I think the point Tom has made twice now over the weekend is for BC service
that might work, for ham service it doesn't.
Until we are going hammer down on these amps for a solid 24/7/365, that
scene doesn't fly.
Gotcha Tom,
You should see what an 8938 does when it gets to 2KW out on 432. It goes
into thermal runaway. Then you really need to turn down the filaments.
That's a really scary deal. You flip from 5V down to about 4.75 or even
4.65 when hot.
Talk about complexity. They didn't even think about that feature on my Hank
3004. I've had it 15 years and used it about twice. Like playing with
neutronium.
That amp also has a zero center grid meter. I get minus 50 grid mils tuning
with a carrier and when whistling on SSB I get positive 50 mils.
Hey Ted whats up? Oh yeah that's completely normal with that amp. Yeah
might want to put that in the manual.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Tom W8JI
Sent: Monday, January 29, 2007 9:28 AM
To: Harold Mandel; amps@contesting.com
Subject: Re: [Amps] Filament voltages
> Lastly, Eimac publishes a technical bulletin for
> maximizing tube life
> through the measurement and control of filament voltage
> whereby
> emission is used as an indicator, with the tube heater set
> at just the
> point where the "knee" in emission increases, and then
> just under,
> maximizing the tube life, and further, Eimac has a plan
> whereby this
> adjustment can be brought to bear in the last hours of
> service, both
> indicating
> a need to start the purchase process for replacement and
> also adding
> many hours of useful life.
Hal,
I think you are mixing models or applications.
I think you are considering things that work for thoriated
tungsten tubes operating at ONE constant power level (like
an FM broadcast transmitter) with another totally different
application.
1.) A metal oxide cathode tube will be poisoned if operated
where peak emission demand reaches available cathode
emission. We NEVER want to run a metal oxide cathode tube
anywhere near peak emission limits, and that's exactly what
we would be doing by adjusting for a power drop and then
adjusting voltage a small amount higher. That is a very bad
idea and very bad advice.
2.) In amateur service peak cathode current varies from band
to band and with drive power and other variables. You would
have to set peak emission under the worse case conditions
for all bands, modes, voltages, and tuning conditions. If
you set it too low with a thoriated tungsten cathode tube
you just lose IM performance and efficiency. If you do it
with an indirectly heated cathode like the 8877, you
splatter and eventually will lose the whole tube by
poisoning the cathode.
I've replaced several low emission 3CX1500A7's and
3CX800A7's, and every single one was in amplifiers that had
life-extending reduced filament voltage! It's a dumb idea to
reduce voltage on an indirectly heated cathode power tube to
just above the point when output power drops. It's just a
useless waste of time, but not harmful, to monitor voltage
to 50mV when the failure mode in amateur service is
virtually never emission life.
73 Tom
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