3-500zg- no output

[John Godfrey]

My brother has an sb-220 Amp, that was retubed with a set of Taylor RFparts
3-500ZG tubes just 3 weeks ago. It has worked fine, yet last week, he said
his power dropped off to half what it should be. He was operating it on 10
meters. I looked at it and the first thing I found was the resister used in
the factory Parasitic suppressor, and going to one tube was cracked and
burned black. I replaced that resister with one of the same, as I had a
couple of extras from a previous order of the same for another fellow. When
I tried to tune it up, bang and smoke from the same. I ordered the Richard
measures suppresser kit for 2 3-500z tube amps and installed per directions.
This time, no pops or bangs, yet still about half the output. So I inserted
a carrier and held it for a minute or two to let the plates began to change
color. One tube starts to turn red on the lower plate rings, but the other
never changes color. Obviously the one making the color is the one making
power. I swapped the tubes and re-installed in the other sockets. The same
tube made color, as before. In other words, I did this to rule out any issue
with the amp other than the tube, and so it shows to be the tube, based on
my test. The tube has not been to Hot, the red ink on it is still fully red
and the plate on the tube seems to look as new. The filament lights as new
of course. My question is why would the tube go from 100% to 0 with no
warning and what is it about the tube that must have went bad.
 
 
73 de  John Godfrey 
KE5NZY BARC Pres.
ASTEN NM - IND CO AEC

 

 

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