Can’t hear any hum from the power transformer.
Checking a bit further as I did not check the right speaker. (thanks for your assumption) The right speaker is quiet. I have the FM3 plugged into the Preamp, with two prong plug. Preamp is plugged into multi power box with two prong plug. ST70 plugged into power box with 3 prong plug.
Thanks for suggestion and link will buy one.
Shorting input made no difference that I could detect.
On checking with a VOM from main ground the builder used , I tested to multiple other ground points around the chassis and boards. I was getting .010-.012 volts.
When I used a two prong plug the hum was a bit louder. Did not notice any difference when
I reversed the plug in socket.
All wiring that went to ground used same ground point. (lug of hold down for power transformer.)
“ So, the dedicated receptacle *here* has same-gauge home-run straight to the panel, but the undedicated receptacle *there* is at the and of a 7-device daisy-chain. So, any device plugged in *there* will be 'above ground' relative to *here*. “
I have always been a bit “hazy” when I watch a youtube videos with people repairing amps and old radios when they mentioned this. That one sentence has made it very clear to me now. Thank you for putting it in a way I could understand.
Understand about a cover board for the driver board. We have three small dogs (20 lbs + -). The
youngest like to jump from any surface she can land on then on to the next. I could see her landing on top of the ST70. I will use the cage I have for the parts chassis if I do not put it in
an audio cabinet with glass front. Also have a grand daughter to worry about when she is around.
On testing my unit the hum is less with a three prong plug. Will be using it unless in the future I find it is causing problems.
Also did following:
I also swapped the output tubes, made no difference. Swapped the ECC82EHs with no difference. Will have to get another 12au7 to test with.
Went over the wiring a few times and could find nothing that looked like it may be casing the hum in a psychical nature.