Not sure about WAG6. AFAIK it consists of a Single Phase transformer with two secondary feeding the thyristor base controlled rectifier converting AC into controllable DC. The circuit configuration is same as the tap changer locos except diode bridge rectified is replaced by thyristor base controlled bridge rectifier. The transformer is step down transformer without having windings for different steps and controlled output is obtained by controlling the firing angle of thyristor.Surely, microprocessor must be involved in this case too (like 3-phase locos). But the role of microprocessor could be something like this : click here

Loco allocations list (part 2).

Loco allocations.