[AMPS] Tube failure witnessed

[km1h @ juno.com]

I just watched a 572B  go kaput. After working on hundreds of amps over
the years I was looking at the right place at the right moment. 

It was in a SB200 that I had picked up for 6M conversion. Upon initial
testing, before I start the mods,  all was fine on 40M ( where I always
start). I then went to 10M and after about  5 seconds into tuning one
tube got much brighter and the Ip went to the pin. 

Without drive and no relay closure there is slight Ip and negative Ig.
Replaced that tube and all is OK.

So what made the increase in filament brightness??  And it is permanent,
I plugged in without HV to verify. There are also no shorts measurable on
a Simpson 260.   

There are no other failure signs, switch, chokes,  resistors, etc are all
OK. Parasitic suppressor R is 54 Ohms (47 Ohm resistor). With the new
tube the amp is running 700W out on 10M, stable as a rock.  

NO, I did not hi-pot it or take apart to play with gold. There is nothing
rolling around loose either ( except maybe myself). Neither was there any
gas glow. 

I have seen this brightness symptom before in 572B's and 3-500Z's that
have come to me after the failure.

So what are the theories??

Tnx....Carl   KM1H

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