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

"K7JEB" wrote in
news:3gPYb.2132$4K3.74@fed1read06:

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




Hi Jim

Sure, plate current went far above the normal 500 mA, particularly on 7
MHz, where the rig blew its fuse.

One point you mentioned is very interesting. You wrote that when a tube
gets gassy, plate current goes up to the point the plate can turn red hot.
I'm no specialist and i would have thought that the gas molecules in the
tube would be an obstacle for the electrons so plate current would
decrease.

Vy 73 de HB9SLV

Ray di Tutto" wrote:

One point you mentioned is very interesting. You wrote that when a tube
gets gassy, plate current goes up to the point the plate can turn red hot.
I'm no specialist and i would have thought that the gas molecules in the
tube would be an obstacle for the electrons so plate current would
decrease.

Vy 73 de HB9SLV

Sorry to be so late getting back to you on this. I had
forgotten I left a thread open over here on .boatanchors...

What really happens is the gas molecules (atoms) become
ionized due to the high plate voltage accelerating the plate-
current electrons to energies above their ionization
potential (typically 10 - 15 electron volts). When
this happens the now-ionized gas becomes a very good
conductor (almost a dead short) and the control grid
is totally ineffective in limiting the cathode-to-plate
current flow. The only thing that can halt the process
is the removal of the plate voltage. Unfortunately,
there is usually enough energy left in the conduction
electrons striking the plate to increase its power
dissipation disastrously.

This process is actually used to good effect in mercury-
vapor rectifiers and thyratrons because that "dead-short"
effect means the losses through the conducting tube are
low. Both of these devices "fire" (ionize) at some
forward voltage and then stay in conduction until the
AC driving them reverses polarity.

Jim Bromley, K7JEB
Glendale, AZ