----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
To: "AMPS" <amps@contesting.com>; "Jim" <w7ry@earthlink.net>
Sent: Wednesday, June 20, 2001 9:26 PM
Subject: Re: [AMPS] Filament voltage/bad advice
>
> > 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?
Just 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),........
True statement.......
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.
True....
>
> 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
73
Jim W7RY
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