I have a backgound hum when listening to short wave on my RS66 radio with
headphones.

I'm not familiar with that radio, but is the hum heard while on battery power?

My DX392 eats batteries but, when using the wall thingie, it has hum on CW/SSB
and hum bars on FAX. So, I built a regulated power supply for it using R.S.
parts.

I'll try this coming year to get all the info about the the power supply posted
as a web page. The supply is usable for any of the portable SW radios, by
adjusting the output voltage and/or changing the power connector wiring.

Bill, K5BY