Some tubes are made to exhibit red color under maximum CCS or ICAS ratings.
The transmitting tube datasheets often contain text such as "the plate
shows a dull red color at maximum CCS ratings", or "The plate shows a bright
cherry red color at maximum CCS ratings" or even "shows an orange read color
at maximum CCS ratings". For, some tubes description says "the plate shows
no color at maximum ratings".
One example is the RCA 4-400 tetrode. (a little less dissipation that the
3-500z triode). Here is a picture of two 4-400s operating as class B
modulators at or near their CCS ratings (in an RCA broadcast transmitter
where they are on 24 hours a day)
http://hawkins.pair.com/atcitynj/atcity14b.jpg
(makes a nice screen background for glass tube enthusiasts)
Sweep tubes were never intended for this sort of operation, and their
maximum dissipation ratings were quite low. An article on what maximum
ratings mean can be found at http://www.tubecad.com/december2000/page17.html
With the 3-500zg, the real cooling issue is the keeping the glass-metal
seals cool enough.Typically, the use of a muffin (or tubeaxial) fan blowing
across the tubes isn't sufficient to allow continuous operation at the
maximum ratings; this is why amp manufacturers (including Ten Tec, Qro etc)
restrict the power to 650 or 800 watts respectively in 'key down' modes like
RTTY, and also limit the maximum transmit time to about 10 minutes. Through
the use of a better cooling system, (like an air system socket and chimneys)
one can take advantage of the higher power operation. The trade off is that
the muffin fan is typically quieter and less expensive and smaller. For
typical amateur use (that is non-rtty) it's fine.
It is also interesting to note that when you just key the amp (no rf drive,
just bias) you'll quickly start to see anode color in the 3-500s biased for
class B or AB(unless you have a circuit that cuts off the tube in the
absence of rf drive). In the case of the Ten Tec, I think the tubes are
dissipating around 300-400w total which is clearly much less than the 1000 w
combined dissipation rating, and yet you will quickly see anode color which
will become a reddish orange.
With respect to tube life, these tubes last a long time (years) if operated
with any reasonable degree of care, which is what makes them so popular. If
you search long enough, you'll find people who have had them running for 20
years, others who lost them at first power up. I think at least 10k hours
depending upon filament voltage is representative. Now, 10k hours is about
1.14 years continuous operation, but in amateur use, that translates to many
years. I've read the record for continuous operation is held by a thoriated
tungsten filament large transmitting tube operating at a Los Angeles
broadcast station for 10 years continuously.
The most common failures are filament failures, gas arcs causing damage (and
you know about anode color and gettering), excessive drive causing grid
damage. There are all kinds of other things that could go wrong.
A great source of information is Eimac's "Care and Feeding of Power Grid
Tubes" which has be reposted in its' entirety at www.eimac.com (click on
care and feeding).
Another article, on filament life extension, is "Extending transmitter tube
life, Eimac application bulleting No. 18", at www.eimac.com (click on
applications)
Also see, "Techniques to extend the Service Life of High Power Vacuum
Tubes", Walter Johnson, VOA,
http://www.burle.com/cgi-bin/byteserver.pl/pdf/tp202.pdf
Also see, "Extending the Life of Power Grid Electron Tubes", Alper;
http://www.svetlana.com/docs/TechBulletins/technoteNo54.html
For some info on tube maintenance from Econco, a tube rebuilder at
http://www.svetlana.com/docs/TechBulletins/technoteNo54.html
My own advice would be, use the amp, enjoy it. The more you read about
obscure failure modes, the less you'll enjoy it. ;-)
73s
Ed
N1TS
----- Original Message -----
From: <jsb@digistar.com>
To: "R. Measures" <r@somis.org>
Cc: "AMPS" <amps@contesting.com>
Sent: Wednesday, December 10, 2003 10:03 AM
Subject: Re: [Amps] 3-500Z plate durability
> On Tue, 9 Dec 2003, R. Measures wrote:
>
> > >I read that the heat generated by the kinetic energy of electrons
hitting
> > >the plates is what creates the heat (not the plate current),
> >
> > ** Electron movement is "plate"/anode current.
>
> Okay - I had read it the wrong way then. I read it as though the energy
> striking the plate (and not the movement through it) was creating the
> heat. I wasn't totally sure about that.
>
> > >Do 3-500Z tubes get "soft" or do they just suddenly drop output and
fail?
> >
> > ** All thoriated-tungsten tubes gradually loose emission as the carbon
> > in the filament's tungsten-dicarbide surface layer is used up while the
> > filament is lit .
>
> Wow - so regardless of transmitting or any other use, while the filament
> is lit, the clock is ticking? Ouch. How long does the 3-500Z
> tungsten-dicarbide surface layer last? 5,000 hours? That's only 208
> days, not taking into account the cycling of powering on and off the
> filament. I used to leave my old Drake gear on for a couple days at a
> time to avoid cycling the filaments - that's disappointing if I have to
> weigh the differences between cycling the filament (which to me is pretty
> bad for it) or leaving them lit for 48 hours at a time, etc.
>
> > >And do they fail because of high grid current causing the grid to warp
and
> > >short or because of some other reason?
> >
> > ** 3-500Zs can fail from a sudden burst of grid-current during an
> > intermittent VHF parasitic-oscillation, but only if the Pi-network tank
> > is of a low pass design so that the VHF energy being generated can not
> > escape to the antenna. In this case, the burst of grid-current from the
> > unloaded condition can cause the unsupported center of the 3-500Z
> > filament/cathode to be bent sideways by the perpindicular EMF and touch
> > the grid-cage. [see photo in September/October 1990 *QST*, "Parasitics
> > Revisited"]
>
> I thought all "proper" modern amplifier designs were geared to eliminate
> VHF from ever making it beyond the amplfier chassis.
>
>
> thanks,
> Jason
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