Lets pretend I have a friend who is in need of some 2.5V @ 25A filament transformers.
It seems you can scour the world and not find an appropriate beast when in theory it is really a pretty simple job.
Simple... Just take an $18 100VA antek toroid and pull off the secondary. Then all you need is 12 turns of #9 wire and you are done. Since the final use needs to do 2.5V @ 25A * 2, I was a bit concerned that the extra 25VA over would be problematic. My first go was using some #10 wire I had which gave me 10 turns for an experiment. The next problem was how does one draw 25A of current? since I didn't have any 0.1 ohm 100W resistors I had to make due. Luckily in my junk bin I had a 10 pound spool of #20 nichrome wire so a few inches should do it. Well, that got really really hot really quickly but three strands in parallel about 8" each did the trick. The #10 showed promise but I was a few turns short so off to the "home despot" I went to get some #10 stranded house wire for round 2.
The stranded wire was a failure. First, the nichrome wire gets really hot and copper being a good conductor of heat gladly wicked the heat back into the transformer. For kicks I was able to flow solder into the junction of the two and the first few inches of copper got so hot that they could cut styrofoam through the insulation. In reality, I suspect the #10 would have been fine but since I was already pushing the VA rating I decided to go for #9. Luckily I have a few spools of #12 and a twisted pair of 12's = a single 9.
The black flecs in on the core are nichrome flakes from the making of the video but I have run the unit for an entire day with both windings drawning 25A and it didn't get hot enough to concern me. As an added plus the voltage didn't change as the units reached full temperature under load.
It is a bit of a shame that the #10 didn't work since apart from the "home despot" green, I also have three different types of #10 Litz wire. Just think of all of the Audiophool Hype you could come up with as you painstakingly strip 420 strands of #36 per connection.