Hum in a sortof 18W amp

[8wattjack]

I see in the schematic that only the normal channel triode uses a sheilded cable. Have you checked that the sheild is not grounded at both ends?

[8wattjack]

Also just checking the grounding scheme in the actual chassis itself. Whats on paper isn't always whats present. I am no expert on the switching problems mentioned but I have fixed a lot of hum and noise issues with checking ground arrangement and looking for ground loops. Sometimes simply foil ending the plate capacitor on that stage can help too. And/or upping the qualty of it. Works in 5150's.

[TexasTone]

I was having hum problems with a relay board. My solution was using a stereo plug instead of a mono plug. I'm not sure what you are using, but in my case the hum went away when using a stereo plug.

Interesting.  The hum is there with or without a plug.  I'll take a look.