I think its MAGNETIC coupling from the power transformer to the audio
transformer.
-Al



Antonio Vernucci wrote:
Most of the strong AM signals received on my National HRO are affected
by hum.

I put a ceramic capacitor in parallel to each of the 80-type vacuum
diodes with no noticeable improvement. Does anyone remember which other
cure was proposed to solve the problem?

73

Tony, I0JX