tube testing oddity

I have four 6550 tubes that are acting a bit crazy. When I first run an emission test the needle goes high into the " good zone " however after about 3 minutes the needle falls back down quite a bit. Still reading good, but not near as good as first registering. I tested other 6550's as well as EL34's and they all test normally. I have never had any tube do this.


I personally have had situations where my tube tester has failed me. I've had tubes test well and than blow a fuse in my amplifier and I would have to locate the bad tube by trial and error and replace it. I never had your situation, but just guessing, there is a possibility that once the tube filament becomes heated, the tube is leaking emissions. I am very curious about this anomaly, I hope someone out there can provide a definitive answer.
I understand that Vintage Tube Services will put your tubes through a battery of tests for $15 each,Might be worth the time and money.
Thanks for the advice but these are not NOS. I can buy new ones for $30 each. Just curious why a matched quad would exhibit such a pattern. I could see it if one of them was doing it, but all four at the same time? Very odd indeed.

Which brand?