15QR40 — High-Voltage Mercury-Vapor Thyratron

The 15QR40 (also written as 15 QR 40) is a mercury-vapor filled, hot-cathode thyratron equipped with a control grid. It is designed primarily for use as a controllable rectifier tube in the high-voltage half-wave rectifier stages of radio transmitters and industrial generators. Additionally, its grid-controlled switching characteristics make it suitable for use as a switching and control tube in industrial regulation systems.

Construction, Heater, and Physical Specifications

The 15QR40 features an indirectly heated oxide cathode designed for robust thermal emission. The physical structure is optimized for high-voltage isolation and vertical operation.

The preheating requirements are critical for the safe operation of mercury-vapor tubes. Under normal operating conditions, the cathode must be heated for at least 300 seconds before anode voltage is applied. Following transport or prolonged shutdown, the preheating time must be extended to at least 3600 seconds (1 hour) to ensure that any condensed mercury on the electrodes is completely vaporized and redistributed to the lower portion of the envelope.

Physically, the tube has a maximum envelope diameter of 150 mm. The maximum overall height is specified between 440 mm and 450 mm, with a base height of up to 30 mm.

Base, Connections, and Socketing

The tube utilizes a specialized 4-pin base and a top anode cap to maximize electrical isolation between the high-voltage anode and the low-voltage grid and cathode connections.

• Base Designation: Special 4-pin base (4-35, TGL 70-76)
• Socket Designation: 4-35, TGL 68-5
• Anode Cap Connection: Type E (TGL 70-123) or clip-on Type C 20 (TGL 4520)

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

Electrical Characteristics and Capacitances

The electrical behavior of the 15QR40 is defined by its gas-discharge properties and interelectrode capacitances:

• Grid-to-Cathode Capacitance ($C_{gk}$): 40 pF
• Anode-to-Grid Capacitance ($C_{ag}$): 15 pF
• Arc Drop Voltage ($U_{\text{arc}}$ / $U_i$): 16 V (measured at an anode current of 12.5 A)
• Ignition Voltage ($U_z$): 2 kV
• Ignition Time ($t_{\text{ign}}$): Maximum 10 \u03bcs
• Deionization Time ($t_e$): Maximum 1000 \u03bcs

Absolute Maximum Ratings

Operating the 15QR40 beyond its maximum ratings can lead to arc-back, cathode damage, or envelope failure. The limiting values are specified as follows:

Rectifier Operating Configurations

In practical high-voltage rectifier applications, multiple 15QR40 tubes are arranged in various circuit configurations. At a maximum peak inverse anode voltage of 15 kV, the typical operating parameters for configurations (excluding transformer and tube losses) are detailed below:

Ignition Characteristics

The ignition characteristic of the 15QR40 defines the relationship between the negative grid bias voltage ($U_g$) and the forward anode voltage ($U_{as}$) required to initiate conduction. Because of manufacturing tolerances and operating conditions, this relationship is expressed as a spread band ("Streubereich") rather than a single curve.

At a nominal heater voltage of 5 V, the ignition threshold varies with the applied anode voltage:

• At high anode potentials near 15 kV, a strong negative grid bias between approximately -7 V and -33 V is required to prevent the tube from firing.
• At lower anode potentials, such as 2 kV, the ignition threshold shifts to a less negative range, typically between -3 V and -10 V. Once the tube ignites, the grid loses control over the anode current, and conduction can only be stopped by reducing the anode voltage below the ionization threshold.

Related Designations and Equivalents

The 15QR40 is electrically and physically compatible with several industry-standard high-voltage thyratrons. It corresponds directly to the following designations:

• ASG 5830
• DCG 7/100
• F 941
• HF 3415
• WL 41

Additionally, the 15QR40 is a direct replacement for the older S 15/40 i tube.