[Eric]

Is your 6v charger putting out 6v under load? Is there a full 6v going into the set? Typically chargers don't have any filtering so the dc output has a lot of ripple. Try adding a 6v car battery or a large cap like 10,000mf @ 20v or so to eliminate the ripple. If you have a full 6v at the set measure the ac voltage from plate to plate of the 0Z4. Should see about 500vac or better. If you do then chuck the 0Z4 and solder in two 1N4007. Tie the banded ends together and solder to the cathode pin of the socket. The unbanded ends go to each of the plate pins. I suspect your 0Z4 is weak.

You should be able to use a pm spkr as the only thing that the fc is doing a providing magnantizum for the spkr, and draining the battery.

Terry.
I get 8vdc when I turn it on it drops to 6v, across the plates of the OZ4
I get 250v  Don't have the diodes but I will get them.