1701 — Radiation-Cooled Mercury-Vapor Thyratron
The 1701 (also designated as 5557/1701) is a radiation-cooled, mercury-vapor, low-voltage thyratron designed for control and switching applications. Classified as a grid-controlled mercury-vapor triode or tetrode, the device is structurally similar to the 866-A mercury-vapor rectifier but incorporates grid control to regulate the onset of conduction.
Filament and Thermal Characteristics
The 1701 utilizes a high-current filament designed for low-voltage operation. The thermal and filament specifications are highly specific, requiring a brief preheating period before anode voltage is applied to ensure proper mercury vaporization and prevent damage to the cathode structure.
The 5-second filament heating time is notably brief compared to larger mercury-vapor thyratrons, allowing for rapid readiness in control circuits. Safe operation requires maintaining the condensed mercury temperature strictly within the range of +30°C to +80°C to ensure stable ionization characteristics and prevent arc-back or failure to control.
Electrical Ratings and Operating Parameters
The 1701 is rated for moderate power switching and control tasks. It features a high peak inverse voltage capability relative to its forward voltage rating, which is characteristic of mercury-vapor devices.
Voltage and Current Limits
- Maximum Peak Forward Anode Voltage: 2500 V
- Maximum Peak Inverse Anode Voltage: 5000 V
- Maximum Peak Anode Current: 1.0 A
- Maximum Average Anode Current: 0.5 A
- Maximum Grid Voltage: -500 V
Switching and Conduction Characteristics
During conduction, the tube exhibits a constant voltage drop, known as the tube drop, which is independent of the current through the device. The switching speed is governed by the ionization and deionization times of the mercury vapor.
- Tube Drop: 16 V
- Ionization Time: 10 microseconds (μs)
- Deionization Time: 1000 microseconds (μs)
The 16 V tube drop represents the typical potential difference across the tube during active conduction. The deionization time of 1000 μs dictates the minimum time required for the grid to regain control after anode current is interrupted, which limits the maximum operating frequency of the circuit.
Structural Design and Applications
The 1701 is a radiation-cooled tube, meaning it relies on natural convection and radiation to dissipate heat generated during operation. Its structural design is closely related to the 866-A half-wave mercury-vapor rectifier, sharing a similar envelope and thermal profile but adding a control grid. This grid allows the tube to function as a controlled rectifier, electronic switch, or sensitive relay driver, where conduction is initiated by a small grid signal and continues until the anode voltage drops below the ionization threshold.
Technical Summary
The 1701 (5557/1701) is a specialized mercury-vapor thyratron featuring a 2.5 V, 5.0 A filament with a rapid 5-second heating time. It is rated for a maximum peak forward voltage of 2500 V, a maximum peak inverse voltage of 5000 V, and can handle a peak anode current of 1.0 A (0.5 A average). With a characteristic tube drop of 16 V, an ionization time of 10 μs, and a deionization time of 1000 μs, this radiation-cooled device provides reliable low-voltage switching and control within a condensed mercury temperature range of +30°C to +80°C.
