Are you sure the hum isn't real? A lot of solid state
devices trash up the power line. Rectifiers are a big
source, lamp dimmers and fluorescent lamps are another.

Barring that, look for heater cathode shorts from the
detector tube back to the power amplifier tube.

-Chuck

My problem is called "tuneable hum". It has nothing to do with filter
capacitors, transformers coupling, cathode to filament leakage, etc.

It only shows up on certain stations. It depends on the path followed by RF
signal (e.g. if it passes through the rectifiers).

Typical cures a

- bypass capacitor across the power supply rectifiers
- bypasscapacitor between mains and ground.

But there still are some stations affected by a considerable hum. I was seeking
some suggestions on what to do more.

73

Tony I0JX