I've figured that the circuit is a standard EL84 push pull with 12AX7 driver, and there are 2 problems:
A - The left channel is working, but the volume is very low.
B - The right channel sounds normal, but it makes white noise (even with the volume turned down).
I have tried the following:
1 - Change all valves for new ones: no difference here
2 - Check the voltages in all the pins of the valves: they are very simmilar in both channels.
3 - Remove the 12AX7 valves: the noise is still there!. So problem B is not coming from the driver stage; it is some component of the output stage that is noisy (and it's not the valve) any clue here?