Of course, Heising is single-ended. In normal Heising, the choke must
handle the DC current of the PA and the modulator, and not saturate in
the process. By using instead a center-tapped winding you can
(nearly) balance the DC, so the choke saturation isn't so much a
problem. If you use an audio output transformer for the job, say 4000
ohms CT to 8 ohms, that's an 11:1 turns ratio from half the primary to
the secondary, and you can add the secondary to the PA side to get a
little higher (a little closer to 100%) modulation. Not that audio
output transformers of an appropriate size are a dime a dozen, but
there's at least some hope of finding one "kicking around" somewhere.

Or--maybe you can find someone taking an old plate-modulated AM
broadcast transmitter out of service and get a really good set of
modulation transformer, modulation choke, and coupling capacitor. ;-)

Tom

August 1956 QST has a class B modulator that works without a modulation
transformer! Two tubes, which goes against the original posters question,
but what the hay. If you don't have to buy the transformer, you can afford
the second tube.

Still looking for my class B/A article on the 304TL Heising bias shift
modulator. Am certain that the author was Bill Orr, W6SAI, cause I called
him on the phone when my 304TH didn't work as advertised. Very nice and
helpful to an almost beginner, but that article doesn't exist in any QST to
back before I would have read it. As I now recall, I think maybe Bill wrote
it up in CQ magazine and got paid for it. Don't have those on CD's.

Regards
W4ZCB