Hallo Prahlad,

I'm not an expert in welding maschines and their current control, but don't forget, that a transformer presents an ideal inductive load. This causes a heavy phase shift between current and voltage, which can result in lots of trouble...

Another issue is asymmetry of positive and negative half wave of mains. If phase control leads to a different, means mismatched phase angle for positive and negative half wave, very low winding resistance makes, that enormeous DC currents are built. Maybe that's the reason, why no single triac is used, but two thyristors? I don't know, I'm not an expert in this dangerous electronics...

It seems to me, that your firing card does not necessarily waste lots of components. I think they are really needed...

Another idea is to use some sort of opto isolation for triggering the two thyristors. There are nice opto diacs available from motorola, maybe they do also have some application notes?

Good luck, and take care!
Kai