two transformers back to back for isolation

[jeff]

 I have an old transformerless tube stereo and I want to build an isolation transformer. I had two identical SS stereos I'm getting rid of so I cut out their transformers to wire back to back.
The transformers have multiple secondaries for different voltages, should I hook all the secondaries up or just pick one secondary?
I imagine if I can hook them all up this will allow me to draw more current than just using one.
If I do connect them all how do I check that they're all in phase?

[kagliostro]

If you want all the VA be disposable you must connect all the secondary windings with the correspondents

K

[kagliostro]

I think he want only do something as to have an existing amp more safe

K

[jeff]

There are two windings with the same color, two blue two red. How can I test phase?
What happens if I connect both secondaries and they're out of phase with each other?
Will being out of phase cause damage or will they cancel out(is that what bucking is???)?
If there's no chance of damage should I just hook them up one way check for 120V and reverse one of the secondaries check for 120V?

[proaudioguy]

Sounds to me he wants a 1:1 isolation transformer for his mains. We use them at work all the time although quite often we get 480V 3 phase and use a step down to get to 120V. The idea is to generate the neutral from the secondary center tap. This eliminates all the nasty stuff on the building neutral caused by industrial freezers and such from getting into our stuff. It also isolates audio from the nasty stuff the lighting guys dump onto their neutral.  I can't imagine using a step down transformer and another reversed to make my own iso but I think that's what he wants.  My buddy is an audiophile with all tube (low power amps with high efficiency speakers) and using an iso is something those guys do. I would personally recomend using DC instead of AC if you want to really get rid off all the AC related noise.  Perhaps simply remoting the power supply would be enough.