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. The problem however is
not due to interfering sources (e.g. lamps, power supplies, etc.) in my house,
but to the fact that the RF signal of certain stations (not all of them) follows
a path where it gets modulated by alternating current. As a matter of fact,
using a simple wire, the ground counterpoise is represented by the house
electrical wiring

While leakage of capacitors is often called equivalent series resistance
(ESR) it can also be parallel resistance. For instance its pretty common for
bad electrolytic caps to look like dead shorts. The same can happen to paper
caps. If the parallel resistance of a capacitor is low paralleling another cap
across it will simply put the low resistance across both. So, if one is
checking for bad caps its best to actually substitute the cap otherwise you
can be mislead.

Still, I am unable to follow your argument. Assume that a screen bypass
capacitor is leaky. Its DC resistance typically is in the range of several
hundreds kohm (should it be lower, the screen DC voltage would be greatly
affected). Having a resistance in parallel to a good capacitor does not impair
its bypassing function. Even a low resistance value would not harm (apart from
its effect on the DC voltage). In the limit, a zero resistance would turn into a
perfect bypass (again, apart from the DC voltage).

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).

73

Tony I0JX