This configuration obtained from a 70's RCA manual shows how to use a triac in the primary of a transformer to obtain a simple variable symmetrical voltage source. The Triac can be any of the ICT series and the diac of any type. The diodes can be 1N4002 and the transformer has a secondary from 12 V to 1 A, according to the desired current in the load. In the diagram we have a quadrac, but we can use separate triac and diac, without problems. The adjustment pot can be 220k and capacitors C1 and C2 can be increased to 220 nF to allow the use of the device in the 220 V power grid.
