On Sat, 29 Mar 1997 08:53:47 -0500 "Tom Rauch (W8JI)"
<W8JItom@worldnet.att.net> writes:
>km1h@juno.com wrote:
>
>> I have to disagree a bit with the above. The standard Eimac or clone
>> tantalum plate tube has a published amplification factor of 130. The
>> Amperex graphite version has an amplification factor of 200
>according to
>> their 1969 spec sheet.
>
>Carl, does the Amprex 3-500Z require a reduction of bias when
>replacing
>a 3-500Z? When you plug in am Amprex tube does the gain increase?
If you mean a change in idling current, I dont know. It has not been
something I really looked for and will remember that for the future. A
higher idling current would cause the tube to be a bit easier to drive
and offer better IMD. I note that the TL-922 ( a great -37dB 3rd order in
the QST review) uses a 7.5V Zener vs the 5.1V in the SB-220.
>Neither of these things occur in any PA's I've seen.
>
>The spec sheet I have indicates the Amprex tube has the same
>amplification factor as a 3-500Z.
I am beginning to believe that the original Amperex was meant as a
3-400Z/8163 replacement; therefore the higher amp factor. I did some
more literature digging and find that Amperex listed the 400 and 500
versions in 1973. Add to the confusion factor is that the ARRL Handbook
lists the 500Z with amp factor of 160 from 1972 until at least 1985.
I'm going out on a limb here but I'm guessing that the 500Z was
introduced in 1969. My earliest Eimac sheet is dated 1973 and has the 130
amp factor.
Lets just assume that the ARRL Handbook and early Amperex specs were a
typo error....makes life a bit easier!
>
>I've never seen any PA get "unstable" from Amprex tubes, but I have
>seen
>plenty of graphite tubes arc from gas. The graphite anode traps gas
>and
>slowly releases it on the shelf. When the tube is plugged in,
>pow....it
>arcs. An arc in the tube has nothing to do with RF instability. That's
>folklore.
VERY INTERESTING point Tom. And something I will have to consider. Past
experience has led me to classify that arc as instability and the tube
was yanked. How many times should the darn thing arc before it calms
down......or does it have to be run red and cooked??
>Labeling a Chinese tube Made in Russia is fraud, as we all agree. And
>it is being done.
>One fellow did weekend jail time for re-marking used 8877's (and other
>tubes)as new. All the new Eimac tubes on the market are not new, some
>of
>the Penta Labs Chinese 8877's were tubes from a batch of rejected
>Eimac
>8877's with cathode and grid defects.
I also had an Alpha 77DX customer that got skunked with Chinese tubes
marked as Eimac. There was about a 1000 Watt difference!
It's been an interesting discussion.
73...Carl KM1H
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