Ohh, this parasitic nonsense again?
> I'm sure using a sledge-hammer to drive paneling nails works, but
> honestly, it's better to do the job right to begin with.
>
> If a parasitic can be determined and suppressed, without the need for
> brute force automatic shutdown of a circuit, then that is IMHO, the
> correct choice (at least until someone shows me that tree 8877's grow
> on).
8877's have had poor service history while they were at Salt Lake.
While there were periods of time in the 80's when you couldn't get
a tube that lasted over a few hours, there were periods when they
were fine.
They just aren't as long lasting a tube as a thoriated tungsten tube,
or a tube with wider internal spacing between grid and cathode.
Tubes in the 80's were so bad we ran 8877's in a tester (like we do
3-500ZG's now) that cycled the filament off and on, and had a
latching SCR between the reversed biased grid and the cathode,
watching for shorts. We'd come in after cycling a batch of tubes
overnight, and six or more fault lights would be glowing out of every
eight tubes.
8877's failed in voltage regulators, in MRI gear, in BC stations, as
well as in Ham gear like AL1500's. They failed in nichrome loaded
amplifiers, and they failed in tube testers that don't even apply HV
to the tube.
As tough as it might be to swallow for some people, tubes are the
weakest and most unreliable link in ANY gear. And if you mistune
a PA, or run it without a load or if the load faults just for a
millisecond, something will often arc. We may like to blame
everything on the parasitic fairy, but there are multiple causes of
failures.
The parasitic nonsense about damaging switches and tubes has
been gone over a thousand times, and those that "believe" are
welcome to believe. It's Christmas, and I'm sure some people really
believe in Santa. After ten or fifteen years of what amounts to only
one person fueling the fires, there is little reason to point out the
same clear and proven technical points over and over again.
73, Tom W8JI
W8JI@contesting.com
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