Art Audio Diavolo hum

I have an old model Art Audio Diavolo.  I like it, but it just started humming on one channel.  I switched all the tubes, and it still hums on that same channel.  Not affected by the volume.  I disconnected inputs, still not affected. Plugged it in to different outlets, used cheater plug, still hums.  Hum is loud, about 30 dbs, and kicks in after 30 or so seconds, like its warming up.  Plays music, but hum sort of ruins it.  Did something burn out?  I'm the third owner, amp is over 15 years old. Thanks.  

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Nice amp you have and most likely just need some new power supply filtering caps. 
C'mon ebm, that IS a nice amp, if a little's really worth fixing. Have it checked out by a competent tech. After all these years it could be as the previous poster said, your caps need replacement.
Agree with above, ignore ebm, time for new caps should be an easy job and should not only fix the hum but give your amp a new lease on life! And better sound as well.
Thank you very much grannyring, roxy54 and jond.  I like the sound, just bought new tubes a month ago, and it was working great until today.  thanks again.  
Very sorry fix it and hold on to it for another 10 years.Enjoy