Hello again,
Sorry for the delay, I've been sick with flu a few days.
So to recap, I've replaced both screen resistors with 470R/5W wire wound ones and when I received the 100x probe for my scope, I took the reading of both power tubes' anodes using a 60W bulb limiter. Here are the results:


I then took out the limiter and switched the amp on with tubes but no guitar. The amp was silent and behaved as expected when tinkering with the controls, channel switch etc, BUT as soon as I plugged the guitar in the noise started again. Switching it off and back on after a few seconds wouldn't "fix" it.
I also checked bias and the DMM showed 62v, so that should be ok I guess.
This kind of behaviour reminds me of bad solder joints I've had in some of my past builds, but I'm not sure that the failing tube could cause that in this amp (all my builds are p2p).
All your help is seriously appreciated

Thanks in advance
Sono