The BTH BT85 (CV381) is a hydrogen thyratron for delivering short duration high current pulses. The intended use is as a Radar (magnetron) modulator and the BT85 can switch 90 A 10 kV pulses at up to 2,500 pulses per second. That equates to 0.9 MW pulses.
The cathode and one side of the heater is connected to the metal ring and the flying leads are identified by coloured ceramic rings. The yellow ring is the other heater connection and the red ring is the grid connection.
The insulator at the base is made of Paxolin and the metal ring around the envelope has fixing lugs. The pinch sits above a wide tube that is re-entrant to the main envelope and it functions as the support for the metal collar that holds the electrodes. In the centre is the small diameter evacuation tube. The anode is a wide top cap with ceramic insulator below. The etched lettering on the glass either side gives the Type and the BTH name and Made in Rugby England as well as the fact that it is a Thyratron.
The base and insulated flying leads. Note the coloured rings.
The anode connection rod is encased in a glass tube from top cap to the main electrode assembly. The anode itself looks to be a thin disc inside the screening can, with mesh sides, that forms the grid. The collar that the electrode supports attach to is secured by a nut and bolt.
The heater connections pass via insulators into the lower cylinder. There is a coil of wire that is connected between the grid connection and a support in the pinch. Looking into the base tube it can be seen that the support was initially connected to a stranded wire that has been cut off at the base. This was, therefore, used only during manufacture.
The wide glass tube envelope is 65 mm in diameter, and including the base is 207 mm tall.