My experience is that the core material is not so critical for a transformer.
In my PA (450W out, variation of the G3YXM design) I use a 'double-U' core
that was slaughtered from the HV transformer of an old TV set. It gets hand
warm after longer QSO's.
I you want to buy a core the 3C85 material (Philips) should be OK.

But be aware the in contradiction to the above the core material is far
more critical for use in low pass filters or any other resonant circuits.
There the 'red' Amidon cores can be used.

73, Rik  ON7YD

At 09:18 3/12/00 -0000, you wrote:

Hi all,

Can anyone point me to any sources that will give me information on core
mixes, properties  and design criteria to assist me in realising output
transformers for an LF PA?
Data on the more esoteric cores available surplus seems to be very thin on
the ground....

Regards to all

Paul G4MD