autoharp bob , they are extremely rare, most of the supposedly tone daef are not tone deaf at all, they just need help and tuition.
   i had a half brother who was a real room clearer, he frequently changed key during a song[ are you there Vic Smith ,you know who I mean] but he was not actually tone deaf, after having fiddle lessons, and some guidance about pitching and interval recognition, his singing improved in as much he was able to hold a tune, and did not change key during a song.