There's a simple voltage reducing trick that Drake used in at least one model
of an emergency marine receiver. To make the 12-volt powered receiver operate
on ships with 24 volts (Europe), a Zener diode was put in series with the d.c.
input. It was installed inside the cabinet, which had plenty of room. Because
of the current requirement, it was a hefty top hat type.

That could be done for civilian SW units, to operate off readily available
regulated 12-volt power supplies. I recall that, after a couple of Zener
failures in customers' receivers, we installed two or three smaller values in
series instead of a single 12-volt Zener.

So, for a 6-volt/500 ma (1/2 amp) receiver like my DX-392, I would use, say,
three 2-volt/1 amp Zeners in series between the power supply output and the
power input to my receiver.

Bill, K5BY