Tunable hum from the power supply where the filtering is good is found
mainly in AC/DC receivers. There are other sources, mainly lack of bypassing
especially of the screens of tubes with AVC on them and heater to cathode
leakage anywhere in the RF path. In either case the tube or tubes affected can
modulate hum onto signals. Since AVC tubes are operating with a non-linear
characteristic they can be pretty good modulators.
If you have access to a good tube checker, one which will indicate shorts
and leakage, test the tubes with it. A tube may test fine for emmision and
transconductance but still be leaky. Bypass condensers can sometimes be
checked by simply paralleling the cap with a good one but, if its got low
enough series resistance this test may not work and only substituting another,
known good, cap will do.
You are working on a receiver of first class design which did not have this
problem designed into it so it must be coming from a defective component. The
tubes and by-pass or decoupling capacitors are the most likely. However, I
would also do a routine check for correct tube socket voltages and resistance
values. These can somtime give you a good clue as to what is wrong.
Note that where tubes on AVC are concerned tunable hum can vary with the
strength of the signal and with the setting of the RF gain control. This may
be another clue.
You have not answered my question about getting the same hum on other
radios at the same location. There are conditions where the actual signal can
be modulated by somthing often high voltage power lines nearby. This is an
effect familiar to those with auto radios.

Hi Dick,

thanks for useful info.

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.

I must also try to connect that same piece of wire to another receiver, to see
whether there is any difference (actually I have a second HRO to compare).

The hum heavily depends on signal strength and on the particular station (there
are some with no hum at all), so I would tend not to attribute the cause to
internal tube leakage.

Your supposition that poor screen / AVC bypassing could be the cause looks
interesting and I will do some tests at that regard.

However I did not fully understand your argument that putting a capacitor in
parallel to an existing one may not a good way to do the test. If I use a
low-ESR capacitor, why is the existing one not out of the game?

Thanks and 73

Tony I0JX