[Amps] 572b Amplifier

[Mel]

Greeting to all.

I built an amplifier out of salvaged parts lying around the workshop; a diagram of it is here (http://s111.photobucket.com/albums/n150/Paws264/?action=view&current=4X572b.jpg).  The amp worked, showing about 12-15 bd gain.  I sold it and told the new Ham owner I would stand behind the amp if he had a problem ('cept for the tubes).
  
It comes back dead and I repair the problem with the antenna transfer relay and the owner orders a new set of "matched 811-H" Tubes from California; these are Russian tubes marked V572b but re-branded by the seller.  I installed each tube individually and checked the idle current and grid current with no drive, one of the tubes was out of line while the other 3 were close.  I searched through his "Old" tubes to find one that was about equal to the rest.

The amp took off and flew as expected; the 4 tubes drew 160 ma with 2200 volts on the plate with no drive.  The amp put out watts and the graphite plates never reached color,  I was confident enough in the amp to "Test" the rest of the tubes.
 
I switched the old tube out and put the fourth "New" tube in, keyed it up a couple of times (soft blue X-rays out of that new tube) and then, all hell broke loose. The A side contacts of relay K1 welded itself shut and immobilized K1-b contacts in a in-between position; the safety protection "fuse/lamp" in the back of my Yaesu FT-101 radio was zapped out by the amp.  The amp boiled for a couple of seconds before I killed it.

I repaired the relay and the amp now has "unity" gain or less.  I electrically checked all tubes for shorts between grid and filaments and plate to grid but found nothing however, one of the tube draws 2 amps plate current when checked with high positive grid current.  Two of the other tubes checked "normally" while the last is around 400 mills of plate amps.

My question is about the "Failure mode" of 572b / 811H tubes; tubes are supposed to be more forgiving than this, I've had better luck out of '2879s that this.  What could have happened to the other tubes causing a major calamity?  All tubes look the same as they came out of the box, equal brightness, no smoke or gas.
 
ko0m
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