Unless something's happened to them, Uncle Albert's has always been considered THE place to have your amp worked on or modified, especially tube amps. I think they're always backed up, though. Just what I've heard. There's a guy out in Plainfield that's supposed to be competent. Not northside, but I'll try to get his name for you. You should try pm-ing guitar junkie, he should know of someone. I'll give him a try, too.
What do you have, and what is it that needs done?
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Try Slowrider, too. I think he's up in your area. I'm on the far westside, Eagle Creek area.