[ETO_Alpha] Re: 3CX800A7 versus 3CPX800A7
Jan C. Robbins
[email protected]
Mon, 21 Jan 2002 07:53:16 -0600
I think your understanding and mine are the same, Dave. You DO have to
run higher pv for higher output. I sure agree that tubes are not as
simple as they seem! Tks for all your info; it has been a great help to
me. 73, Jan N0JR
Dave Haupt wrote:
> 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
> > >
> > > __________________________________________________
> > > Do You Yahoo!?
> > > Send FREE video emails in Yahoo! Mail!
> > > http://promo.yahoo.com/videomail/
> > > _______________________________________________
> > > ETO_Alpha mailing list
> > > [email protected]
> > > 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."
> >
> >
> > _______________________________________________
> > ETO_Alpha mailing list
> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/eto_alpha
>
> __________________________________________________
> Do You Yahoo!?
> Send FREE video emails in Yahoo! Mail!
> http://promo.yahoo.com/videomail/
> _______________________________________________
> ETO_Alpha mailing list
> [email protected]
> 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."