The DX66 takes 4 type C batteries so the supply voltage must be 6V.
I should point out that there is a hum without the headphones but it is
less notisable when using the speaker. I became aware of the hum
after using headphones.

The schematic shows each end of transformer secondary connected to
a diode ...a filter cap across it ...and a transistor with a zener
diode on the base at the output of the power supply.

Can I modify this with something to get rid of the hum?

WShoots1 wrote in article
...
What Charles said, "author." The "wall wart" was the problem I had with
my
DX392, so I built a regulated supply.

What's the supply voltage for that DX66?

Bill, K5BY