Interesting - you inspired me to fix the hum on my PRS Custom 50. Lived with it since I bought it off Bernie Marsden's tech, and thought it was "normal". Although I've never had amp generated hum with anything other than a Marshall before thisMy Tucana 3 started humming pretty bad. I just dealt with it for the past several months. But being at home under the Covid-19 situation I decided to trouble shoot it. Narrowed it down to the effects 5814 effects loop tube. Just got an NOS Sylvania this morning - hum instantly gone. Not sure if its possible from this particular point in the preamp, but the amp seems to sound better. Less crisp, a bit warmer. Some of the strident nature of my Forte 3D 1x12 is gone.
Could be my brain... but I think its sounding better.
PS. @fusionbear what does the effects loop tube do in the circuit besides drive the effects loop? If it was a bad tube could it have affected the overall tone of the amp?
Did the usual swaps with known good glass, until I got to V6 which had a JJ ECC 82 in it. Checked the PRS tube guide which called for a 12DW7, which is a JJ ECC 832
I've not come across 12DW7's before. They are apparently 2 mismatched triodes in the same tube: half an ECC83 & 82
Replacement is on the way, so we'll see if that ECC82 caused the hum