[morzh]

Found this

https://antiquewireless.org/wp-content/u...del_84.pdf

It says the same thing:
Filament Balancing Pot
When the set is tuned off station, there is enough gain so that you hear a small
hum. This hum has nothing to do with filter capacitors; it is due to capacitive pick up
from the filament/heater circuits. I found it loud enough to be annoying.
There is a way to reduce this hum that has been used in audio amplifiers for many
years. A small pot is connected to the two sides of the filament/heater circuit and
the arm of the pot is grounded. The pot is adjusted for minimum hum. Adding this
pot balances the ac pickup on one side of the circuit against that from the other
side, effectively nulling it out.
An incentive to make this addition is that local Radio Shack stores stock 25-ohm 3-
watt pots that are ideal for this application (Catalog number 271-265). Purists will
have to decide whether it is ok to add something to the radio that was not there
originally

-----------------------


I thought of the same and since before yesterday trying to get an offer through on some 10 ohms 5W small pots. Those are the cheapest I found. But my guess was correct: the hum is inherent to the circuitry.