Special thanks to Calvin at the DIY Audio Forum for sharing his experience using inexpensive 230v/6v power toroids in ESL's. They sound fantastic at about one-tenth the price of the specialized ESL toroids from Plitron or Aplimo.
Cheap toroidal power transformers work great for ESL's but you can't always trust their winding polarities will match the phasing dots on their data sheets or my schematics.
One of the VTX transformers I tested had its red/black leads reversed, such that one 6v winding was out of phase. So when I assembled the transformer pair their windings cancelled and I got no sound. Once I figured out the phasing error and reversed the leads, they worked fine and sounded wonderful.
Winding polarities can be checked with a DVM and jumper wire by applying 115VAC to the primary and measuring the voltages out of the secondaries, as shown here: Transformer Polarity Test
Finding suitable tranformers at a decent price is a problem in the US, where the preferred single primary 230 volt toroids are not available.
Although I prefer a single 230 volt winding over two 115 volt windings, I have sound tested several dual 115v primary transformers and all of them sounded very good.
At this time the only source I know of for single primary 230v transformers that will accept a US credit card and ship to the US are these 230V/2x6V toroids at Rapid Online. I have tested them and they sound great.
I have also tried these dual primary 2x115V/2x6V toroids from Mouser in the US and they sound great too:
Triad Magnetics VTP12-4170
Yet another transformer option would be a quad setup of these Antek AN206's, wired as shown in the schematic below. Although I have not personally tested them, I see no reason why these transformers would not work fine and they are readily available in the US.
Below are my simple bias supply with tandem VTX 50VA 230v/2x6v transformers. The bias supply is laid out on a Radio Shack #276-147 (4.5x 6.625) universal PC board from which I cut the two 4.5" x 2.125" boards shown.
Below: Bias supply and VTX transformers
The schematics below are color coded for the transformers shown. If using different brand transformers, follow the phasing dots and make sure the (+) phase output goes to the rear stator. This will ensure the woofer will be in phase with the panel. Even then, you should verify the woofer/panel phasing as described here.
Schematic with preferred VTX transformers:
Schematic with Triad Transformers
Bias supply wired for 230 volt input: