The Ubiquitous Radio Hum

[Electronic M]

There are several causes of hum in radios such as poorly filtered B+, capacitive / inducive pickup of the power wiring by adjacent parts on the chassis, coupling of hum from heater/filament to the signal elements of a tube, pickup of ambient hum (ever touched a phono input of and amp and heard the amp hum...Your body was acting as an antenna for 60Hz).

Isolating the main source is useful to fix it. pulling the plug to see if it is clear for the second or 5 that it runs off stored filament heat and cap charge will tell you if it is coming off the line or from the air. Replacing one on the power supplies (A or B) with a known pure DC source (like a battery) will help diagnose which supply is the issue.

If you ever decide to build a tube phono pre-amp from scratch you will be forced to learn these techniques...I helped a friend, who is a fellow engineer, make one. I gave him all the basic parts to make the circuit he found and a basic power supply and helped assemble the prototype...Some months later he gave it back because no matter what he did he could not get the hum down to an acceptable level...It was fun/educational whipping that thing into shape.

In vintage equipment some hum may be part of the design (especially cheap stuff and pre-octal based sets)...I look at it this way: if it is not easy to notice at normal listening levels then it is probably close to it's design standards.