[ETO_Alpha] Re: 3CX800A7 versus 3CPX800A7

[Dave Haupt]

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

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