(03-07-2016, 02:27 PM)goltigut Wrote: another question : i don't have a scope but can i measure the power supply rail ripple measuring the ac voltage on the rails ? if yes, what readings should i be getting for a "clean" rail ?
I don't know off hand.
It's simply good to know about how much ripple you have...it starts as AC, it's rectified to DC...not that you don't know that. But no power supply is going to be perfect.
Current drawn will be a factor, as is your target/actual measurable DC voltage level.
From This page: http://www.valvewizard.co.uk/smoothing.html
"The ripple voltage is often expressed as a percentage of the maximum DC voltage. A typical figure might be 10% for a push-pull amp or 5% for a single-ended amp, though this is highly dependent on individual circuit requirements of course."
seeing how/where it's smoothing out could be useful. Or not--we don't quite know the problem...but having the info won't hurt now or in the future (if you resolve the hum, then later run into other issues etc), and if you amp is already open for measuring where it's at right now, etc.
It can also help, as if you know where it's at on the power supply, but get really weird DC readings or ripple at various other places (at triode pins, etc) you'll be tracking down some of where the hum is being injected...though right now it sounds like the answer to that is "everywhere."
Obviously you will see most ripple at the power amp's plates, as it's considered "least critical." Next will be the screen tap...and so on, per the schematic. It should be decreasing with each r-c filter in the power supply chain.