[TenTec] Failure Modes for 811a tubes?

[Steve Berg]

I bought a nice Centaur amplifier from a list member a few years ago, 
and finally got around to putting it on the air.  I took out the 572B 
tubes that were installed in the amp, and put the original 811A tubes in 
there so that I would be less likely to blow up by 600 watt autotuner. I 
have since bought a new 238C tuner from Ten Tec.

It was working fine, until the net last Sunday.  Suddenly, the output 
power dropped.  I noticed later that the heater in the center tube was 
out.  I cycled the power switch, and it came back on for a while, then 
went dark again.  I have two sets of 572B tubes for the amp, but I am 
curious about this being a common problem with 811A tubes.

Any pointers?

Steve WA9JML