transformers

[JC@]

Is there any reason for not using a step-up or step-down mains transformer of the toroid type?
Thanks

[Steve Conner]

Err, because you're allergic to toroids and they bring you out in hives? Or maybe your dog thought it was a donut and chewed it to destruction?

Can't really think of any other reasons, except that toroids aren't as tolerant of DC imbalances as E-I core transformers. The voltage doubler power supply circuit, such as used in the old Ampeg SVT, is an example of this. Any current you draw from the half-voltage node puts a DC imbalance on the core and makes it growl. But I plan to do it anyway in my upcoming SVT clone project with a toroidal PT. :P

[JC@]

OK then, toroids rule.
I can't find 230 to 117 (or close to it), if you find a source please drop me a line.
Thanks

[Steve Conner]

I had trouble finding a suitable toroidal PT too.

In the end I bought an old 500VA medical isolation transformer off a friend who dumpster-dived it. It has two 120V windings either side of the isolation barrier, so it can go from 120 to 120, 120 to 240, 240 to 120, 240 to 240, or whatever. I'll add a heater winding and use another small transformer to get bias.

[JC@]

Yes those medical isolation transformers look great.
I just found a factory located in UK after doing some google.