1956 VM sound fading out??

[rob Cashin]

I have a 1956 Voice of Music 1285 Tri-O-Matic. It plays beautiful, i bought a new needle and cartridge for it, greased it up and it works great. It has been recapped recently so the old owner told me. Only problem is after about 15 min of being on, the sound starts to fade out and sounds like its being turned on and off. Ive been told its a tube going bad. I dont have a tube tester and ive found new replacements for it. a couple concerns, for the price of the 3 new tubes for it seems cheap, is that anything to worry about? And does it make a difference if i use GE or RCA tubes instead of genuine VM tubes? No idea if those exist either way. Any input helps, thanks a lot.