>"## leaving the fil on the 8877 tube in the 9500 means excess wear and
tear on the tube itself"
Early on, I was more concerned about another issue. Some ceramic tubes
cannot be left in an indefinite filament-only state for long periods of
time. Eventually, a semi-metallic coating builds inside the tube that
affects grid circuit tuning, particularly with grid driven amps running at
VHF and above.
After college, I worked at RKO Radio in Chicago. Two Harris 20KW FM
transmitters were used in an alternate/main fashion atop Sears Tower. At
the time, Sears was the tallest building in the world. The IPA stage used a
pair of 4CX250B in parallel. We attempted to keep one transmitter on as a
hot-standby with filaments lit at all times. But after a couple months, the
IPA would require retuning. It would then reach a point where efficient
tuning was impossible and output power dropped. It wasn't a one-time event.
It occurred any time we ran the 4CX250B pair with filaments on and no plate
voltage applied. The effect was finally documented in a joint effort
between Eimac and RKO Engineering. The exact detuning mechanism escapes me
but I can likely get the documentation from my old boss who recently retired
from WLS-AM in Chicago. Ultimately, both transmitters were replaced with
Harris FM-25K models that used a solid-state IPA. Problem solved.
Now, I don't have any evidence that the same detuning mechanism occurs with
the 8877. Possibly the internal structures are different enough that it
really isn't a concern. But, having gone through a dozen 4CX250s during
the RKO experiment is concern enough to at least raise the issue. Keep in
mind that during the experiments, the 4CX250Bs would run without plate
voltage applied perhaps for several months. -- until a transfer was
initiated by us. At our present remote site, the 9500 is used on a daily
basis, from minutes to a couple hours per day. So, that in and of itself
may mitigate the problem if it were to otherwise occur -- and, the amp
operates at HF only.
Paul, W9AC
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