omega$). Under a $60\text{ mA}$ load, the output voltage is approximately $270\text{ V}$.

These curves demonstrate that higher source resistance ($R_t$) and higher load current ($I_o$) result in a predictable and significant voltage drop, which must be accounted for when designing the power supply filter network.

Technical Summary

The 1805 is a classic directly heated, high-vacuum full-wave rectifier designed for A.C. mains receivers. Operating with a standard 4.0 V, 1.0 A filament supply, it provides reliable high-voltage D.C. conversion. The tube is highly adaptable, offering a maximum output of 100 mA at $2 \times 300\text{ V}{\text{eff}}, **75 mA** at \2 \times 400\text{ V}{\text{eff}}, and **60 mA** at its maximum input rating of \2 \times 500\text{ V}_{\text{eff}}$. Its physical construction utilizes an A35 or 4-pin base, making it a standard component for mid-20th-century receiver power supplies.