{Collins} 4CX250B Behavior in my KWS-1K

["Roger Shultz" <nj2r@xxxxxxxxxxx>]

I finished up the restoration of my "K" and to reward it for being the
easiest restoration I have ever done, I thought it might like some new
4CX250B's.

So, I installed a nice matched 1998 date code pair of Eimac's and
neutralized the PA's according to normal. All seemed well. I had let the
tubes "cook" for about 6 hours before applying HV.

On 80 meters, I tuned and loaded the PA as normal and still all seemed well.
Suddenly while whistling up on LSB, I heard the dreaded "WHUMP" and lost HV.
The nice expensive HVB-1 had gone. I did not notice a sudden spike in PA
plate current and it had never blown a fuse previously during all my testing
and alignment.

I hoped it was a fluke but the next day it did the same thing this time
however I noticed the screen current drop to zero from its usual 20 ma or so
level.

Today I wanted to see if one or both tubes had a problem so I tried one at a
time with one of the original good Eimacs. In one case one seemed to have a
slightly negative drop in current but neither resulted in blown HVB-1's.

So group of knowledge...what do you think was going on? Should I just keep
the new tubes and not use them together? Such a mystery!!!

73, Roger, NJ2R