I used to live at 4170 N Marine, on LSD, and used the old guy at 21st Century TV Repair. Its a few store fronts down (heading west) Montrose from Montrose & Ashland intersection, next door to a killer hispanic restaurant 'La Sierra'. There's a dynamite little bakery across the street too.
The guy has thousands of vacuum tubes & is as old skool as it gets. His shop is hysterical, an experience every time. He has a tube tester also.
You might also ask Jon Van L, he has an audio store in that area. The guys at Decibel in Wicker Park are good too, as someone else mentioned.
matt