ORIGINAL MESSAGE:
On Tue, 19 Oct 2010 20:34:53 -0700, Kevin Normoyle
<knormoyle@surfnetusa.com> wrote:
>
>Now: It may be true that none of these common zener failures
>are due to overtemp. If so, then what is causing them? Is
>RF causing them? or what?
REPLY:
If not overtemp, it's almost certainly arcing inside the tube. Even good
tubes arc occasionally with no harm to the tube, but there can be harm
to components in the HV- to HV+ path and that includes the bias
zener(s).
IMO, for a legal-limit amp a ten watt zener is marginally adequate. Even
if it can dissipate the cathode current x zener voltage, there isn't
much left to absorb arcs. I would recommend a 50 watt stud mount type.
There is a certain amount of guessing in the rating because you never
know how massive the arc is going to be. If the 50 watt zener ever
blows, they do make them larger but with any luck, 50 will do.
73, Bill W6WRT
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