> I was able to increase the final tube life in a UHF television transmitter
> (channel 63, 717.0 MHz 1000 watts peak output) running the filament at the
> lowest setting....Just where the power dropped off then increase a
> little....4+ years of continuos transmit with the same tube..
Did you run several sample comparisons, or is this just an opinion
based on one tube?
Doing that with an indirectly heated tube, like an 8877, is a
disaster waiting to happen.
> remember the tube number. It was a tetrode running 3800 volts on the plate
> and about 800Ma of plate voltage and 375 volts of screen voltage....
>
> I have spoken with many broadcast engineers and that is how they do it
> too.....In low band TV (channel 2 and 4) and also in 100 MHz fm
> transmitters 25-35 kW range.
Reducing voltage on a single-band single-mode transmitter is fine,
as long as you don't go below the minimum factory recommended
voltages for MOX cathode tubes. It is NOT a good idea to use that
method for amplifiers that operate multiple modes and bands
UNLESS you know for sure you are not emission-starving the tube
on the worse band and worse mode.
Since very few failures in amateur service are from emission life
problems (the tubes simply don't see that many hours), it makes
no sense at all to go below the recommended operating range. It is
a good idea to not go above the maximum voltage, but people
forget excessively low heater voltage can decrease life in some
tubes, and will increase IMD in other cases.
By far the largest problems are from cycling the tubes off and on,
from outgassing or seal leakage, from poor operating, and from
normal tube manufacturing defects.
Correcting a non-problem can really create problems, although it is
fashionable to pretend we are doing something constructive with
our reduced filament voltage tricks.
73, Tom W8JI
W8JI@contesting.com
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