Jim, if you operate your GU74B tube at no more than 800W CW or PEP output
in
AB1 SSB service then the tube will have a very long service life. DO NOT
push it up to 1000 or 1200 Watts Po like some ham-radio clowns do!
For your proposed 3 holer, that equates to quite a healthy 2400 W output,
or
circa 1500 to 1600 Watts for popular two-tube amps like the fine
ACOM-2000A.
As I remarked in an earlier post, one of my amps with a single GU74B has
been used as a station workhorse for over 10 years at the 800W and under
level and it's still the original tube. The amp today happily produces
800W+
Po and exhibits no sign of power output degradation; so at that rate the
attainable tube life when used at or slightly above the original
conservative Russian language data sheet spec appears to be indefinite!
Precisely as one would expect from a rugged mil-spec purpose designed SSB
tube!!
There is so much nonsense and misinformation in ham circles about this
very
fine tube. If you want to know how to optimally deploy these tubes, go
look
at the ACOM-2000A specifications and carefully study the schematics.
Salient
features are cathode NFB, and an efficacious tri-state EBS system to
achieve
a small desktop footprint with adequate blower CFM and quite low fan
noise.
When it come to the outstanding performance of the GU74B's bigger brother,
the 2500W plate dissipation GU84B, then look no further than those
exemplary
amps designed and produced by the Slovakian company OM Power.
Moral of the story is very simple: don't push any tube beyond its sensible
ratings as per the manufacturer's data sheet. Work within those parameters
and excellent results will be achieved, especially in respect of the
GU74B.
The only folk disappointed by the GU74B are those who flogged them too
hard
and tried to deploy them for purposes never intended. No surprises here.
Yes, best results with EBS stem from using PTT mode. Yes, keep an eye on
maximum plate/cathode current and always keep it within published ratings,
and use sufficient CFM cooling.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Carl
Sent: Saturday, 27 October 2012 1:26 AM
To: Jim Garland; amps@contesting.com
Subject: Re: [Amps] Additional comments, re GU-74B/4CX800A
++SNIP++
** How do you justify that tube life statement Jim?
From Alpha on down owners have had reliability problems when pushing those
tubes.
In the real world few hams run them at 1500W a pair and instead drive with
the typical 100W xcvr and tune for full bore in the 2200-2400W range. The
higher power rigs "may" be backed down to 100W but they could also get the
Magnum version. A pair of 3CX800A7's with a real 800W rating can handle
that
for years.
Incidentally, although some folks have asserted that the GU-74B life
expectancy will be extended if the tube is not pushed to, e.g., 1000W
output, I don't see why that is necessarily true. As the above numbers
indicate, at 1000W output, the tube is only dissipating 500W, and the
other parameters are well within maximum ratings. Assuming adequate
cooling is supplied and that the filament voltage is maintained, I'd
think
the biggest killer of tube life would be drawing excessive cathode
current. At 0.6A cathode current, the tube is being operated
conservatively and still producing 1000W of RF.
** You are making a lot of assumptions there Jim. One is that the tube is
actually rated for long life at its intended military service. In
actuality
tubes are changed when a certain number of hours are reached. The GU-74B
commercial ratings in AB1 MF/HF service is 550W output at 2000V with a
300V
screen. The all Russian original spec sheet is the same as the later one
that is Russian and English and aimed at capturing some of the commercial
market. Then Svetlana simply made the specs to fit their needs.
The "Limit Operating Values" paragraph is there for a reason as it is in
tubes from all countries. The 600W Pd is an absolute max, not daily
operating.
You have enough tubes, set one up in a test jig as a monoband amp and run
30WPM CW thru it 24/7 and graph the Pout degradation at an initial 1200W
out. Also do IR tests on the anode using the typical ham amp cooling.
When you were involved with the ARD-230 did you and Jerry conform to
Eimacs
3CX800A7 specs or push them a bit?
Carl
KM1H
73,
Jim W8ZR
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