"Antonio Vernucci" wrote in message
...
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).

Tony

You have been offered several suggestions, and I offered some advice
and never heard a reply back to some questions I asked regarding the
antenna system and grounding.

Tuneable hum can indeed be caused by poor cathode RF bypass caps
in the RF and IF stages. The lack of adequate bypass capacitors would
allow any RF/IF stage with cathode/filament leakage to be modulated
by the AC signal on the filament. No signal, no hum.

Pete k1zjh