Thank you all for your support and opinions. Unfortunately, I do not have an oscilloscope, so I could not make the measurements, however I have no hum when I use other rectifiers (RGN1064, RGN1054, RGN2004, U14) so I can rule out hum due to magnetic coupling from power transformer and chokes.
Originally Posted by Alan4411
I thought the MU12 was a 'direct rectifier' and the filament is the cathode like the RGN10xx rectifiers. In which case it should not matter which end of the filament is used as the DC feed to the first capacitor.
But if as you say your MU12 is a sort of indirect type, you have to use the end of the filament connected to the cathode to feed the first capacitor. If you don't you will have the 4 volt ac supply superimposed on the DC output.
The UU5 is an equivalent with a cathode on pin 3, check the valve socket, and make sure pin 3 (not pin 4) is connected to the first capacitor.
The other difference is that the MU12 needs 2.5A at 4 volts, slightly more than the RGN10xx ones, is the filament transformer big enough? Failing that perhaps the valve is not so well balanced and you have a little half wave hum?
Thank Alan for the tip, I already tried to change the connection to the first cap to pin 3 (previously, I used pin 4) but there's no change in the hum
All in all, after some trials without success, I came to the conclusion that the hum is probably somehow due to the defective of the MU12 tube rectifier.