PY 81 as power rectifiers?

[Terry]

Done the transformer thing, the best orientation for lowest coupling is with the normal mounting surface of the mains transformer at 90° to the output one..

Yes, the magnetic field is at its minimum at 90°.

Even better is to mount them at 90° in two planes. This achieved by using a 'drop through' method of mounting the mains transformer by cutting a rectangular hole in the chassis for the bobbin and mounting the transformer so that the laminations are flat against the chassis and at 90° to those of the output transformer mounted upright in the conventional manner.

An alternative, if transformer and chassis sizes are suitable, mount the mains transformer on the top of the chassis as normal and mount the smaller output transformer horizontally under the chassis on the front or back.

You can probably think of other ways of achieving the same objective.