You just need phase angle referenced source to tell the SCR's when to turn on in relation to what the line voltage is at that instant in regards to the sine wave.
Basically all you need is a zero crossing reset of a simple timer that will delay the turn on of the SCR's from front to back time wise.
Turning them on latter in the sine wave gives you a lower voltage turning them on at the front of the sine wave gives you a higher voltage.
A simple UJT oscillator thats set up with an adjustable delay from 0 to 1/120 of a second will work as a timing reference source. Each time the sine wave crosses the zero point it resets the UJT. Then so many milli seconds later the UJT fires and sends power to the Opto coupler that fires the SCR gates.
This is the basic way to be able to use phase angle voltage control to get a variable voltage with a inductive load like a transformer. By varying the one time of each half of the sine wave you get a basic PWM type effect. the inductance of the transformer smoothers out the chopped wave form and then gives a reasonable smooth sine wave on the secondary as a result.
By adding a feed back loop to either vary the output voltage or to limit the output current you can get a stable and constant voltage or a constant current. Or with the right circuits a combination of both. Voltage limiting up to a preset current and then decreasing voltage as the current is limited.