Antonio Vernucci wrote:
I have to carry more tests on my HRO, including that of connecting my
Yagi antenna to it instead of a short indoor piece of wire.

Using my HF Yagi antenna instead of the indoor wire all problems
completely disappeared and there is no hum at all on any station.
....

BTW, have you checked to see if the hum changes when you go to
manual RF gain control? There may be a clue there if it does.

The hum-to-sound ratio was almost independent of the RF gain control
setting (a potentiometer of the tubes cathode).


Both of the above statements indicate that the hum was caused by emissions
from your receiver, or your shack.

Classically, on a solid state rectified device, the rectifiers don't switch
instantly, and hand up for an instant at every zero crossing of the AC waveform.
That hang is a 1 usec long short circuit, and emits 60Hz glitches at every crossing.

It will radiate 60 Hz modulated broadband hash over a frequency range that includes
the AM broadcast band.

When you switched to an external antenna, you did two things, one, you got the
antenna away from the interfering source, and two, you increased the signal
strength of the received stations.

Probable sources of such hash are compact fluorescent lamps, regular fluorescent
lamps, the power supplies in any test equipment, or soldering irons in your shop,
and the radio itself.

-Chuck