Filament power supply improvement of the TRIUMF RF system

The TRIUMF RF system, operating at a fixed frequency of 23.06 MHz, employs four push-pull power amplifiers using eight EIMAC 4CW250,000 tetrodes. The filament power supplies for the above tetrodes uses simplified three phase SCR control circuits to slowly ramp up the output voltage. Failure of SCR´s during ramping and also during power bumps has been a source of RF downtime. A new SCR control scheme has been developed and tested which relies on the fact that the secondary output voltage from a three phase transformer will be 57.7% of it´s rated output voltage if only two phases of the primary are connected. In this scheme, one phase is connected directly and the second phase is connected through two SCR´s of which the conduction angle is controlled by a slow ramp. The third phase is initially blocked by two SCR´s which are turned on fully after a fixed delay thereby limiting the surge current of the filament during start up. A greatly simplified firing circuit and smaller number of components has improved the reliability and the performance of such a power supply. Also, an alternate scheme of a power supply with series resistors in the primary of the transformer is described