Thanks for your info! I'll check the tube soon. I just opened the amp (I was out of use for at least 2 years) and found some cables in very bad condition. One of the guesses I got was about a bad connection which somehow was fixed after shorting the circuit with the speaker cables. I'll repare the cables and see what happens, if it still does not work I'll check more bad connections and the tube.

Thanks a lot for your help!! I'll post the results here.

Best regards,

Jose