A simple way to make a transformer for a vacuum tube project
The first thing you need to do is measure the current through the primary winding of a transformer with NO load attached to the secondary winding(s).
If you are not 100% confident with mains electricity, STOP NOW! This project will output high voltages as it is so it's probably not the project for you.
Set your digital multimeter to the 10A AC setting and wire it in series with the transformer. I highly recommend an isolation transformer for extra protection, and please, earth the transformer case!
Switch everything on and take a note of the current that flows through the primary. It should be a few tens of milliamps. Write this down.
Dismantle a transformer. Start by undoing the four lugs underneath it, remove the piece of metal that's being held there and remove the shroud surrounding the rest of the iron.
The transformer might (should... some cheap ones aren't) be coated in varnish. Start by running a butter knife or similar around an iron lamination at one end to split it off the stack. You'll notice that it won't budge. This is the tricky bit... you need to get the butter knife and tap the iron core out gently. The iron should be packed in fairly tightly.