Antonio Vernucci wrote:
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.

Tunable hum is easy: most of the time it is caused by either heater-cathode
leakage on the AGC, or pentodes in the IF amplifier, or bypass capacitors on
the AGC, or IF tubes.

Tunable hum is a modulation issue, the small amount of ripple that comes from
the heater-cathode leakage is amplified greatly by the AGC circuitry, and
AM modulates the IF signal with power-line hum.

A little tube swapping very often finds the problem. Start with the AGC/detector
tube, and then go to the tubes that are controlled by the AGC line.

Make sure that you put any tube that isn't bad back in its original position.

If you look at the power supply, you will probably see a multi section electrolytic
capacitor that has resistors between the sections. Usually, the AGC tube is powered
from the most filtered section of the power supply filter, so if the hum is coming
from there, it is usually everywhere.

-Chuck