Angel Vilaseca, HB9SLV, wrote:

-why was one of the tubes plate glowing red?
-Was it the good or the bad tube? Which one of the two can I keep as a
spare?
-And why dit it glow red? Could it be that the grid could not stop the
electron flow and the power dissipated in the red hot plate was DC power?

w4udx:

replace them both with matched 6146W's and forgeddaboudit .....

HB9SLV:

Sure, that is what I did and it worked, but I am
still left with some questions about what happened
exactly and above all: will it happen again??

My guess is that one of the tubes got gassy and started
drawing a lot of plate current, hence the red-hot plate,
and possibly the reduction in output power. You didn't
mention whether you monitored the plate current while
tuning up; you only said you looked at output power.

I wouldn't keep either of the old tubes as a spare.
One is completely shot and the other is well on its
way. In this application, it is probably best to
replace both tubes even if only one goes bad.

But I'm glad you're back on the air with no more
problems.

Jim, K7JEB