when i use a variac as transformer, with a bridge- from where do i get the ground, which will be connected to the mid of the capacitors?
When i use a "normal" transformer, it`s symmetrical, that means the ground i need is the connection of two cables, coming out of the transformer, plus the two ac - cables, which are connected to the bridge.
what do you want to do exactly? 2X120VAC to +/-100VDC? If so that is going to involve a lot of power dissipation. How much current do you plan to draw?
If it's more than 200mA you better buy a new transformer. The variac alternative will work but it's awfully clunky. Plus you would be limited in the A of your VA rating.
To get +/- 100DC unregulated you will need a trafo more in the order of 2x60VAC.
Look in Geoff site he's got one adjustable regulator circuit that kind of works.
Actually you just need to adjust the voltage prior to loading and it's fine.
You could change a few components to make it siutable for your application.
If you really want to drop from ~170 rect. to 100 regulated and get 500mA out of it you are going to need some big mama transistors. 500mA at 70 V is probably dangerously close to the SOAR limit of the 2N3055. You should be able to do it with more transistors in parallel followed by a 0.33 ohm emitter resistor or with a MJ150XX-type transistors. In any case, the transistor on each side of the PS should be equipped with a heatsink able to dissipate ~50W continuously.