The Valve Wizard web page is about tube amplifiers. They have a very good document on grounding issues in tube circuits. So read their 16 page Grounding document to learn about grounding (both general information and tube amplifier specific information).
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Tomi Engdahl says:
Two Types of Tube Amplifier Hum and How to Determine the Source
https://www.youtube.com/watch?v=GrVtX0QGNls
In this video, I discuss two types of hum found in tube amplifiers, how to easily differentiate between the causes, and how to effectively remedy the situation. The technical aspects of this discussion are explained in basic, comprehensible language. I hope you find it to be both interesting and informative. NOTE: The frequencies stated in the video (60Hz and 120Hz) refer to 60Hz primary AC. If your AC source is 50Hz, then the frequencies would be 50Hz and 100Hz.
Tomi Engdahl says:
Let’s try to isolate the problem. Take RCA cable. Plug in one end to power amplifier inputs. Short on both L and R channels the center pin of RCA to RCA shied. Make sure that L and R ends do not touch each other. I expect that the amp is quiet now. Connect L and R grounds together on cable end not connected to amp -> if noise comes back it is loop noise coupled to cable and/or amplifier inside. Disconnect L&R grounds again on cable end and do L to R ground connection shorting on amplifier RCAs -> if noise comes back the problem is inside power amplifier ground arrangements. Make sure you have proper single point grounding and proper routing of signal cables.
Tomi Engdahl says:
I all else has been attempted, a possible common mode rejection issue could be the source. Below is essential reading for all DIY, repair techs and system installers. https://web.mit.edu/jhawk/tmp/p/EST016_Ground_Loops_handout.pdf
Tomi Engdahl says:
Ham Busts The Myth Of Ground
https://hackaday.com/2024/06/04/ham-busts-the-myth-of-ground/
Ground is a Myth! | 2024 ARRL National Convention
https://www.youtube.com/watch?v=VvE570NPmmI