141 — High-Voltage Rectifier Tube
The ML-141 (also designated as the 141) is a high-vacuum thermionic rectifier tube designed for high-voltage, high-power-level applications. It is particularly suited for electrostatic particle precipitation service with moderately high power requirements, where its design offers a combination of low initial cost, long operating life, and high current capacity.
Construction and Mechanical Design
The construction of the ML-141 incorporates specialized design features engineered for high-power operation, ensuring physical ruggedness and thermal stability.
Cathode and Anode Structure
The tube utilizes a directly heated, low-wattage, thoriated-tungsten filament of a catenary design. This catenary configuration allows for close spacing between the anode and cathode without risking filament distortion from strong electrostatic forces. The anode is a cylindrical structure made of molybdenum, which provides a high rate of heat dissipation and includes a substantial safety factor to protect against accidental thermal overload.
Physical Dimensions and Mounting
The ML-141 is designed for flexible installation and can be mounted in any position (mounting position is optional). The physical dimensions of the tube are as follows:
The cathode base of the tube is designed to fit a Machlett mounting socket, part number P-8835. The net weight of the tube is 14.5 ounces.
Cooling and Insulating Medium
The tube can operate in either oil or air as an insulating medium, which directly affects its voltage ratings and cooling requirements:
- Operation in Oil: The tube relies on the surrounding oil for insulation and cooling.
- Operation in Air: When operated in air, the cooling method must combine radiation and forced-air. A forced-air flow of 50 cubic feet per minute (cfm) must be directed at the glass bulb to ensure adequate heat dissipation.
Electrical and Filament Characteristics
The ML-141 requires specific filament operating conditions to maximize its service life and ensure stable emission.
- Filament Voltage: 5.5 Volts. The applied filament voltage must be maintained within ±5% of this rated value under all operating conditions. For maximum tube life, it should be kept as close to 5.5 Volts as possible.
- Filament Current: Approximately 6.5 Amperes when operating at the nominal 5.5 Volts.
- Filament Heating Time: A minimum heating time of 2 seconds is required after lighting the filament before any anode voltage is applied.
- Tube Voltage Drop: The maximum internal voltage drop across the tube is 650 Volts when the instantaneous plate current is at 0.750 Ampere.
Maximum Ratings and Circuit Applications
The maximum ratings of the ML-141 vary depending on the insulating medium and the specific electrical circuit configuration. Operating the tube within these limits prevents premature failure and damage.
Absolute Maximum Ratings
- Peak Inverse Anode Voltage (Oil Insulation): 125,000 Volts (125 PKV)
- Peak Inverse Anode Voltage (Air Insulation): 80,000 Volts (80 PKV)
- Peak Anode Current: 0.75 Ampere (750 mA)
- Anode Dissipation: 100 Watts
Average D-C Load Current Ratings
The allowable average direct-current load capacity depends on the rectifier circuit configuration and whether the circuit is unfiltered (based on a sine-wave input and a pure resistance load without inductive or capacitive effects) or filtered (based on a sine-wave voltage input and an inductive choke-input filter):
Plate Current Characteristics
The relationship between the peak plate voltage and the resulting peak plate current demonstrates the emission capability of the thoriated-tungsten filament. Under typical operating conditions, the peak plate current increases with the applied peak plate voltage as follows:
- At 200 peak volts, the peak plate current is approximately 0.12 Ampere.
- At 400 peak volts, the peak plate current rises to approximately 0.35 Ampere.
- At 600 peak volts, the peak plate current reaches approximately 0.75 Ampere.
- At 800 peak volts, the peak plate current is approximately 1.05 Amperes.
- At 1000 peak volts, the peak plate current is approximately 1.22 Amperes.
- At 1200 peak volts, the peak plate current levels off near 1.28 Amperes.
These values show that the tube can deliver currents well in excess of its 0.75 Ampere maximum peak rating when transient voltage drops occur, though continuous operation must be kept within the specified limits.
Radiation Safety Warning
When operated at high voltages, the ML-141 is capable of producing x-rays. To prevent harmful exposure to personnel, equipment designers must incorporate adequate rayproof shielding around the tube and associated high-voltage circuitry.
Technical Summary
The ML-141 is a high-vacuum, high-voltage rectifier tube featuring a thoriated-tungsten catenary filament and a cylindrical molybdenum anode. It is rated for a maximum peak inverse voltage of 125,000 Volts in oil (80,000 Volts in air) and a maximum peak anode current of 0.75 Ampere. Operating with a 5.5 V, 6.5 A filament supply, the tube requires a minimum of 2 seconds of preheating before anode voltage is applied. When operated in air, it requires a forced-air flow of 50 cfm directed at the bulb. The ML-141 is primarily intended for electrostatic particle precipitation and other high-voltage rectification applications, requiring proper x-ray shielding during operation.
