Voltage Adapters as power supplies - Safety?

[Don Moose]

I found this 85W Euro->US voltage adapter transformer and instantly thought Preamp PSU for 5$!

Busting the thing out of its case and probing around, I find that all of the windings/wires have continuity, but nothing smells burnt. There are 2 heavy black wires that are effectively shorted (< 1 ohm) and one heavy RED and one heavy WHT that are the 240 and 120 VAC taps (tens of ohms to each other and to the BLK wires). There is also a thin black wire connected to a resistor/LED as pilot light.

I'm gathering the idea that this is an auto-transformer, and therefor not safe as a preamp B+ source. Unless, that is, I feed it from the second 120VAC primary on the filament tranny I still have to buy.

How wrong am I?

[dai h.]

I've done a little bit of googling on auto-(step-up) transformers, and what I found is that some are single winding, and some are dual(i.e. there is a primary and a secondary). Single winding helps make it less expensive but of course you would not want to use it for a B+ trafo since you have no isolation. There were a couple of models of small 220V to 100V transformers (with Pri. and Sec.) from Toei(? IIRC) that some DIYers were using backwards for preamps and such. I don't think they were that expensive (not much difference from 5 bucks if memory serves--couple bucks higher), so I'd imagine that if you looked you would find the same in the States or elsewhere. There was an article on Eric Barbour's site also detailing how to use back to back trafos to obtain B+ and heater supply as well.

[Steve Conner]

These things are always autotransformers, because an autoformer needs less iron and copper than a two-winding transformer for the same VA, and the customer looks for VA per dollar.

[Don Moose]

If they're autoformers, and this one sure seems to be, do I get the isolation I need for safety if I get a dual-primary filament trafo and feed this puppy from the second primary?

[Steve Conner]

Sure, but you could just hook up a full-wave doubler to that spare primary, throw the autoformer away, and still have the same DC voltage and the same isolation. If you're wondering what that is, it's the rectifier hookup with two diodes and two capacitors, as used in the Fender 300PS: giving 2.8* Vac instead of 1.4.

Note that the isolation between primaries on a dual-primary transformer is often not as good as the isolation between primaries and secondaries. It probably won't meet the requirements of electrical safety codes.

[Don Moose]

Understood. And appreciated. This thing won't be for sale, so I'm really only worried about my own patootie.