Filament transformer operating properly?

[JoeTele]

I just picked up a Hammond 100-watt, 500ma filament transformer for use as a wide varying negative bias supply in a tube tester I'm building. I was proto-boarding the bias circuit earlier, and I noticed when I was taking things down, the transformer was slightly but noticeably warm, despite the fact that I was monitoring current the whole time and never even drew 10ma from the secondary. Is this normal?

Thanks!

Joe

[Malcolm Irving]

Probably OK. As well as heating from current in the windings, a transformer also has heating directly in the iron core due to its AC magnetisation. This 'iron loss' heating only depends on the AC voltage applied and is independent of the current.

[Stevem]

Warm is fine, but more than 30 degrees above body temp if it's sitting out in the open maybe a concern!
Do you have a Theramal gun you can check it with?

[JoeTele]

I do. I'm going to wire it up again tonight or tomorrow night, let it sit for a while, and take the temp.

Thanks as always!

Joe