> > 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.
Point well taken, Carl. I am making a lot of assumptions! Obviously, there
are many ways a tube can fail: faulty vacuum seal, grid contamination,
overheating, inadequate filament voltage, excessive grid or screen current,
etc. The point I was making is just that, other things being equal, the
worry from pushing a tube for maximum output comes from possible overheating
(signifying inadequate airflow) or excessive cathode current (which can ruin
the cathode emission). If the tube is adequately cooled and the cathode
emission isn't pushed beyond specs, then I don't see that running the tube
below its ratings gains much in terms of life expectancy. However, your
point, which is that the conversative maximum ratings of the original
Russian GU-74B are more realistic than the "liberalized" ratings of the
Svetlana rebranded 4CX800A, may be quite correct. In that case, I agree that
one will get maximum life by following the original specs.
I'm interested in your cathode feedback experiment with your NCL-2000. I
note Svetlana recommends adding cathode resistance "to provide degeneration
for improved linearity and reduced zero signal plate current." It's pretty
clear why doing so would reduce zero signal plate current, but less so why
it should improve linearity. Your measurements would suggest that it
doesn't.
Re the ARD-230A, Jerry K8RA and I ran the 3CX800A7s at 2200V plate voltage,
which was the limit Eimac published at the time. I forget what the grid and
plate current trip points were, but the amp would not let one push it much
beyond 1500W output. It would run key down all day at that level, and the
3-tube ARD-230C version (with a much beefier plate xfmr and heavier tank
circuit) would produce 2500W continuously. The ARD amps were very nicely
built, with first-class components. The problem with them is that they used
DC motors instead of stepper motors to turn the bandswitch (a model 86). The
inertia in the motor and gearing meant that the wiper often wasn't centered
in the switch contacts, which led to frequent switch arcing and failure.
Stepper Motor technology wasn't advanced enough in those days, which is a
shame, because the ARDs would have been great amps if they had used
steppers.
73,
Jim W8ZR
>
> 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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> >
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