3-500Z tubes have been notorious for short shelf (storage) life since Eimac first started making them. Almost everybody who's designed and manufactured amplifiers and used thousands of these tubes recommends: Don't keep spares on hand unless you rotate them through a working amplifier, and get them "very hot" (anodes red/orange) about every six months. Otherwise spares often go bad on the shelf, becoming gassy beyond the possibility of repair by the gettering process, which only occurs when the anode gets very hot (filaments won't help).