I don't mess with triacs or silicon controlled rectifiers much.. but.. Think of the SCR as a solid state switch. You need to supply a small current at the gate terminal and then hook up anode and cathode as for a diode application. Add some resistance to the gate line.. it doesn't need a huge jolt. Once the scr is triggered, it will remain on till the current thru the SCR drops below a minimun threshold.. which will be identified as Ih or holding current listed on the package.
So you will need a small power supply, a 'exciter; possibly momentary push button switch, and then run a wire from that switch to all 4 gates in your full wave bridge. The kicker is.. you will have to push the exciter to get the arc started, and as soon as you loose the arc, you will need to re-excite. I'm not sure if you could put a large value resistor across the + and - of the load to keep holding current flowing... might have to expirement with a resistor that allowed enough holding current to flow.. yet wasn't a heavy parasitic load that would burn up with long welding runs. You might expirement with just keeping the gates excited with a on/off switch. DO read up on the characteristics of yor SCR.. see if they will tolerate constant gate voltage.. and if so.. try to provide not much mor ethan the minimum to excite them...
Soundguy
Soundguy