240V > 120V stepdown converter issue

[boroman]

I am using a lot U.S. gear here in Europe - mainly ampeg amps, tube testers etc. Been using an old HEAVY 2000W one, but decided to buy 500W, way smaller and fits the lower wattage stuff well.

But I'm confused.

Bought new unit, and between NEUTRAL and PHASE (so both holes) it has 120V, between NEUTRAL and GROUND - 120V and between PHASE and GROUND - 240V (!)
I quickly broguht back the bigger one that I was using til this day, and it has close to 0V between PHASE and GROUND.

Was it just a faulty unit?
I know that with 2-prong equipment it will be okay, because I have 120v betwen holes and not using ground, but with 3-prongs?

[Helmholtz]

I guess you're talking about magnetic transformers and not SMPS converters.

It seems your new transformer is an autotransformer. These are typically smaller and cheaper than isolating ones.
An autotransformer only has one winding with a tap for the lower voltage.
So input and output are not separated/isolated.
Autotransformers are not safe, especially if the power plug is reversible (like in Germany).

An isolating stepdown transformer should be safe as it has separate, isolated primary and secondary windings (making it more expensive and larger)
I.e. if its built and tested according to safety standards. Look for approvals.