[ETO_Alpha] Re: 3CX800A7 versus 3CPX800A7

[Jan C. Robbins]

Very nice write-up, Dave!  Many tks. Your info corresponds with mine,
EXCEPT that it is Eimac that claims the higher output of the 3CPX's, and
the longer tube life, and the former is commonly observed (none of us
have been around long enough to observe the latter yet, and in any case
longer tube life is pretty difficult to demonstrate except in controlled
operating conditions over very long periods of time, which few of us
could manage even if we wanted to).  Happy New Year and vy 73 to all!
Jan N0JR

Dave Haupt wrote:

> Greetings ETO/Alpha listmates,
>
> The short answer is that you can certainly use the
> 3CPX800A7 in place of a 3CX800A7 tube.
>
> I have designed commercial amplifiers using the
> 3CPX800A7 and have seen over a thousand go out the
> door with no troubles at all.  I have visited the
> Eimac factory and watched both tubes being built.
>
> As Jan says, the overt datasheet difference between
> the tubes is that Eimac rates the "P" version to
> withstand higher anode voltage.  Due to the different
> anode-to-grid ceramic insulator, the anode-to-grid
> capacitance is a little different (less, if I remember
> correctly) than the anode-to-grid capacitance of the
> non-pulse rated tube.  The "P" version is also rated
> to a higher frequency (from memory, I believe it's 500
> MHz instead of 350 MHz), so it can be expected to
> deliver more gain in a 432 MHz amplifier.
>
> The cathode/grid structures of the two tubes are
> identical.  Since the wearout mechanism for this tube
> is cathode depletion, there is no reason to believe
> that there will be any difference in lifetime if they
> are used in the same circuit.
>
> The exterior dimensions are identical, so they can be
> used interchangeably in any ham amp.
>
> Due to the slightly different anode-to-grid
> capacitance, I would expect that an amp will tune with
> a slightly different "tune" capacitor position, but
> idential load cap position.
>
> If anybody experiences higher gain with the "P"
> version in an amplifier operating at or below 30 MHz,
> it is probably because they received tubes with a
> better batch of cathode/grid structures.  There is no
> intended difference between the tubes that would lead
> you to expect more gain from the "P" version.
>
> When you study the 3CX800A7 (not the "P" version)
> datasheet carefully, and do some calculations, it is
> very easy to come to the conclusion that the tube
> isn't designed all that carefully.  The anode
> dissipation is too high for the combination of anode
> voltage and cathode current, or the anode withstand
> voltage is too low for the dissipation and cathode
> current - however you want to look at it.  Basically,
> if you design an amplifier that really causes 800
> watts of plate dissipation, and is operated inside the
> anode voltage limit, you'll find that you're
> outrunning the cathode's maximum current rating.  When
> used in commercial equipment, the 3CX800A7 users
> always wanted to see higher anode voltage capability.
> Thus was born the "P" version.
>
> >From what I saw at the factory, the "P" version
> requires no more elaborate assembly or testing, and
> there are no more exotic materials in it.  So, if I
> were Eimac, I'd want to cease production of the
> 3CX800A7 and instead manufacture and sell only the "P"
> versions from now on.  My suspicion is that's their
> objective.
>
> So, to summarize: yes, the "P" version is objectively
> better, in terms of being capable of higher anode
> voltage.  In practical terms, if the circuit was
> designed for the plain (non "P") version of the tube,
> you will experience fewer high voltage events
> (internal tube arcs), and the tune capacitor will be
> adjusted a bit differently for tuneup, but that is the
> only difference you should see.  In commercial pulse
> service, I saw 15,000 to 20,000 hours of lifetime out
> of both tubes.  One FM broadcast transmitter model,
> which pushed the 3CX800A7 very hard, only averaged
> 5,000 hours, but it was running the cathode over the
> current limit spec.
>
> I've replaced now about a dozen 3CX800A7s with
> 3CPX800A7s and have experienced no deleterious
> effects.  In one homebrew 432MHz amplifier, the owner
> did see more gain, as can be expected when you replace
> a 350 MHz tube with a 500 MHz tube.
>
> Alphas of recent manufacture used one of two tubes.
> The 86, 87A and 89 use the 3CX800A7, and you should be
> able to use the 3CPX800A7 just fine in those amps.
> The 91B and 99 use the 4CX800A7, and you cannot
> substitute the 3C anything for that tube.
> Fortunately, it is easy to find surplus tubes from the
> Russian military to use in place of the 4CX800A7.  I
> think they carry the number GI-74 or GU-74.  You can
> find that information from, among other places, W4TH's
> website (www.tomstubes.com).
>
> I obviously cannot speak for the specific tubes being
> offered, but I can vouch for the efficacy of the
> 3CPX800A7 as a product.
>
> 73,
>
> Dave W8NF
>
> --- "William P. Osborne" <wosborne@utdallas.edu>
> wrote:
> > Has anyone on this list purchased samples of these
> > tubes and if so
> > what was your experience?
> >
> > Thansk bill, K5ZQ
>
> __________________________________________________
> Do You Yahoo!?
> Send FREE video emails in Yahoo! Mail!
> http://promo.yahoo.com/videomail/
> _______________________________________________
> ETO_Alpha mailing list
> ETO_Alpha@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/eto_alpha

--
"There is no end to what you can accomplish
if you don't care who gets the credit."