[ETO_Alpha] Re: 3CX800A7 versus 3CPX800A7
Dave Haupt
[email protected]
Sun, 20 Jan 2002 22:20:09 -0800 (PST)
Jan,
I'm only aware of Eimac's application bulletin #25 on
their website. This application bulletin suggests
that you can achieve higher output from the "P"
version of the tube by operating it at higher anode
voltage. The note implies that a re-design of the
transmitter is in order to achieve this.
I'm referring to this application note:
"Application Bulletin #25: 3CPX800A7 Recommended for
FM Broadcast" http://www.cpii.com/eimac/ab25.htm
If there's a different application note which suggests
that you can get more power out of the "P" version
while operating under the same anode voltage, I would
be most interested in a link to it. I'm always
finding new things I had never known before! Tubes
are not all that simple!
Thanks in advance,
73,
Dave W8NF
--- "Jan C. Robbins" <[email protected]> wrote:
> 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" <[email protected]>
> > 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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